03 advanced routing 2011

7/31/2019 03 Advanced Routing 2011

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ADVANCED ROUTING


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Outline

Basic Routing

Routing Information Protocol (RIP)

Open Shortest Path First (OSPF)

Border Gateway Protocol (BGP)


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Routing and Forwarding

Routing How to determine the routing table

entries

carried out by routing daemon Forwarding

Look up routing table & forward packetfrom input to output port

carried out by IP layer

Routers exchange information using routing

protocols to develop the routing tables


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Route Construction

Static

Listed Manually: change route slowly

not robust: reachability is independent of network condition

stable Dynamic

Learn route via routing protocols

React to topology, traffic or configuration changes directly

Might not converge or oscillate Might have loop


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Static routing Used on hosts or on very small networks

Manually tell the machine where to send the packets for each prefix% netstat -nr

Routing Table:

Destination Gateway Flags Ref Use Interface

------------- ------------ ----- ---- ----- ---------

130.207.7.0 130.207.7.27 U 1 9090 ce0

130.207.6.0 130.207.7.1 UG 1 2058

130.207.102.0 130.207.7.1 UG 1 101

130.207.97.0 130.207.7.1 UG 1 351

130.207.3.0 130.207.7.1 UG 1 15961

130.207.99.0 130.207.7.1 UG 1 1705

130.207.98.0 130.207.7.1 UG 1 201

130.207.29.0 130.207.7.1 UG 1 18

130.207.28.0 130.207.7.1 UG 1 779

130.207.26.0 130.207.7.1 UG 1 524

130.207.117.0 130.207.7.1 UG 1 433

130.207.116.0 130.207.7.1 UG 1 14667

130.207.23.0 130.207.7.1 UG 1 4724

130.207.119.0 130.207.7.1 UG 1 4406

130.207.114.0 130.207.7.1 UG 1 5489

224.0.0.0 130.207.7.27 U 1 0 ce0

default 130.207.7.1 UG 1 44950

127.0.0.1 127.0.0.1 UH 7 2344869 lo0

U-Route is up H-route is to host (else route is to network)

G-route to gateway (else direct connection)


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Forwarding Procedure

Does routing table have entry that matchescomplete destination IP address? If so, usethis entry to forward

Else, does routing table have entry thatmatches the longest prefix of the destinationIP address? If so, use this entry to forward

Else, does the routing table have a default

entry? If so, use this entry.

Else, packet is undeliverable


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Hierarchical Routing

scale: with 200 million

destinations:

cant store all dests in

routing tables! routing table exchange

would swamp links!

administrative autonomy

internet = network of

networks

each network admin maywant to control routing in its

own network

Our routing study thus far - idealization

all routers identical

network flat

nottrue in practice


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Autonomous Systems

Global Internet viewed as collection of autonomoussystems.

Autonomous system (AS) is a set of routers ornetworks administered by a single organization

Same routing protocol need not be run within the AS But, to the outside world, an AS should present a

consistent picture of what ASs are reachablethrough it

Stub AS: has only a single connection to the outsideworld.

Multihomed AS: has multiple connections to the outsideworld, but refuses to carry transit traffic

Transit AS: has multiple connections to the outsideworld, and can carry transit and local traffic.


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Peering and Inter-AS connectivity

Tier 1 ISP (Transit AS)

Non-transit ASs (stub & multihomed) do not carry transit traffic

Tier 1 ISPs peer with each other, privately & peering centers

Tier 2 ISPs peer with each other & obtain transit services from Tier1s; Tier 1s carry transit traffic between their Tier 2 customers

Client ASs obtain service from Tier 2 ISPs

AS

Content or Application

Service Provider

(Non-transit)

Tier 1 ISP (Transit AS)

Tier 2 (transit AS)

Tier 2 (transit AS)

AS AS

Tier 2 (transit AS)

AS AS AS AS

Peering Center


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AS Number

For exterior routing, an AS needs a globally uniqueAS 16-bit integer number

Currently, there are about 17,000 registered ASs in

Internet (and growing)

Stub AS, which is the most common type, does notneed an AS number since the prefixes are placed at

the providers routing table

Transit ASneeds an AS number Request an AS number from the ARIN, RIPE and

APNIC


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3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

Intra-AS

Routing

algorithm

Inter-AS

Routing

algorithm

Forwarding

table

3c

Interconnected ASes

Forwarding table is

configured by both

intra- and inter-AS

routing algorithm

Intra-AS sets entries

for internal dests

Inter-AS & Intra-As

sets entries for external

dests


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3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

Inter-AS tasks Suppose router in

AS1 receivesdatagram for which

dest is outside of AS1

Router should forward

packet towards one ofthe gateway routers,

but which one?

