feature description - ip routing(v600r001c00_03)

HUAWEI NetEngine80E/40E RouterV600R001C00

Feature Description - IP Routing

Issue 03Date 2010-03-31

HUAWEI TECHNOLOGIES CO., LTD.

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Contents

1 RIP.................................................................................................................................................1-11.1 Introduction to RIP..........................................................................................................................................1-21.2 References.......................................................................................................................................................1-21.3 Principles.........................................................................................................................................................1-2

1.3.1 RIP-1......................................................................................................................................................1-31.3.2 RIP-2......................................................................................................................................................1-31.3.3 Timer......................................................................................................................................................1-41.3.4 Split horizon...........................................................................................................................................1-41.3.5 Poison Reverse.......................................................................................................................................1-51.3.6 Triggered Update....................................................................................................................................1-51.3.7 Route Aggregation.................................................................................................................................1-61.3.8 Multi-process and Multi-instance...........................................................................................................1-71.3.9 Hot Backup.............................................................................................................................................1-7

1.4 Terms and Abbreviations................................................................................................................................1-72 RIPng............................................................................................................................................2-1

2.1 Introduction to RIPng......................................................................................................................................2-22.2 References.......................................................................................................................................................2-22.3 Principles.........................................................................................................................................................2-2

2.3.1 RIPng Packet Format.............................................................................................................................2-32.3.2 Timer......................................................................................................................................................2-42.3.3 Split Horizon..........................................................................................................................................2-42.3.4 Poison Reverse.......................................................................................................................................2-52.3.5 Triggered Update....................................................................................................................................2-52.3.6 Route Aggregation.................................................................................................................................2-62.3.7 Multi-process and Multi-instance...........................................................................................................2-62.3.8 Hot Backup.............................................................................................................................................2-7

2.4 Terms and Abbreviations................................................................................................................................2-73 IS-IS..............................................................................................................................................3-1

3.1 Introduction to IS-IS........................................................................................................................................3-23.2 References.......................................................................................................................................................3-23.3 Principles.........................................................................................................................................................3-3

3.3.1 Basic Concepts of IS-IS.........................................................................................................................3-4

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3.3.2 IS-IS Multi-instance and Multi-process...............................................................................................3-223.3.3 IS-IS Route Leaking.............................................................................................................................3-233.3.4 IS-IS Fast Convergence........................................................................................................................3-243.3.5 Priority-based IS-IS Convergence........................................................................................................3-253.3.6 IS-IS LSP Fragment Extension............................................................................................................3-253.3.7 IS-IS Administrative Tag.....................................................................................................................3-283.3.8 Dynamic Hostname Exchange Mechanism..........................................................................................3-293.3.9 IS-IS HA...............................................................................................................................................3-303.3.10 IS-IS 3-Way Handshake.....................................................................................................................3-313.3.11 IS-IS GR.............................................................................................................................................3-313.3.12 IS-IS NSR...........................................................................................................................................3-393.3.13 IS-IS for IPv6.....................................................................................................................................3-393.3.14 IS-IS MT............................................................................................................................................3-403.3.15 IS-IS TE..............................................................................................................................................3-423.3.16 IS-IS Shortcut (AA) and Advertise (FA)...........................................................................................3-463.3.17 IS-IS Wide Metric..............................................................................................................................3-483.3.18 IS-IS Local MT..................................................................................................................................3-503.3.19 IS-IS LDP Synchronization................................................................................................................3-543.3.20 BFD for IS-IS.....................................................................................................................................3-563.3.21 IS-IS Auto FRR..................................................................................................................................3-593.3.22 IS-IS Authentication...........................................................................................................................3-61

3.4 Terms and Abbreviations..............................................................................................................................3-634 OSPF.............................................................................................................................................4-1

4.1 Introduction to OSPF...................................................................................................................................... 4-24.2 References.......................................................................................................................................................4-24.3 Principles.........................................................................................................................................................4-4

4.3.1 Fundamentals of OSPF...........................................................................................................................4-44.3.2 OSPF GR..............................................................................................................................................4-134.3.3 OSPF TE..............................................................................................................................................4-164.3.4 OSPF VPN...........................................................................................................................................4-184.3.5 OSPF NSSA.........................................................................................................................................4-244.3.6 OSPF Local MT...................................................................................................................................4-254.3.7 BFD for OSPF......................................................................................................................................4-264.3.8 OSPF GTSM........................................................................................................................................4-284.3.9 OSPF Smart-discover...........................................................................................................................4-294.3.10 OSPF-BGP Association.....................................................................................................................4-294.3.11 OSPF-LDP Association......................................................................................................................4-304.3.12 OSPF Database Overflow..................................................................................................................4-324.3.13 OSPF Hot Standby.............................................................................................................................4-334.3.14 OSPF Smart Timer.............................................................................................................................4-334.3.15 OSPF MIB..........................................................................................................................................4-344.3.16 OSPF Mesh-Group.............................................................................................................................4-35

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4.3.17 Priority-based OSPF Convergence.....................................................................................................4-374.4 OSPF Applications........................................................................................................................................4-37

4.4.1 OSPF GR..............................................................................................................................................4-374.4.2 OSPF GTSM........................................................................................................................................4-37

4.5 Terms and Abbreviations..............................................................................................................................4-385 OSPFv3.........................................................................................................................................5-1

5.1 Introduction to OSPFv3..................................................................................................................................5-25.2 References.......................................................................................................................................................5-25.3 Principles.........................................................................................................................................................5-3

5.3.1 Principle of OSPFv3...............................................................................................................................5-35.3.2 OSPFv3 GR............................................................................................................................................5-95.3.3 Association Between OSPFv3 and BGP..............................................................................................5-125.3.4 Comparison Between OSPFv3 and OSPFv2.......................................................................................5-13

5.4 Terms and Abbreviations..............................................................................................................................5-156 BGP...............................................................................................................................................6-1

6.1 Introduction to BGP........................................................................................................................................6-26.2 References.......................................................................................................................................................6-36.3 Principles.........................................................................................................................................................6-4

6.3.1 Basic Principle of BGP..........................................................................................................................6-66.3.2 Route Import........................................................................................................................................6-106.3.3 Route Aggregation...............................................................................................................................6-116.3.4 Route Dampening.................................................................................................................................6-116.3.5 Community Attribute...........................................................................................................................6-126.3.6 Route Reflector....................................................................................................................................6-146.3.7 BGP Confederation..............................................................................................................................6-156.3.8 MP-BGP...............................................................................................................................................6-166.3.9 BGP GR................................................................................................................................................6-176.3.10 BGP Security......................................................................................................................................6-186.3.11 BGP 6PE............................................................................................................................................6-196.3.12 BFD for BGP......................................................................................................................................6-196.3.13 BGP Tracking.....................................................................................................................................6-206.3.14 BGP Auto FRR...................................................................................................................................6-216.3.15 Active-Route-Advertise.....................................................................................................................6-226.3.16 BGP Dynamic Update Peer-Groups...................................................................................................6-226.3.17 BGP NSR...........................................................................................................................................6-246.3.18 4-Byte AS Number.............................................................................................................................6-246.3.19 Next-Hop Iteration Based on the Specified Routing Policy..............................................................6-26

