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.

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 that describe 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.How Does IS-IS Protocol Work
  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 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 I have missed any major step that you think should have been added in the preceding post. We love to hear you through the comments.

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2 comments on “How Does IS-IS Protocol Work – Step By Step Explaination
  1. Cool says:

    Great information. Please also share the configuration of IS-IS protocol.

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