AS1 needs:

1. to learn which dests

are reachablethrough AS2 and

which through AS3

2. to propagate this

reachability info to all

routers in AS1

Job of inter-AS routing!


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Inter and Intra Domain Routing

R

R

R

RR

R

RR

AS A

AS B

AS C

IGPEGP IGP

IGP

Interior Gateway Protocol (IGP): routing within AS

RIP, OSPFExterior Gateway Protocol (EGP): routing between ASs

BGPv4

Border Gatewaysperform IGP & EGP routing


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Intra-AS Routing

Also known as Interior Gateway Protocols (IGP)

Most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco

proprietary)

IS-IS: Intermediate System to Intermediate

System


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Inter-AS Routing Protocols

Use EGP in NSFNET


BGP-4: de facto standard

Path Vector Algorithm


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Administrative Distances

The administrative distance (AD) is used torate the trustworthiness of routing information

received on a router from a neighbor router.

An administrative distance is an integer from0 to 255, where 0 is the most trusted and 255

means NO traffic will be passed via this

route.


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Default Administrative Distances

Route Source Default AD

Connected Interface 0

Static Route 1

EIGRP 90

IGRP 100

OSPF 110

RIP 120

External EIGRP 170

Unknown 255 (no traffic)


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The Three Classes of RoutingProtocols

Distance Vector finds the best path to a remotenetwork using hop count. (RIP, IGRP)

Link State (also called shortest-path-first

protocols) the routers each create three separate

tables. 1) keeps track of directly attached neighbors,

2) topology of network, 3) the routing table. (OSPF,

IS-IS)

Hybrid uses aspects of both distance vector and

link state. (EIGRP)


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Outline

Basic Routing





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RFC 1058

RIP based on routed, route d, distributed in BSDUNIX

Uses the distance-vector algorithm

Runs on top of UDP, port number 520

Metric: number of hops

Max limited to 15

suitable for small networks (local area environments)

value of 16 is reserved to represent infinity

small number limits the count-to-infinityproblem



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RIP Operation

Router sends update message to neighbors every30 sec

A router expects to receive an update message fromeach of its neighbors within 180 seconds in theworst case

If router does not receive update message fromneighbor X within this limit, it assumes the link to Xhas failed and sets the corresponding minimum costto 16 (infinity)

Uses split horizon with poisoned reverse Convergence speeded up by triggered updates

neighbors notified immediately of changes in distancevector table


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Figure 14.8 Example of a domain using RIP


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Figure 14.9 RIP message format


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Figure 14.10 Request messages


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Figure 14.11 shows the update message sent from router R1 to

router R2 in Figure 14.8. The message is sent out of interface

130.10.0.2.

See Next Slide

The message is prepared with the combination of split horizon

and poison reverse strategy in mind. Router R1 has obtainedinformation about networks 195.2.4.0, 195.2.5.0, and 195.2.6.0

from router R2. When R1 sends an update message to R2, it

replaces the actual value of the hop counts for these three

networks with 16 (infinity) to prevent any confusion for R2.

The figure also shows the table extracted from the message.Router R2 uses the source address of the IP datagram carrying

the RIP message from R1 (130.10.02) as the next hop address.

i 14 11 S l i l 1


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Figure 14.11 Solution to Example 1


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RIP problems Counting-to-infinity problem:

Simple configuration A->B->C. If C fails, B needs to update

and thinks there is a route through A. A needs to update

and thinks there is a route thru B.

No clear solution, except to set infinity to be small (eg 16

in RIP)

Split-horizon: If As route to C is thru B, then A advertises

Cs route (only to B) as infinity.