6.4 Terms and Abbreviations..............................................................................................................................6-277 Routing Policies..........................................................................................................................7-1

7.1 Introduction to Routing Policies.....................................................................................................................7-27.2 References.......................................................................................................................................................7-2

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7.3 Principles.........................................................................................................................................................7-27.3.1 Basic Principle of Routing Policies........................................................................................................7-27.3.2 Application Environment.......................................................................................................................7-47.3.3 BGP to IGP.............................................................................................................................................7-4

7.4 Terms and Abbreviations................................................................................................................................7-58 IP FRR and VPN FRR................................................................................................................8-1

8.1 Introduction to FRR........................................................................................................................................ 8-28.2 References.......................................................................................................................................................8-28.3 Principles.........................................................................................................................................................8-2

8.3.1 Principle of IP FRR................................................................................................................................8-28.3.2 Principle of VPN FRR............................................................................................................................8-38.3.3 Comparison Between IP FRR and VPN FRR........................................................................................8-48.3.4 Comparison Between IP FRR and Load Balancing...............................................................................8-4

8.4 Applications.................................................................................................................................................... 8-48.4.1 Typical Application of IP FRR.............................................................................................................. 8-48.4.2 Typical Application of VPN FRR..........................................................................................................8-5

8.5 Terms and Abbreviations................................................................................................................................8-59 Priority-based Route Convergence.........................................................................................9-1

9.1 Introduction to Priority-based Route Convergence.........................................................................................9-29.2 References.......................................................................................................................................................9-29.3 Principles.........................................................................................................................................................9-2

9.3.1 Priority-based Route Convergence.........................................................................................................9-29.4 Applications.................................................................................................................................................... 9-39.5 Terms and Abbreviations................................................................................................................................9-3

10 Indirect Next Hop...................................................................................................................10-110.1 Introduction to Indirect Next Hop...............................................................................................................10-210.2 References...................................................................................................................................................10-210.3 Principles.....................................................................................................................................................10-210.4 Application..................................................................................................................................................10-410.5 Terms and Abbreviations............................................................................................................................10-6

A Appendix List of Port Numbers of Common Protocols................................................... A-1

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Figures

Figure 1-1 RIP-1 packet format............................................................................................................................1-3Figure 1-2 RIP-2 packet format............................................................................................................................1-3Figure 1-3 Schematic diagram of split horizon....................................................................................................1-4Figure 1-4 Schematic diagram of poison reverse.................................................................................................1-5Figure 1-5 Schematic diagram of triggered update..............................................................................................1-6Figure 2-1 RIPng packet format...........................................................................................................................2-3Figure 2-2 Format of the next hop RTE...............................................................................................................2-4Figure 2-3 Format of the IPv6-prefix RTE..........................................................................................................2-4Figure 2-4 Schematic diagram of split horizon....................................................................................................2-5Figure 2-5 Schematic diagram of poison reverse.................................................................................................2-5Figure 2-6 Schematic diagram of triggered update..............................................................................................2-6Figure 3-1 OSI model...........................................................................................................................................3-4Figure 3-2 Schematic diagram of the address structure of IS-IS..........................................................................3-6Figure 3-3 Format of a Level-1 or Level-2 LAN IIH...........................................................................................3-8Figure 3-4 Format of a P2P IIH............................................................................................................................3-8Figure 3-5 Format of a Level-1 or Level-2 LSP...................................................................................................3-9Figure 3-6 Schematic diagram of LSDB overload.............................................................................................3-10Figure 3-7 Format of a Level-1 or Level-2 CSNP.............................................................................................3-10Figure 3-8 Format of a Level-1 or Level-2 PSNP..............................................................................................3-11Figure 3-9 CLV format.......................................................................................................................................3-11Figure 3-10 IS-IS topology I..............................................................................................................................3-13Figure 3-11 IS-IS topology II.............................................................................................................................3-14Figure 3-12 DISs and adjacencies in an IS-IS broadcast network.....................................................................3-15Figure 3-13 Networking diagram of a broadcast link........................................................................................3-16Figure 3-14 Process of establishing a neighbor relationship on a broadcast link..............................................3-16Figure 3-15 Process of establishing a neighbor relationship on a P2P link.......................................................3-17Figure 3-16 Process of updating LSDBs on a broadcast link.............................................................................3-19Figure 3-17 Process of updating the LSDB on a P2P link.................................................................................3-20Figure 3-18 Networking for route leaking.........................................................................................................3-23Figure 3-19 Networking for LSP fragment extension........................................................................................3-27Figure 3-20 Format of the Restart TLV.............................................................................................................3-33Figure 3-21 Process of IS-IS restarting..............................................................................................................3-35Figure 3-22 Process of IS-IS starting.................................................................................................................3-37

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Figure 3-23 Application of GR on the provider network...................................................................................3-38Figure 3-24 Networking for IS-IS MT...............................................................................................................3-41Figure 3-25 Diagram of separate IPv4/IPv6 topologies.....................................................................................3-41Figure 3-26 IS-IS route defects..........................................................................................................................3-42Figure 3-27 Relationships between MPLS TE, CSPF, and IS-IS TE................................................................3-44Figure 3-28 Diagram of IS-IS TE networking...................................................................................................3-45Figure 3-29 Principle of IS-IS Shortcut (AA) and Advertise (FA)....................................................................3-46Figure 3-30 TE tunnel scenario..........................................................................................................................3-51Figure 3-31 Local MT Topology........................................................................................................................3-52Figure 3-32 Local MT addressing the conflict between multicast and a TE tunnel...........................................3-53Figure 3-33 Networking for IS-IS LDP synchronization...................................................................................3-54Figure 3-34 LDP-IGP synchronization state machine........................................................................................3-55Figure 3-35 Networking for IS-IS BFD.............................................................................................................3-58Figure 3-36 IS-IS Auto FRR link protection......................................................................................................3-60Figure 3-37 IS-IS Auto FRR link-node dual protection.....................................................................................3-60Figure 3-38 Networking for IS-IS authentication on a broadcast network........................................................3-62Figure 4-1 Router type.........................................................................................................................................4-6Figure 4-2 Non-Backbone Area Not Connected to the Backbone Area............................................................4-12Figure 4-3 OSPF virtual link..............................................................................................................................4-13Figure 4-4 OSPF GR process.............................................................................................................................4-15Figure 4-5 Function of OSPF in the MPLS TE architecture..............................................................................4-17Figure 4-6 Running OSPF between PEs and CEs..............................................................................................4-19Figure 4-7 Configuring OSPF areas between PEs and CEs...............................................................................4-20Figure 4-8 OSPF VPN routing loops.................................................................................................................4-21Figure 4-9 OSPF sham link................................................................................................................................4-23Figure 4-10 NSSA..............................................................................................................................................4-24Figure 4-11 OSPF Local MT..............................................................................................................................4-26Figure 4-12 BFD for OSPF................................................................................................................................4-27Figure 4-13 OSPF-BGP association...................................................................................................................4-30Figure 4-14 OSPF-LDP association...................................................................................................................4-31Figure 4-15 Flooding of LSAs when OSPF mesh-group is disabled.................................................................4-35Figure 4-16 Flooding of LSAs when OSPF mesh-group is enabled..................................................................4-36Figure 4-17 Interfaces failing to be added to a mesh group...............................................................................4-36Figure 4-18 OSPF GR........................................................................................................................................4-37Figure 4-19 OSPF GTSM...................................................................................................................................4-38Figure 5-1 Router type.........................................................................................................................................5-5Figure 5-2 OSPFv3 virtual link............................................................................................................................5-8Figure 5-3 OSPFv3 planned-GR process (reset ospfv3 graceful-restart)...........................................................5-10Figure 5-4 OSPFv3 unplanned-GR process (active/standby switchover)..........................................................5-10Figure 5-5 Traffic traversing a BGP network.....................................................................................................5-12Figure 5-6 Packet loss during the restart of the router not enabled with association between OSPFv3 and BGP.............................................................................................................................................................................5-13Figure 6-1 Application scenario of BGP..............................................................................................................6-3

