IS-IS explained (Part1)

• IS-IS was developed for OSI routing then extended to support IP by Integrated IS-IS.
• IS-IS is an IGP used for routing within a single administrative domain.
• IS-IS is a Link state routing protocol and uses the SPF algorithm for computing best paths just like OSPF.
• Below is a comparison between IS-IS and OSPF:

Similarities:

1. Both are link-state routing protocols.
2. Both are using the SPF algorithm to calculate the best paths.
3. Both are classless routing protocols.
4. Both build adjacencies with their direct connected neighbors.
5. Both support the concept of areas to allow hierarchical network topology.
6. Both elect a DR on for broadcast multiaccess networks.
7. Both routers support authentication.
8. Cisco routers allow multiple instances per device of both protocols.

Differences:

1. IS-IS router can only belong to one area while an OSPF router can belong to multiple areas.
2. IS-IS uses CLNS to send information between routers; OSPF is a pure IP routing protocol.
3. IS-IS allows DR preemption while OSPF does not.
4. IS-IS backbone is the contiguous Level2 routers; OSPF uses the concept of Area 0 for this purpose.
5. IS-IS all routers form adjacencies with the DIS and each other while  for OSPF the routers form adjacencies with the DR only.

IS-IS operation overview:

• Routers running IS-IS will send hello packets out all IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
• Routers sharing a common data link will become IS-IS neighbors if their hello packets contain information that meets the criteria for forming an adjacency. The criteria differ slightly depending on the type of media being used (p2p or broadcast). The main criteria are matching authentication, IS-type and MTU size).
• Routers may build a link-state packet (LSP) based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.
• Generally, routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. However, there are different forms of flooding and also a number of scenarios in which the flooding operation may differ.
• All routers will construct their link-state database from these LSPs.
• A shortest-path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.