How Does IS-IS Protocol Work – Step By Step Explaination

In the previous post, we have discussed the basics of IS-IS routing protocol. We recommend to you to read the previous post if you have missed that one. In this post, we will discuss how does IS-IS protocol work.

Also read: RDifferences between OSPF and IS-IS

The complete IS-IS protocol operation process is pretty long, requires a lot of patience, and a good understanding of the IS-IS protocol terminology. We have gone through the detailed IS-IS protocol operation process and summarized the following high-level steps. These steps will help you to understand how does IS-IS protocol work.

1. Routers running IS-IS protocol send out IS-IS Hello (IIH) packets to all IS-IS-enabled interfaces. This helps IS-IS routers to discover neighbors and form adjacencies among them. The following figure displays the basic IS-IS network topology.
2. IS-IS routers that share a common data link become the IS-IS neighbors, but only when their IIH packets contain information that meets the certain parameters (conditions) for forming an adjacency. The conditions may be a little bit different depending on the type of link being used. The main conditions that must meet to form an adjacency are authentication, IS-type, and MTU size.
3. A link between two IS-IS routers may be either point-to-point such as a serial line, or a broadcast such as Ethernet and Token Ring.
4. Once the adjacency is formed, now the IS-IS routers may construct a Link-State Protocol Data Units (link-state PDUs or LSP) based upon their local interfaces that are configured for IS-IS. These link-state PDUs define the topology of an IS-IS area. An LSP contains the IP route, checksum, and some other important information.
5. The IS-IS routers flood LSPs to all neighboring routers except that one from which they received the same LSP. There are different types of flooding and a number of scenarios may change the flooding operation.
6. Once the LSP packets are flooded, now all the IS-IS routers use these LSP packets to construct their Link-State Database (LSDB). An LSDB stores the routes information about the network topology.
7. After constructing the LSDB, each IS router calculate the Shortest-Path Tree (SPT) that helps to build the routing table.

That’s all about “how does IS-IS protocol work”. We know that the ultimate goal of all the routing protocols is building the routing table and helping the packets to reach their destination. So does the IS-IS routing protocol.

You are welcome to provide suggestions to improve the posts and articles. Let us know if we have missed any major step that you think should have been added in the preceding post. We would love to hear you through the comments.