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Figure 6-2 BGP operating modes.........................................................................................................................6-6Figure 6-3 Diagram of BGP route dampening...................................................................................................6-12Figure 6-4 Networking diagram of configuring BGP communities...................................................................6-13Figure 6-5 Diagram of RR..................................................................................................................................6-14Figure 6-6 Diagram of a confederation..............................................................................................................6-16Figure 6-7 Networking diagram of 6PE tunnels................................................................................................6-19Figure 6-8 Networking diagram of BFD for BGP.............................................................................................6-20Figure 6-9 Networking diagram of BGP tracking..............................................................................................6-21Figure 6-10 Networking for BGP Auto FRR.....................................................................................................6-21Figure 6-11 Networking for the international gateway......................................................................................6-23Figure 6-12 Networking for the RR with many clients......................................................................................6-23Figure 6-13 Networking for a PE connecting multiple IBGP neighbors...........................................................6-24Figure 6-14 Networking for the application of 4-byte AS numbers..................................................................6-26Figure 6-15 Next-hop iteration based on the specified routing policy...............................................................6-27Figure 7-1 Networking diagram of BGP to IGP.................................................................................................. 7-5Figure 8-1 Configuring the IP FRR function....................................................................................................... 8-5Figure 8-2 Configuring the VPN FRR function...................................................................................................8-5Figure 9-1 Networking for priority-based route convergence..............................................................................9-3Figure 10-1 Schematic diagram before indirect next hop is adopted.................................................................10-3Figure 10-2 Schematic diagram after indirect next hop is adopted....................................................................10-3Figure 10-3 Networking diagram of IBGP route iteration.................................................................................10-4Figure 10-4 Networking diagram of VPN route iteration..................................................................................10-5

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Tables

Table 3-1 The following table lists the references of this document....................................................................3-2Table 3-2 Concepts in OSI and IP........................................................................................................................3-5Table 3-3 PDU types............................................................................................................................................ 3-7Table 3-4 PDU types and CLV names...............................................................................................................3-12Table 3-5 IS Alias ID TLV.................................................................................................................................3-26Table 3-6 Comparison between mode-1 and mode-2.........................................................................................3-28Table 3-7 Description of the fields of the restart TLV.......................................................................................3-33Table 3-8 Sub-TLVs defined in Extended IS reachability TLV.........................................................................3-44Table 3-9 List of modes of receiving and sending.............................................................................................3-49Table 4-1 OSPF packet type.................................................................................................................................4-5Table 4-2 OSPF LSA type....................................................................................................................................4-5Table 4-3 OSPF router type..................................................................................................................................4-6Table 4-4 OSPF route type...................................................................................................................................4-7Table 4-5 OSPF area type.....................................................................................................................................4-7Table 4-6 OSPF network type..............................................................................................................................4-8Table 4-7 Advertisement of default routes.........................................................................................................4-10Table 4-8 Differences between inter-area LSA learning and route learning......................................................4-12Table 4-9 Cause that a router exits from GR......................................................................................................4-15Table 4-10 Comparison between the GR mode and non-GR mode...................................................................4-16Table 4-11 Domain ID........................................................................................................................................4-20Table 4-12 Routing loop prevention...................................................................................................................4-22Table 4-13 BFD for OSPF..................................................................................................................................4-27Table 4-14 OSPF Smart-discover.......................................................................................................................4-29Table 4-15 OSPF-LDP association.....................................................................................................................4-31Table 4-16 OSPF database overflow..................................................................................................................4-32Table 5-1 Router types and descriptions.............................................................................................................. 5-5Table 5-2 Types of OSPFv3 routes...................................................................................................................... 5-6Table 5-3 Types of OSPFv3 areas........................................................................................................................5-6Table 5-4 Types of OSPFv3 networks................................................................................................................. 5-7Table 5-5 Comparison between the GR mode and the non-GR mode...............................................................5-11Table 6-1 References............................................................................................................................................6-4Table 6-2 List of BGP features.............................................................................................................................6-4Table 6-3 Well-known communities of BGP routes..........................................................................................6-13

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Table 8-1 Comparison Between IP FRR and VPN FRR......................................................................................8-4Table 8-2 Comparison Between IP FRR and Load Balancing.............................................................................8-4Table 9-1 Default convergence priorities of public routes...................................................................................9-2Table 10-1 Comparison between route iteration and tunnel iteration................................................................10-4Table A-1 Port number of routing protocols.......................................................................................................A-1Table A-2 Port number of application layer protocols........................................................................................A-1

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1 RIPAbout This Chapter

1.1 Introduction to RIP1.2 References1.3 Principles1.4 Terms and Abbreviations

HUAWEI NetEngine80E/40E RouterFeature Description - IP Routing 1 RIP


1-1

1.1 Introduction to RIPDefinition

RIP is short for Routing Information Protocol. RIP is a simple Interior Gateway Protocol, mainlyused in small-scale and simply-structured networks such as campus networks and regionalnetworks. RIP is not suitable for complex environments or large-scale networks.RIP is based on the Distance-Vector (DV) algorithm. It exchanges routing information throughUser Datagram Protocol (UDP) packets. The port number used by RIP is 520.RIP employs Hop Count (HC) to measure the distance to the destination. The distance is calledthe metric value. In RIP, the default HC from a router to its directly connected network is 0,andthe HC from a router to a network that is reachable through another router is 1, and so on. Thatis to say, the HC equals the number of routers passed from the local network to the destinationnetwork. To speed up the convergence, RIP defines the HC as an integer that ranges from 0 to15. The HC equal to or greater than 16 is defined as infinity, that is, the destination network orthe host is unreachable. RIP, therefore, is not applied to large-scale networks.To improve the performance and to prevent routing loops, RIP supports split horizon and poisonreverse.