Slow convergence after topology change:

Due to count to infinity problem

Also information cannot propagate thru node until

it recalculates routing info.


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RIP problems (contd) Black-holes:

If one node goes broke and advertises route of zero to

several key networks, all nodes immediately point to it.

How to install a fix in a distributed manner ?? Require protocol to be self-stabilizing I.e even if some

nodes are faulty, once they are isolated, the system should

quickly return to normal operation

Broadcasts consume non-router resources

Does not support subnet masks (VLSMs) No authentication


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Basic RIP

Host A Host B

Router B advertises 192.168.3.0 to Router Awith a Metric of 1 Hop.

Router A installs 192.168.3.0 in its table with a

Metric of 1. Advertises with a Metric of 2.

R o u te r A R o u te r B

R IP

S er ial0

1 9 2 . 1 6 8 . 1 . 1

S er ia l0

1 9 2 . 1 6 8 . 1 . 2

e t h 0

1 9 2 . 1 6 8 . 2 . 1

e t h 0

1 9 2 . 1 6 8 . 3 . 1

e t h 0

1 9 2 . 1 6 8 . 3 . 2

e t h 0

1 9 2 . 1 6 8 . 2 . 2

1 9 2 .1 6 8 .2 .0 /2 4 1 9 2 .1 6 8 .1 .0 /2 4 1 9 2 .1 6 8 .3 .0 /2 4


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Before RIP

Router ARouter-A#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate def

U - per-user static route, o - ODR

Gateway of last resort is not set

C 192.168.1.0/24 is directly connected, Serial0

C 192.168.2.0/24 is directly connected, Ethernet0


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Before RIP

Router BRouter-B#sh ip route











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With RIP

Router ARouter-A#sh ip route










R 192.168.3.0/24 [120/1] via 192.168.1.2, 00:00:15, Serial0


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With RIP

Router BRouter-B#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B BGP

[edited for brevity]



R 192.168.2.0/24 [120/1] via 192.168.1.1, 00:00:12, Serial0


RIP Debug Output:

00:40:48: RIP: sending v1 update to 255.255.255.255 via Ethernet0(192.168.3.1)

00:40:48: network 192.168.1.0, metric 1

00:40:48: network 192.168.2.0, metric 2

00:40:48: RIP: Update contains 2 routes


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RIP Network FailureRouter-A#sh ip route

Codes: [edited for brevity]




R 192.168.3.0/24 is possibly down, routing via 192.168.1.2, Serial0

RIP Debug output:

00:44:41: RIP: sending v1 update to 255.255.255.255 via Ethernet0(192.168.2.1)

00:44:41: network 192.168.1.0, metric 1

00:44:41: network 192.168.3.0, metric 16


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RIPv2 Why ? Installed base of RIP routers

Provides:

VLSM support

Authentication

Multicasting

Wire-sharing by multiple routing domains,

Tags to support EGP/BGP routes.

Uses reserved fields in RIPv1 header. First route entry replaced by authentication

info.

Figure 14 13 RIP version 2 format


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Figure 14.13 RIP version 2 format

Figure 14 14 Authentication


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Figure 14.14 Authentication

Interior Gateway Routing Protocol


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Interior Gateway Routing Protocol(IGRP)

Cisco-proprietary distance-vector routingprotocol (must use only Cisco routers).

Classful

Default max hop count = 100. Can be used in large networks.

Uses a different metric than RIP IGRP usesbandwidth and delay of line by default. This iscalled a composite metric. Reliability, load, and MTU can also be used,

although they are not by default.

Enhanced Interior Gateway


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Enhanced Interior GatewayRouting Protocol (EIGRP)

Cisco proprietary Classless

Uses autonomous system numbers

A number assigned to a group of routers undermutual administration.

Referred to as a hybrid routing protocol

Provides support for IP, IPX, and Appletalk

Best path selection using the DiffusingUpdate Algorithm (DUAL)

Communication via Reliable Transport

Protocol (RTP)


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EIGRP

Before EIGRP routers exchange routes with eachother, they must become neighbors.

