We're going to spend the next few minutes introducing the IS-IS routing protocol. IS-IS stands for intermediate system to intermediate system and it's the OSI routing protocol for CLNS traffic specified originally in ISO 10589 document. It's a link state protocol with two-level hierarchical architecture and it has type length value options to enhance the protocol. Of most interest to us in the Internet today is RFC 1195 which added IP support to IS-IS and is known as integrated IS-IS although we just talked about IS-IS, the notable thing about IS-IS is that it runs on top of the data link layer unlike IP based protocols which run on IP. IS-IS is known as a link state routing protocol and the other well-known one which we will also cover is OSPF each node in the network computes the map of connectivity through the network. Both IS-IS and OSPF use Eska Dijkstra's algorithm producing shortest path tree through a graph and you can read more about Dijkstra's algorithm in the reference. Sure. The other type of routing protocol is distance vector and the common distance vector protocols are EIGRP which is exclusive to Cisco and RIP where each node shares its view of the routing table with other nodes in the network. Routers with IS-IS enabled on them look for neighboring routers also running IS-IS. Hello Protocol Data Units or PDUs are exchanged. The "Hello" packet includes the list of known neighbors and details such as the "hello interval" and the "router dead interval". The hello interval is how often the router will send hellos. Hello intervals are usually of the order of a few seconds 5, 10, 15, depending on the vendor defaults. The router dead interval is how long to wait before deciding the neighboring router has disappeared. The router dead interval is usually a multiple of the hello interval. So maybe three times hello intervals or in other words three hellos have been missed and that allows IS-IS to determine if the neighbor has disappeared The values of Hello interval and router dead interval do not need to match on both neighbors unlike in the case for OSPF. When a neighboring router responds with matching details a neighbor relationship is formed. A relationship is formed between neighboring routers for the purpose of exchanging routing information and this is what we call an adjacency. Once an adjacency is formed neighbors will share the link state information. Information goes in a link state PDU, or more commonly called LSP. LSPs are flooded to all neighbors. New information received from neighbors is used to compute a new view of the network. When a link fails new LSPs are flooded and the routers recompute the routing table. IS-IS operating across a network. All routers across the network form neighbor relationships with the directly attached neighbors. Each router computes the routing table. Once each router has the same view of the network, the network has converged. The IGP design for a network is crucially important to ensure scalability and rapid convergence The general rule: the fewer the prefixes, the faster the convergence. Network operators pay close attention to the IGP design to minimize the number of prefixes so they ge t the fastest possible convergence of the IGP.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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