PurposeAs an earliest IGP, RIP is used in small-scale networks that support RIP. The implementationof RIP is simple. The configuration and maintenance of RIP are easier than those of the OpenShortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS) protocols.RIP is thus widely used.

1.2 ReferencesThe following table lists the references of this document.

Document Description Remarks

RFC1058 This document describes RIP protocol, describes theelements, characteristic, limitation of RIP version 1.

RFC2453 This document specifies an extension of the RoutingInformation Protocol (RIP), as defined in [1], to expandthe amount of useful information carried in RIPmessages and to add a measure of security.

1.3 PrinciplesRIP is based on the Distance-Vector (DV) algorithm. It forwards packets through User DatagramProtocol (UDP). RIP uses three timers to guarantee advertisement, update, and aging of routinginformation. To maximumly avoid loops that may caused by RIP defects, RIP supports threefeatures: split horizon, poison reverse, and triggered update.

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In addition, RIP periodically advertises its routing table to neighbors, and thus it uses RIP routeconvergence to minimize the routing table.1.3.1 RIP-11.3.2 RIP-21.3.3 Timer1.3.4 Split horizon1.3.5 Poison Reverse1.3.6 Triggered Update1.3.7 Route Aggregation1.3.8 Multi-process and Multi-instance1.3.9 Hot Backup

1.3.1 RIP-1RIP-1, that is, RIP version 1, is a classful routing protocol. It supports the advertisement ofprotocol packets only in broadcast mode. Figure 1-1 shows the packet format.A RIP packet cancarry a maximum of 25 entries. RIP is based on UDP, and a RIP-1 data packet cannot be longerthan 512 bytes. The RIP-1 protocol packet does not carry any mask, so it can identify only theroutes of the natural network segment such as Class A, Class B, and Class C. RIP-1, therefore,does not support route aggregation or discontinuous subnet.

Figure 1-1 RIP-1 packet format

Metric

0 7 15 31

Address family identifierIP address

Must be zeroVersion

Must be zeroMust be zero

Must be zeroRouteEntries

Header Command

1.3.2 RIP-2RIP-2, that is, RIP version 2, is a classless routing protocol. Figure 1-2 shows the packet format.

Figure 1-2 RIP-2 packet format

Metric

0 7 15 31Command

Address Family IdentifierIP Address

Must be zeroVersion

Next HopSubnet Mask

Route Tag

RouteEntries

Header



1-3

Compared with RIP-1, RIP-2 has the following advantages:l It supports route tag and can flexibly control routes on the basis of the tag in the routing

policy.l Its packets contain mask information and support route aggregation and Classless Inter-

domain Routing (CIDR).l It supports the next hop address and can select the optimal next hop address in the broadcast

network.l It uses multicast routes to send update packets. Only RIP-2 routers can receive protocol

packets. This reduces the resource consumption.l To enhance the security, RIP-2 provides two authentication modes to enhance security:

plain-text authentication and MD5 authentication.

1.3.3 TimerRIP mainly uses three timers:l Update timer: The timer triggers the sending of update packets every 30s.l Age timer: If a RIP router does not receive any update packet from its neighbors in the

aging time, the RIP router considers the route to its neighbors unreachable.l Garbage-Collect timer:If the route is no longer valid after the timer times out, the entry is

removed from the RIP routing table.The following describes the relationship among the three timers:The advertisement of RIP routing update is triggered by the update timer every 30 seconds. Eachentry is associated with two timers, the age timer and the garbage-collect timer. When a routeis learned and installed in the routing table, the age timer is initialized. If no Update packet isreceived from the neighbor for 180 seconds, the metric of the route is set to 16. At the same time,the garbage-collect timer is initialized. If no Update packet is received for 120 seconds, the entryis deleted after the garbage-collect timer times out.

1.3.4 Split horizonThe principle of split horizon is that a route learnt by RIP on an interface is not sent to neighborsfrom the interface. This reduces bandwidth consumption and avoids route loops.

Figure 1-3 Schematic diagram of split horizon

RTA RTB

10.0.0.0/2

10.0.0.0/2

As shown in Figure 1-3, Router B sends a route to 10.0.0.0 to Router A and Router A does notsend the route back to Router B.




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1.3.5 Poison ReverseThe principle of poison reverse is that RIP sets the cost of the route learnt from an interface ofa neighbor to 16 (specifying the route as unreachable) and then sends the route from the interfaceback to the neighbor. In this way, RIP can delete useless routes from the routing table of theneighbor.Poison reverse of RIP can also avoid route loops.

Figure 1-4 Schematic diagram of poison reverse

RouterA RouterB10.0.0.0/16

10.0.0.0/2

As shown in Figure 1-4, if poison reverse is not configured, Router B sends Router A a routethat is learnt from Router A and the cost of the route from Router A to network 10.0.0.0 is 1. Ifthe route from Router A to network 10.0.0.0 is unreachable and Router B keeps sending RouterA routes to network 10.0.0.0 because Router B fail to receive the route update packet from RouterA, a route loop forms.If Router A sends Router B a message that the route is unreachable after receiving a route fromRouter B, Router B no longer learns the reachable route from Router A, thus avoiding routeloops.If both poison reverse and split horizon are configured, simple split horizon (the route learntfrom an interface is not sent back through the interface) is replaced by poison reverse.

1.3.6 Triggered UpdateTriggered update occurs when the local routing information changes and the local routerimmediately notifies its neighbors of the changes of routing information by sending the triggeredupdate packet.Triggered update shortens the network convergence time. When the local routing informationchanges, the local router immediately notifies its neighbors of the changes of routing informationrather than waiting for periodical update.



1-5

Figure 1-5 Schematic diagram of triggered update

11.1.0.0

RouterARouterB

RouterC

11.4.0.0

E0S0 S0 S1

S0E011.3.0.0

11.2.0.0

The network to11.4.0.0 fails.



As shown in Figure 1-5, when network 11.4.0.0 is unreachable, Router C learns the informationfirst. Usually, the route update message is sent to neighbors every 30s. If the update message ofRouter B is sent to Router C when Router C is waiting for the route update message, Router Clearns the faulty route to network 11.4.0.0 from Router B. In this case, the routes from RouterB or Router C to network 11.4.0.0 point to Router C or Router B respectively, thus forming aroute loop. If Router detects a network fault and immediately sends a route update message toRouter B before the new update interval reaches. Consequently, the routing table of Router B isupdated in time, and routing loops are avoided.There is another mode of triggering updates: The next hop of the route is unavailable becausethe link is faulty. The local Router needs to notify neighboring Router about the unreachabilityof this route. This is done by setting the cost of the route as 16 and advertising the route. Thisis also called route-withdrawal.