There are three conditions that must be met for

neighborship establishment:

1) Hello or ACK received

2) AS numbers match

3) Identical metrics

Exchange info between different AS

Internal (AD=90 )& external (AD=170) EIGRP route

redistribution


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EIGRP

Feasible distance full distance Reported distance as given by the neighbor

Neighbor table

Topology table Feasible successor topology table

Successorrouting table

Reliable multicast (class D 224.0.0.10 +unicast)


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EIGRP Feature Comparison

Link-state Features

Converges quickly

Discovers neighbors viaHello packets

Builds topology table

After learning its

neighbors routes, onlychanges to the routing

table are propagated.

Distance-vector Features

Uses autonomoussystem number (like

IGRP) Uses metric based on

bandwidth & delay (alsoload & reliability)

Advertises entire routingtable to new neighbors.


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Outline

Basic Routing





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Link State Routing

Building Routing Tables1. Creation of the states of the links by each node, called

the link state packet or LSP

2. Dissemination of LSPs to every other router, called

flooding, in an efficient and reliable way

3. Formation of a shortest path tree for each node

4. Calculation of a routing table based on the shortest

path tree


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Link State Routing


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Link State Routing

Flooding of LSPs The creating node sends a copy of LSP out of

each interface

A node that receives an LSP compares it with the

copy it may already have

Sequence number of the copy > sequence number of

the arrived LSP = discard the arrived LSP

Otherwise

Discard the old LSP and keeps the new one

Send a copy of it out of each interface except the one from

which the packet arrived


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RFC 2328 (v2) Fixes some of the deficiencies in RIP

Enables each router to learn complete network

topology

Each router monitors the link stateto each neighborand floods the link-state information to other routers

Each router builds an identical link-state database

Allows router to build shortest path tree with routeras root

OSPF typically converges faster than RIP when

there is a failure in the network

Open Shortest Path First


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OSPF Logic

1) Each router discovers its neighbors on eachinterface. The list of neighbors is kept in a

neighbor table.

2) Each router uses a reliable protocol toexchange topology information with its

neighbors.

3) Each router places the learned topology

information in its topology database.


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OSPF Logic (cont.)

4) Each router runs the SPF algorithm againstits own topology database to calculate the

best routes to each subnet in the database.

5) Each router places the best route to eachsubnet in the IP routing table.


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OSPF terminology

RID router ID highest IP (virtual, physical) DR - designated router (highest priority,

highest RID)

No. SPtree = No. areas for all interfaces Metric = cost

Advertises list of connections

Multicast hellos

AllSPF dest = 224.0.0.5

AllDRs dest = 224.0.0.6


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Multiple routesto a given destination, one per type ofservice

Support forvariable-length subnettingby including thesubnet mask in the routing message

More flexible link costwhich can range from 1 to 65,535 Distribution of traffic overmultiple pathsof equal cost

Authenticationto ensure routers exchange informationwith trusted neighbors

Uses notion of areato partition sites into subsets

Designated routerto minimize table maintenanceoverhead

OSPF Features

E l OSPF T l


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Example OSPF Topology

10.5.1.1

10.5.1.3 10.5.1.5

10.5.1.2 10.5.1.410.5.1.6

At steady state:

All routers have same LS database

Know how many routers in network

Interfaces & links between routers Cost of each link (1-65.535)( CISCO ->

108/bandwidth)

Occasional Hello messages (10 sec) & LS

updates sent (30 min)


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To improve scalability, AS may be partitioned into areas Area is identified by 32-bit Area ID

Router in area only knows complete topology inside area & limits

the flooding of link-state information to area

Area border routerssummarize info from other areas

Each area must be connected to backbone area(0.0.0.0)

Distributes routing info between areas

Internal routerhas all links to nets within the same area

Area border routerhas links to more than one area Backbone routerhas links connected to the backbone

Autonomous system boundary (ASB) routerhas links toanother autonomous system.

OSPF Network


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OSPF Features

Metric OSPF protocol allows the administrator to assign

a cost, called the metric, to each route

Based on a type of service (minimum delay,

maximum throughput, and so on)

A router can have multiple routing tables, each

based on a different type of service.


