ISIS part 2

ISIS router has 2 network types which are broadcasts (LAN and Multipoint WAN) and point-to-point (everything else). If the router configuration is point-to-point, the adjacency will be formed and the routers will send a complete sequence number package, which contain the routers link-state database and can therefore synchronize with the other. If the routers are in a broadcast network, the network will elect a DIS (Designated Intermediate System) to send the latest configuration to the other routers. The DIS is elected to the router with the highest ISIS interface priority (default = 64). If the interface priorities are the same, the highest SNPA (Sub network Point of Attachment) will determine the DIS. The SNPA is the highest MAC address if it is a LAN configuration, and the highest DLCI number in a Frame Relay configuration. A DIS does not sync with all other routers. Therefore, a pseudonode (a virtual router) is created by the DIS. In this way, all routers forms an adjacency with the pseudonode, including the DIS. The DIS can detect problems quickly because it sends hello packets every 3.3 seconds, whereas CSNP sends hello packets every 10 seconds.

ISIS has 4 types of PDU which are Hello, LSP, CSNP and PSNP packets.

Hello packets consists of 3 different packets:

1. - ESH (End System Hello) which is sent by an ES (host) to an IS (router)

2. - ISH (Intermediate System Hello) which is sent by IS and listened by ES.

3. - IIH (IS-IS Hello) which is send from an IS to another IS to form and adjeceny.

The router is able to send multiple hello packets at the same time (ISH and IIH).

LSP (Link State Packets) is used to build a link-state database sent from adjacent routers, which are necessary to build a routing table together with the SPF algorithm. There are 2 types of LSP which are the L1 LSP and L2 LSP.

CSNP (Complete Sequence Number PDU) contains a complete list of LSP, sent to adjacent routers to update and sync the adjacent router. This list is sent by the DIS every 10 seconds.

PSNP (Partial Sequence Number PDU) is used to request LSP and to acknowledge if an LSP has been received.

-- 1 March 2011 --