1.3.7 Route AggregationWhen different subnet routes in the same natural network segment are transmitted to othernetwork segments, these routes are aggregated into one route of the same segment. This processis called route aggregation. RIP-1 packets do not carry mask information, so RIP-1 can advertiseonly the routes with natural masks. RIP-2 packets carry mask information, so RIP-2 supportssubnetting.RIP-2 route convergence can improve extendibility and efficiency and minimize the routingtable of a large-scale network.Route convergence is classified into two types as follows:l Classful convergence based on RIP processes:

Aggregated routes are advertised with natural masks. When split horizon or poison reverseis configured, classful aggregation becomes invalid due to the following reasons: split




Issue 03 (2010-03-31)

horizon and poison reverse suppress routes to be advertised and when classful aggregationis configured, an aggregated route may be the aggregation result of routes from differentinterfaces. As a result, a conflict occurs on the aggregated route in advertisement.For example, router 10.1.1.0 /24 (metric=2) and router 10.2.2.0 /24 (metric=3) areaggregated as an aggregated route (10.0.0.0 /8(metric=2)) in the natural network segment.RIP-2 aggregation is classful, thus obtaining the optimal metric.

l Interface-based aggregation:A user can specify an aggregation address.For example, router 10.1.1.0 /24(metric=2) and router 10.2.2.0 /24 (metric=3) areaggregated as an aggregated route (10.0.0.0 /16(metric=2)).

1.3.8 Multi-process and Multi-instanceFor easy management and effective control, RIP supports multi-process and multi-instance. Themulti-process feature allows a set of interfaces to be associated with a specific RIP process. Thisensures that the specific RIP process performs all the protocol operations only on this set ofinterfaces. Thus, multiple RIP processes can work on a single router and each process isresponsible for a unique set of interfaces. In addition, the routing data is independent betweenRIP processes; however, routes can be imported between processes.For the routers that support the VPN, each RIP process is associated with a specific VPN instance.In this case, all the interfaces attached to the RIP process should be associated with the RIP-process-related VPN instance.

1.3.9 Hot BackupRouters with distributed architecture support the RIP Hot Standby (HSB) feature. RIP backs updata from the Active Main Board (AMB) to the Standby Main Board (SMB). Whenever theAMB fails, the SMB becomes active. In this manner, RIP, being free from active/standbyswitchover, proceeds to work normally.RIP supports only the backup of RIP configurations. RIP performs Graceful Restart (GR) toresend a routing request to neighbors and synchronize route database.

1.4 Terms and AbbreviationsTerm

Term ExplanationPoison reverse RIP sets the cost of the route learnt from an interface to 16 (specifying the

route as unreachable) and then sends the route from the interface toneighbors.

Split horizon A route learnt by RIP on an interface is not sent to neighbors from theinterface.



1-7

AbbreviationAbbreviation Full SpellingRIP Routing Information Protocol




Issue 03 (2010-03-31)

2 RIPngAbout This Chapter

2.1 Introduction to RIPng2.2 References2.3 Principles2.4 Terms and Abbreviations

HUAWEI NetEngine80E/40E RouterFeature Description - IP Routing 2 RIPng


2-1

2.1 Introduction to RIPngDefinition

RIPng is an IPv6 extension of RIP-2 on the original IPv4 network. Most RIP concepts can beapplied to RIPng.RIPng, based on the Distance Vector (D-V) algorithm, is a routing protocol that measures thedistance (metrics or cost) to the destination host by Hop Count (HC). According to RIPng, theHC from a router to its directly connected network is 0, and the HC from a router to a networkthat is reachable through another router is 1, and so on. When the HC reaches 16, the destinationnetwork or host is defined as unreachable.For adaption to the IPv6 network, RIPng is derived from RIP with changes as follows:l UDP port number: RIPng uses UDP port number 521 to send and receive routing

information.l Multicast address: RIPng uses FF02::9 as the multicast address of a RIPng router in the

local scope of the links.l Prefix length: RIPng uses a 128-bit (the mask length) prefix in the destination address.l Next hop address: RIPng uses a 128-bit IPv6 address.l Source address: RIPng uses the local link address FE80::/10 as the source address to send

RIPng update packets.

PurposeRIPng is developed by extending RIP to support IPv6.

2.2 ReferencesThe following table lists the references of this document.

Document Description Remarks

RFC 2080 This document specifies a routing protocol for an IPv6 Internet.It is based on protocols and algorithms currently in wide use onthe IPv4 Internet.

2.3 PrinciplesRIPng is developed by extending RIP-2 on the IPv6 network and uses the same timer as RIP-2.RIPng supports split horizon, poison reverse, and triggered update to avoid routing loops.2.3.1 RIPng Packet Format2.3.2 Timer2.3.3 Split Horizon

2 RIPngHUAWEI NetEngine80E/40E Router



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2.3.4 Poison Reverse2.3.5 Triggered Update2.3.6 Route Aggregation2.3.7 Multi-process and Multi-instance2.3.8 Hot Backup

2.3.1 RIPng Packet FormatA RIPng packet is composed of a header and several route table entries (RTEs). In a RIPngpacket, the maximum number of RTEs is determined by the MTU value of the interface.Figure 2-1 shows the basic format of a RIPng packet.

Figure 2-1 RIPng packet format

---------

0 7 15 31Command Must be zeroVersion

Route table entry 1 (20 octets)

Route table entry N (20 octets)

A RIPng packet contains two types of RTEs as follows:l Next hop RTE: is located before the IPv6-prefix RTEs that have the same next hop. It

defines the IPv6 address of the next hop.l IPv6-prefix RTE: is located after a next-hop RTE. Several different IPv6-prefix RTEs can

exist after the next-hop RTE. It describes the destination IPv6 address and the cost in theRIPng routing table.

Figure 2-2 shows the format of the next-hop RTE.



2-3

Figure 2-2 Format of the next hop RTE0 7 15 31

Must be zero Must be zero 0xFF

IPv6 next hop address (16 octets)

Figure 2-3 shows the format of the IPv6-prefix RTE.

Figure 2-3 Format of the IPv6-prefix RTE0 7 15 31

Route tag Prefix len Metric

IPv6 prefix (16 octets)

2.3.2 TimerRIPng uses the following three timers:l Update timer: The timer triggers the sending of update packets every 30s. This timer

synchronizes RIPng routes on the network.l Age timer: If a RIPng router does not receive any update packet from its neighbors in the

aging time, the RIPng router considers the route to its neighbors unreachable.l Garbage-Collect timer: If the route is no longer valid after the timer times out, the entry is

removed from the RIPng routing table.The following describes the relationship among the three timers:The advertisement of RIPng routing update is triggered by the update timer every 30 seconds.Each entry is associated with two timers, the age timer and the garbage-collect timer. When aroute is learned and installed in the routing table, the age timer is initialized. If no Update packetis received from the neighbor for 180 seconds, the metric of the route is set to 16. At the sametime, the garbage-collect timer is initialized. If no Update packet is received for 120 seconds,the entry is deleted after the garbage-collect timer times out.