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Hierarchical OSPF


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ASB: 4ABR: 3, 6, and 8

IR: 1,2,7

BBR: 3,4,5,6,8

Area 0.0.0.1Area 0.0.0.2

Area 0.0.0.3

R1

R2

R4 R5

R7

N1

N2

N3

N4

N5

N6

N7

To another AS

Area 0.0.0.0

R = router

N = network

R8

R3 R6

OSPF Areas

Neighbor, Adjacent & Designated


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Neighbor routers: two routers that have interfaces to acommon network

Neighbors are discovered dynamically by Hello protocol

Adjacent router: neighbor routers become adjacent

when they synchronize topology databases byexchange of link state information

Neighbors on point-to-point links become adjacent

Routers on multiaccess nets become adjacent only to

designated & backup designated routers Reduces size of topological database & routing traffic

e g bo , djace t & es g atedRouters


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Link state info exchanged by adjacent routers to allow area topology databases to be maintained

inter-area & inter-AS routes to be advertised

Router link ad: generated by all OSPF routers state of router links within area; flooded within area only

Net link ad: generated by the designated router

lists routers connected to net: flooded within area only

Summary link ad: generated by area border routers 1. routes to dest in other areas; 2. routes to ASB routers

AS external link ad: generated by ASB routers describes routes to destinations outside the OSPF net

flooded in all areas in the OSPF net

Link State Advertisements

Figure 14.19 Areas in an autonomous system


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Figure 14.20 Types of links


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Figure 14.21 Point-to-point link


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Figure 14.22 Transient link


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Designated router

Receives updates and distributes them toeach segment router

DR and BDR are elected on the basis of

highest OSPF priority, and highest IPaddress

Default priority is 1 and a priority of 0 prevents a

router from being elected

DRs are stable

Figure 14.23 Stub link


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Figure 14.24 Example of an AS and its graphical representation in OSPF


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Figure 14.25 Types of OSPF packets


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Figure 14.26 OSPF common header


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Figure 14.27 Link state update packet


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Figure 14.28 LSA general header


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Figure 14.29 Router link


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Figure 14.30 Router link LSA


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Table 14.2 Link types, link identification, and link data


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Give the router link LSA sent by router 10.24.7.9 inFigure 14.31.

Solution

This router has three links: two of type 1 (point-to-

point) and one of type 3 (stub network). Figure 14.32

shows the router link LSA.

See Next Slide

See Figure 14.32

Figure 14.31 Example 3


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Figure 14.33 Network link


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Figure 14.34 Network link advertisement format


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Give the network link LSA in Figure 14.35.

Solution.

See Next Slide

See Figure 14.36

Figure 14.35 Example 4


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In Figure 14.37, which router(s) sends out router link LSAs?

Solution

All routers advertise router link LSAs.

a.R1 has two links, N1 and N2.b.R2 has one link, N1.

c.R3 has two links, N2 and N3.

See Next Slide

Figure 14.37 Example 5 and Example 6


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In Figure 14.37, which router(s) sends out the network linkLSAs?

Solution

All three network must advertise network links:a.Advertisement for N1 is done by R1 because it is the only

attached router and therefore the designated router.

b.Advertisement for N2 can be done by either R1, R2, or R3,

depending on which one is chosen as the designated router.

c. Advertisement for N3 is done by R3 because it is the only

attached router and therefore the designated router.

Figure 14.38 Summary link to network


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Figure 14.39 Summary link to network LSA


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Figure 14.40 Summary link to AS boundary router


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Figure 14.41 Summary link to AS boundary router LSA


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Figure 14.42 External link


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Figure 14.43 External link LSA


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Figure 14.44 Hello packet


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Figure 14.45 Database description packet


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Figure 14.46 Link state request packet


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Figure 14.47 Link state acknowledgment packet


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Areas & LSA propagation


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Areas & LSA propagation

OSPF Issues


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OSPF --- Issues

Processor intensive Hackers can still spoof bogus route updates

Load balancing between equal metric paths isdifficult

Flooding traffic Complexity

Five Messages

Hello, exchange, request, flood update and flood ack

Three algorithms (Dijkstra, flooding, exchange) A lot of code

03 advanced routing 2011

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