IS-IS in nutshell

1, IS-IS is using NASP address to build up adjacency between each node, not IP address.

2, IS-IS is used to solve L1 and L2 routing problem described in

Especially L1 is similar with OSPF Not So Stub Totally Stub area, L1 area has no information about l2 routing and any other l2 area’s routing, it has only a default router (L1L2 router) which will connect this L1 area to L2 area; L2 area is similar with OSPF backbone area, which has information of both L2 and L1 areas.

When OSI network model is used, L2 will carry only area id info because L2 only need to forward the traffic towards correct L1 area, it does not have to know all ES address in L1 area. When IP is used, L2 will have IP information for networks directly connected in L2 router, and also the networks routes learned from L1 route calculation.

L1L2 router will have two IS-IS link databases, one is for L1 area which it connects to, the other is for L2. L1 and L2 databases are separated and can not be shared with each other, but L1L2 router will advertise routes learned according to L1 calculation to L2 database.

3, In IS-IS there are two types of network link: point-to-point and broadcast link. While point-to-multipoint can be simulated as several point-to-point links.

All interfaces in broadcast link will participate in DIS selection, like DR and BDR selection in OSPF broadcast link, but unlike DR and BDR role in OSPF, DIS in IS-IS is not the only one router which can send out LSP update, instead, all routers in  IS-IS broadcast link can send out LSP update. DIS is used to:

  • Help routers on a broadcast segment to synchronize
  • Representing the broadcast segment in the link-state database as a standalone object- The Pseudonode

4, Unlike OSPF which has several types of LSA, in IS-IS each router generate only one LSP, which includes all routes information of the router, LSP can be very long and need to be segmented to transfer in Layer two network, a sequence number will be used to identify the LSP packet sent by each router.  Router selected as DIS will generate 2 LSP: One is the general LSP for routing information, the other is Pseudonode LSP to indicate broadcast segment information. LSPID will be system ID + Pseudonode ID(local circuit ID of the interface)

5,  IS-IS has 3 adjacency states:

  • Down: the initial state
  • Initializing: IIHs have been received from the neighbout, but it is not certain that the neighbor is properly receiving this routers IIH
  • Up: IIHs have been received and also it is certain that the neighbor is properly receiving this router’s IIHs

6, IS-IS is a true multiprotocol routing protocol in the sense that it does not require any particular Layer 3 routing to carry it packets, and in a single instance, it can carry informaton of destination described by different address families, for example IPv4 and Ipv6 can be carried at the same time a single LSP and maintained in the same link state datebase.