IS-IS

End system HostIntermediate system RouterCircuit SubnetLSP LSACSNP Database descriptionPSNP ACKDesignated IS Designated routerNSAP Interface address

An IS-IS router is fully within an area and area boundaries occur between routers

L1 area exchenge within the areaL2 area exchange between the areas

PrefenceL1 = 15L2 = 18

configure router level 1 or level 2

IS-IS OPERATION-Hello PDU out every IS-IS enable interface.-L1/L2 are formed with neighbors.An adjacency is up when it sees its identifier in a neighbor's HELLO PDU.-On broadcast DIS is chosen-Routers exchange CSNP that descrive the link-state database-Based on the CSNP received, neighbors send LSPs to update the neighbors database.-On P2P link PSPN is sent to acknowledge LSP-Routers exchange CSNPs to maintain database synchronisation-Routers exchange HELLO PDU to maintain adjacencies-Topology change, the router affected transmit an update LSP that reflects the change. Every routers update its link state databaseand run SPF to recalculate the path-LSP age in the link state database for 1200 seconds

IS-IS default metric 10To operate the same manner os OSPF,IS-IS can use a reference bandwidth to calculate the interface metric

IS-IS had only 6 bt in the LSPfor carrying the metric then has been improve to support wide metric.

Broadcast:DIS is elected and originates and LSP describing the broadcast network as a psedudonode.

P2P=only one neihgbor active exchanging update.

4 differents types of IS-IS PDU2 link state database L1 and L2

IS-IS Routers are identified with an OSI NSAP.

OSI NSAP fields:

-IDP Initial domain part composed of AFI and IDI.-AFI=indicate the authority which the address was assigned.

39=ANSI47=ISO49=Privately assigned-IDI Initial domain identifier authority that assigned the address.-DSP domain specific partis the portion of the address managed by the network operator.HO-DPSP, System ID and NSEL are comprised of it

NSAP address are simplified in 7750 as the IDP and HO-DSP are assigned by area ID.6 byte system ID is assigned from the router ID and NSEL is 0

System name often used for the system ID

Router ID is transformed into 6-byte value 0100.1001.0001 (4 parts of the router ID as 3 digits)IS-IS � NSAP Addressing- Level 1 routing uses the system ID.- Level 2 routing uses the area address.- 2 nodes cannot have the same NSAP address.- 2 nodes within an area cannot have the same system ID.- The minimum NSAP using local authority is 8 bytes (1 for area, 6for system, 1 for SEL).- The area ID must be minimum 1 byte.-The AFI should be set to 49 for locally administered IS-ISconfigurations.

Item ValueMaximum metric value assignable to a link 16 777 215Maximum metric value for a path 4 261 412 864All L1 IS multicast address 01-80-C2-00-00-14All L2 IS multicast address 01-80-C2-00-00-15SAP for IS-IS on 802.3 LANs FEProtocol discriminator for IS-IS 131NSAP selector for IS-IS 00Size of LSP, which all IS routers must be able to handle 1492Maximum age 1200Zero life age 60Maximum number of area addresses in a single area 3

An IS-IS router also periodically re-floods its own LSPs with an incremented sequence number to refresh the LSDB of the other routers.

IS-IS PDU use Ethernet II TVL provide method of defining data fields in protocol messages. It is composed by:-Type -Length-Value

Protocol discriminator assigned by ISO to identify IS-IS and contains the value of 131

4 Type of IS-IS PDUs-HELLO= maintained adjacency-LSP=Information about neighbors and links, generated by all L1 and L2 routers-PSNP=Specific requests and responses about links, generated by all L1 and L2 routers-CSNP=Complete list of LSPs exchanged to maintain databaseconsistency

IS-IS adjecency is considering to be formed as soon as they have successfully exchanged HELLO PDU

A router with L1 and L2 will form 2 adjancency and have the commmon area ID

L1 LAN HELLO=broadcast network 01-80-C2-00-00-14L2 LAN HELLO=broadcast network 01-80-C2-00-00-15P2P HELLO=multicast address 09-00-2B-00-00-05

Holding time does not have to match on both ends of the link.Router with the highest priority is chosen.

DIS represents the broadcast network as a pseudonode and generates an LSP to represent the pseudonode in the network topology.DIS is the router with highest priority interface. MAC address is used as tiebreakDIS is preemptive,new router with higher priority take over and become the DIS with new LSP for the LAN.Id DIS disappear there is a new election.

DIS timers:Hello 3 seconds 9 seconds hold timeother routers 9 seconds and 27n hold time

After an adjacency has been establish there is an exchange of CSNPs,PSNPs, LSPs to bring the database update.

CSNP FormatCSNPs used to maintain consistency of link-state databaseContains list of router�s LSPs and their sequence numbers.A router that receives a CSNP that includes out-of-date LSPswill transmit up-to-date LSPs.- CSNPs are exchanged at router initialization andperiodically afterward to maintain synchronization.Every 10 seconds on broadcast networkEvery 5 seconds on point-to-point link-For each LSP in its database, the CSNP contains:Remaining life of the LSP, in secondsLSP IDLSP sequence number Checksum value

PSNPs are used by routers to request a specific LSP.PSNPs are also used on point-to-point links to acknowledgethe receipt of an LSP (but not on a broadcast link).A PSNP is similar to a CSNP except that it is a subset of theLSPs from the database.-A PSNP describes one or more LSPs and contains thefollowing information for each:Remaining life of the LSP, in secondsLSP IDLSP sequence numberChecksum value

Large PDUs are divided into fragments that are independentlyflooded.Separate databases are maintained for L1 and L2 LSPs.The ATT bit is set on LSP rtr2.00-00 to indicate that the LSP was received from an L2 router.

Pseudo node � When a broadcast subnetwork has n connected ISs, the broadcast subnetwork itself is considered to be apseudo node. The pseudo node has links to each of the n ISs and each of the ISs has a single link to the pseudo node(rather than n-1 links to each of the other ISs). Link-state PDUs are generated on behalf of the pseudo node by the DIS

Large PDUs are broken into fragments, then flooded.

Start LSP ID and End LSP Id indicate the range of LSP ID carried in the PDU.LSP Entries TLV contains the lifetime,sequence number and checksum

Each router originates an LSP.

Prefix information is carried in the TE IP Reach TLV, (or Extended IP Reachability TLV). Notice the prefLen field which indicates the length of the prefix. Router ID is the unique router ID and is not necessarily a routeable IP address. I/F Addresses lists the IP addresses for interfaces included in IS-IS, but this is not the prefix information. TE IS Nbrs identifies the interfaces where the router has formed an adjacency with another IS-IS router, but this is not the prefix information for that link. A number of the fields are described as TE because these TLVs are defined in RFC 5305 which describes the TE extensions to IS-IS.