2.3.3 Split HorizonThe principle of split horizon is that a route learnt by RIPng on an interface is not sent to neighborsfrom the interface. This reduces bandwidth consumption and avoids route loops.




Issue 03 (2010-03-31)

Figure 2-4 Schematic diagram of split horizon

RouterA RouterB

123::45/64

123::45/64

As shown in Figure 2-4, Router B sends a route to network 123::45 to Router A and Router Adoes not send the route back to Router B.

2.3.4 Poison ReverseThe principle of poison reverse is that RIPng sets the cost of the route learnt from an interfaceof a neighbor to 16 (specifying the route as unreachable) and then sends the route from theinterface to the neighbor. In this way, RIPng can delete useless routes from the routing table ofthe neighbor.Poison reverse of RIPng can also avoid route loops.

Figure 2-5 Schematic diagram of poison reverse

RouterA RouterB

123::0/64metric=16

123::0/64metric=1

As shown in Figure 2-5, if poison reverse is not configured, Router B sends Router A a routethat is learnt from Router A. The cost of the route from Router A to network 123::0/64 is 1.When the route from Router A to network 123::0/64 becomes unreachable and Router B doesnot receive the update packet from Router A and thus keeps sending Router A the route fromRouter A to network 123::0/64, a route loop occurs.If Router A sends Router B a message that the route is unreachable after receiving a route fromRouter B, Router B no longer learns the reachable route from Router A, thus avoiding routeloops.If both poison reverse and split horizon are configured, simple split horizon (the route learntfrom an interface is not sent back through the interface) is replaced by poison reverse.

2.3.5 Triggered UpdateTriggered update occurs when local routing information changes and then the local routerimmediately notifies its neighbors of the changes of routing information by sending the triggeredupdate packet.Triggered update shortens the network convergence time. When local routing informationchanges, the local router immediately notifies its neighbors of the changes of routing informationrather than waiting for periodical update.



2-5

Figure 2-6 Schematic diagram of triggered update

E0S0 S0 S1

S0E011:1::0

11:2::0

The network to123::0 fails.



As shown in Figure 2-6, when network 123::0 is unreachable, Router C learns the informationfirst. Usually, the route update message is periodically sent to neighbors. For example, RIPngsends the route update message every 30s. If the update message of Router B is sent to RouterC when Router C is waiting for the route update message, Router C learns the faulty route tonetwork 123::0 from Router B. In this case, the routes from Router B or Router C to network123::0 point to Router C or Router B respectively, thus forming a route loop. If Router detectsa network fault and immediately sends a route update message to Router B before the new updateinterval reaches. Consequently, the routing table of Router B is updated in time, and routingloops are avoided.There is another mode of triggering updates: The next hop of the route is unavailable becausethe link is faulty. The local Router needs to notify neighboring Router about the unreachabilityof this route. This is done by setting the cost of the route as 16 and advertising the route. Thisis also called route-withdrawal.

2.3.6 Route AggregationRIPng route aggregation is implemented by aggregating all routes advertised on an interfaceaccording to the longest match rule.RIPng route aggregation can improve extendibility and efficiency and minimize the routing tableof a large-scale network.Implementation of route aggregation:For example, RIPng advertises two routes, 11:11:11::24 Metric=2 and 11:11:12::34 Metric=3,from an interface, and the aggregation route configured on the interface is 11::0/16. In thismanner, the finally advertised route is 11::0/16 Metric=2.

2.3.7 Multi-process and Multi-instanceFor easy management and effective control, RIPng supports multi-process and multi-instance.The multi-process feature allows a set of interfaces to be associated with a specific RIPng




Issue 03 (2010-03-31)

process. This ensures that the specific RIPng process performs all the protocol operations onlyon this set of interfaces. Thus, multiple RIPng processes can work on a single router and eachprocess is responsible for a unique set of interfaces. In addition, the routing data is independentbetween RIPng processes; however, routes can be imported between processes.For the routers that support the VPN, each RIPng process is associated with a specific VPNinstance. In this case, all the interfaces attached to the RIPng process should be associated withthe RIPng-process-related VPN instance.

2.3.8 Hot BackupRouters with distributed architecture support the RIPng Hot Standby (HSB) feature. RIPng backsup data from the Active Main Board (AMB) to the Standby Main Board (SMB). Whenever theAMB fails, the SMB becomes active. In this manner, RIPng, being free from active/standbyswitchover, proceeds to work normally.RIPng backs up RIPng configurations only. After the SMB is activated, RIPng resends a routerequest to neighbors and synchronizes routing database.

2.4 Terms and AbbreviationsTerm

Term ExplanationPoisonreverse

RIPng sets the cost of the route learnt from an interface to 16 (specifying thedestination of the route as unreachable) and then sends the route from theinterface to neighbors.

Split horizon A route learnt by RIPng on an interface is not sent to neighbors from theinterface.

AbbreviationAbbreviation Full SpellingRIPng RIP next generation



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3 IS-ISAbout This Chapter

3.1 Introduction to IS-IS3.2 References3.3 Principles3.4 Terms and Abbreviations

HUAWEI NetEngine80E/40E RouterFeature Description - IP Routing 3 IS-IS


3-1

3.1 Introduction to IS-ISDefinition

The Intermediate System-to-Intermediate System (IS-IS) is a dynamic routing protocol initiallydesigned by the International Organization for Standardization (ISO) for its ConnectionlessNetwork Protocol (CLNP).To support IP routing, the Internet Engineering Task Force (IETF) extends and modifies IS-ISin RFC 1195. This enables IS-IS to be applied to TCP/IP and OSI environments. This type ofIS-IS is called Integrated IS-IS or Dual IS-IS.IS-IS stated in this document refers to Integrated IS-IS, unless otherwise stated.

PurposeAs an Interior Gateway Protocol (IGP), IS-IS is used in Autonomous Systems (ASs). IS-IS is alink state protocol. It uses the Shortest Path First (SPF) algorithm to calculate routes.

3.2 ReferencesTable 3-1 The following table lists the references of this document.

Document Description RemarksISO 10589 ISO IS-IS Routing Protocol ISO 8348/Ad2 Network Services Access Points RFC 1195 Use of OSI IS-IS for Routing in TCP/IP

and Dual EnvironmentsMultipleauthenticationpasswords are notsupported.

RFC 2763 Dynamic Hostname ExchangeMechanism for IS-IS

RFC 2966 Domain-wide Prefix Distribution withTwo-Level IS-IS

RFC 2973 IS-IS Mesh Groups RFC 3277 IS-IS Transient Blackhole Avoidance RFC 3373 Three-Way Handshake for IS-IS Point-

to-Point Adjacencies

RFC 3567 Intermediate System to IntermediateSystem (IS-IS) CryptographicAuthentication

RFC 3719 Recommendations for InteroperableNetworks using IS-IS

3 IS-ISHUAWEI NetEngine80E/40E Router



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Document Description RemarksRFC 3784 IS-IS extensions for Traffic

Engineering

RFC 3786 Extending the Number of IS-IS LSPFragments Beyond the 256 Limit

RFC 3787 Recommendations for Interoperable IPNetworks using IS-IS

RFC 3847 Restart signaling for IS-IS RFC 3906 Calculating Interior Gateway Protocol

(IGP) Routes Over Traffic EngineeringTunnels

RFC 4444 Management Information Base for IS-IS

draft-ietf-IS-IS-ipv6-05.txt Routing IPv6 with IS-IS draft-ietf-IS-IS-wg-multi-topology-11.txt

M-IS-IS: Multi Topology (MT)Routing in IS-IS

draft-ietf-isis-admin-tags-02(Admin Tag).txt

Admin Tag

3.3 Principles3.3.1 Basic Concepts of IS-IS3.3.2 IS-IS Multi-instance and Multi-process3.3.3 IS-IS Route Leaking3.3.4 IS-IS Fast Convergence3.3.5 Priority-based IS-IS Convergence3.3.6 IS-IS LSP Fragment Extension3.3.7 IS-IS Administrative Tag3.3.8 Dynamic Hostname Exchange Mechanism3.3.9 IS-IS HA3.3.10 IS-IS 3-Way Handshake3.3.11 IS-IS GR3.3.12 IS-IS NSR3.3.13 IS-IS for IPv63.3.14 IS-IS MT



3-3

3.3.15 IS-IS TE3.3.16 IS-IS Shortcut (AA) and Advertise (FA)3.3.17 IS-IS Wide Metric3.3.18 IS-IS Local MT3.3.19 IS-IS LDP Synchronization3.3.20 BFD for IS-IS3.3.21 IS-IS Auto FRR3.3.22 IS-IS Authentication

3.3.1 Basic Concepts of IS-ISDevelopment of IS-IS

CLNP is a Layer 3 protocol in the OSI model posed by the ISO. IS-IS is initially designed bythe ISO and is used as a routing protocol based on CLNP addressing.

Figure 3-1 OSI model

Application

Presentation

Session

Transport

DataLink

Physical

Presentation Service/Presentation Protocal

Session Service/Session Protocal

TP0 TP1 TP2 TP3 TP4

IEEE802.2 X.25

IEEE 802.5Token Ring FDDI

IEEE802.3

IEEE 802.3Hardware

Token RingHardware

FDDIHardware

X.25Hardware

OSI ReferenceModel

ASESCMIP DS FTAM MHS VTP

OSI Protocol Suite

Network CONP/CMNS CLNP/CLNSIS-IS ES-IS

ACSE ROSE RTSE CCRSE ......

OSI adopts systemized (or hierarchical) addressing. The services on the transport layer in OSIcan be addressed through the Network Service Access Point (NSAP).The following lists the commonly used terms in OSI:l CLNS: indicates the Connectionless Network Service.l CLNP: indicates the Connectionless Network Protocol.




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l CMNS: indicates the Connection-Mode Network Service.l CONP: indicates the Connection-Oriented Network Protocol.OSI implements CLNS through CLNP, and implements CMNS through CONP.CLNS is implemented through the following protocols:l CLNP: is similar to the IP protocol in TCL/IP.l IS-IS: is the routing protocol of an intermediate system.l ES-IS: is the protocol used between a host system and an intermediate system. It is similar

to ARP or ICMP in IP.

Table 3-2 Concepts in OSI and IPAbbreviation

Concepts in OSI Concepts in IP

IS Intermediate System RouterES End System HostDIS Designated Intermediate System Designated Router (DR) in

OSPFSysID System ID Router ID in OSPFPDU Protocol Data Unit IP packetLSP Link state Protocol Data Unit OSPF LSANSAP Network Service Access Point IP address

With the popularity of TCP/IP, the IETF extends and modifies IS-IS in RFC 1195 to support IProuting. This enables IS-IS to be applied to TCP/IP and OSI environments. This type of IS-ISis called Integrated IS-IS or Dual IS-IS.

Address Structure of IS-ISIn OSI, the NSAP is an address used to locate resources. The ISO adopts the address structureshown in Figure 3-2, that is, NSAP. NSAP is composed of the Initial Domain Part (IDP) andthe Domain Specific Part (DSP). The IDP is equal to the network ID in the IP address, and theDSP is equal to the subnet number and host address in an IP address.As defined by the ISO, the IDP consists of the Authority and Format Identifier (AFI) and theInitial Domain Identifier (IDI). The AFI specifies the address assignment mechanism and theaddress format; the IDI identifies a domain.The DSP consists of the High Order DSP (HODSP), system ID, and NSAP Selector (SEL). TheHODSP is used to divide areas; the system ID identifies a host; the SEL indicates the servicetype.The lengths of the IDP and the DSP are variable. The maximum length of the NSAP is 20 bytesand its minimum length is 8 bytes.



3-5

Figure 3-2 Schematic diagram of the address structure of IS-IS

AFI IDI High Order DSP System ID SEL(1 octet)

DSPIDP

Area Address

l Area address

The IDP together with the HODSP of the DSP can identify a routing domain and the areasin a routing domain; therefore, the combination of the IDP and HODSP is referred to as anarea address, which is equal to an area number in OSPF. There cannot be the same areaaddress in a routing domain. and the Level-1 area addresses of the routers in the same areamust be the same.In general, a router can be configured with only one area address. The area address of allnodes in an area must be the same. In the implementation of NE80E/40E, an IS-IS processcan be configured with a maximum of three area addresses for supporting seamlesscombination, division, and transformation of areas.

l System IDA system ID uniquely identifies a host or a router in an area. In the NE80E/40E, the fixedlength of the system ID is 48 bits (6 bytes).In actual applications, a router ID corresponds to a system ID. If a router takes the IP address168.10.1.1 of Loopback 0 as its router ID, its system ID used in IS-IS can be obtained inthe following manners: Extend each part of the IP address 168.10.1.1 to 3 bits and add 0 to the front of the part

that is shorter than 3 bits. Divide the extended address 168.010.001.001 into three parts, with each part consisting

of four decimal digits. The reconstructed 1680.1000.1001 is the system ID.There are many ways to specify a system ID. You need to ensure that the system ID uniquelyidentifies a host or a router.

l SELThe role of an SEL (also referred to as NSAP Selector or N-SEL) is similar to that of the"protocol identifier" of IP. A transport protocol matches an SEL. The SEL is always "00"in IP.

l NETA Network Entity Title (NET) indicates the network layer information of an IS itself. Itdoes not contain the transport layer information (SEL = 0). A NET can be regarded as aspecial NSAP. The length of the NET field is the same as that of an NSAP. Its maximumlength is 20 bytes and its minimum length is 8 bytes. When configuring IS-IS on a router,you can configure only a NET instead of an NSAP.




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In general, an IS-IS process is configured with only one NET. When an area needs to beredefined, such as being combined with other areas or divided into sub-areas, you canconfigure the router with multiple NETs to ensure the correctness of routes.An IS-IS process can be configured with a maximum of three area addresses, and thus amaximum of three NETs can be configured. When configuring multiple NETs, ensure thattheir system IDs are the same.For example, there is a NET ab.cdef.1234.5678.9abc.00, in which, the area is ab.cdef, thesystem ID is 1234.5678.9abc, and the SEL is 00.

NOTE

The routers in an area must have the same area address.

IS-IS PDU FormatThe types of PDUs for IS-IS include Hello, LSPs, CSNPs, and PSNPs.

Table 3-3 PDU typesTypeValue

PDU Type Name

15 Level-1 LAN IS-IS Hello PDU L1 LAN IIH16 Level-2 LAN IS-IS Hello PDU L2 LAN IIH17 Point-to-Point IS-IS Hello PDU P2P IIH18 Level-1 Link State PDU L1 LSP20 Level-2 Link State PDU L2 LSP24 Level-1 Complete Sequence Numbers PDU L1 CSNP25 Level-2 Complete Sequence Numbers PDU L2 CSNP26 Level-1 Partial Sequence Numbers PDU L1 PSNP27 Level-2 Partial Sequence Numbers PDU L2 PSNP

l Hello packet formatHello packets, also called the IS-to-IS Hello PDUs (IIH), are used to set up and maintainneighbor relationships. Among them, Level-1 LAN IIHs are applied to the Level-1 routerson broadcast LANs; Level-2 LAN IIHs are applied to the Level-2 routers on broadcastLANs; P2P IIHs are applied to non-broadcast networks. Packets in different networks havedifferent formats.Figure 3-3 shows the format of a Hello packet in a broadcast network (the part in blue isthe common header).



3-7

Figure 3-3 Format of a Level-1 or Level-2 LAN IIHNo. of Octets

PDU TypeVersion

Reserved

R

1

Intradomain Routeing Protocol DiscriminatorLength Indicator

ID LengthVersion/Protocol ID Extension

Maximum Area Address

11

1111

R R

1

PDU LengthPriority

LAN IDR

ID LengthReserved/Circuit Type

Holding TimeSource ID

Variable Length Fields

1

221ID Length+1

Figure 3-4 shows the format of a Hello packet in a P2P network.

Figure 3-4 Format of a P2P IIHNo. of Octets

PDU TypeVersion

Reserved

R

1

Intradomain Routeing Protocol Discriminator



11

1111

R R

1

Length Indicator

PDU LengthLocal Circuit ID

ID LengthHolding Time

Source ID


221

Reserved/Circuit Type 1




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As shown in Figure 3-4, most fields in a P2P IIH are the same as those in a LAN IIH. TheP2P IIH does not have the priority and LAN ID fields, but has a local circuit ID field. Thelocal circuit ID indicates the local link ID.

l LSP packet formatLink State PDUs (LSPs) are used to exchange link-state information. There are two typesof LSPs, that is, Level-1 LSPs and Level-2 LSPs. Level-1 IS-IS transmits Level-1 LSPs;Level-2 IS-IS transmits Level-2 LSPs; Level-1-2 IS-IS can transmit both Level-1 andLevel-2 LSPs.Level-1 and Level-2 LSPs have the same format, as shown in Figure 3-5.

Figure 3-5 Format of a Level-1 or Level-2 LSP

Sequency NumberChecksum

No. of Octets

PDU Length

LSP IDRemaining Lifetime


22

42

PDU TypeVersion

Reserved

R

1




11

1111

R R

1

ATT IS TypeOLP 1

ID Length+2

The main fields are described as follows: OL: indicates LSDB overload.

LSPs with the overload bit are still flooded on the network, but the LSPs are not usedwhen routes that pass through a router configured with the overload bit are calculated.That is, after a router is configured with the overload bit, other routers ignore the routerwhen performing the SPF calculation. Only the direct routes of the router are considered.As shown in Figure 3-6, packets from Router A to Router C are all forwarded by RouterB. If the OL field is set to 1 on Router B, however, Router A considers that the LSDBof Router B is incomplete. Router A then forwards the packets to Router C throughRouter D and Router E, but the packets to the destination that is directly connected toRouter B are forwarded normally.



3-9

Figure 3-6 Schematic diagram of LSDB overload

RouterD RouterE

OverloadRouterC

RouterB

RouterA

IS Type: indicates the type of IS-IS generating the LSP.

It is used to specify whether the level of IS-IS is Level-1 or Level-2 (01 indicatesLevel-1; 11 indicates Level-2).

l SNP FormatSequence Number PDUs (SNPs) describe the LSPs in all or part of the databases tosynchronize and maintain all LSDBs.An SNP consists of a complete SNP (CSNP) and a partial SNP (PSNP). They are furtherdivided into a Level-1 CSNP, a Level-2 CSNP, a Level-1 PSNP, and a Level-2 PSNP.A CSNP contains the summary of all LSPs in an LSDB. This maintains LSDBsynchronization between neighboring routers. On a broadcast network, the DIS periodicallysends CSNPs. The default interval for sending CSNPs is 10 seconds. On a point-to-pointlink, CSNPs are sent only when the neighbor relationship is established for the first time.Figure 3-7 shows the CSNP format.

Figure 3-7 Format of a Level-1 or Level-2 CSNP

End LSP IDID Length+2

No. of Octets

PDU Length

Start LSP IDSource ID


2

PDU TypeVersion

Reserved

R

1




11

1111

R R

1

ID Length+1

ID Length+2




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The main fields are described as follows: Source ID: indicates the system ID of the router that sends the SNP. Start LSP ID: indicates the ID of the first LSP in the CSNP. End LSP ID: indicates the ID of the last LSP in the CSNP.A PSNP lists only the sequence number of recently received LSPs. A PSNP canacknowledge multiple LSPs at a time. If an LSDB is not updated, the PSNP is also used torequest a neighbor to send a new LSP.Figure 3-8 shows the PSNP format.

Figure 3-8 Format of a Level-1 or Level-2 PSNPNo. of Octets

PDU LengthSource ID


2

PDU TypeVersion

Reserved

R

1




11

1111

R R

1

ID Length+1

l CLV

The variable length fields in a PDU are the multiple Code-Length-Values (CLVs). Figure3-9 shows the CLV format. A CLV is also called the Type- Length-Value (TLV).

Figure 3-9 CLV formatNo. of Octets

Length

CodeLengthValue

11

CLVs vary with PDU types, as shown in Table 3-4.



3-11

Table 3-4 PDU types and CLV namesCLV Code Name Applied PDU Type1 Area Addresses IIH and LSP2 IS Neighbors (LSP) LSP4 Partition Designated Level2 IS L2 LSP6 IS Neighbors (MAC Address) LAN IIH7 IS Neighbors (SNPA Address) LAN IIH8 Padding IIH9 LSP Entries SNP10 Authentication Information IIH, LSP, and SNP128 IP Internal Reachability Information LSP129 Protocols Supported IIH and LSP130 IP External Reachability Information L2 LSP131 Inter-Domain Routing Protocol

I

feature description - ip routing(v600r001c00_03)

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