Troubleshooting the Intermediate System to Intermediate System (IS-IS) Protocol

By Edward Tetz

Troubleshooting issues with the Intermediate System to Intermediate System (IS-IS) protocol is once again pursued thru an investigation of the show and debug commands and their options.

Viewing the show options

To start, here are the different options available in the show command:

Router2#show isis ?
  *            All IS-IS address families
  database     IS-IS link state database
  hostname     IS-IS Dynamic hostname mapping
  ipv6         IS-IS IPv6
  lsp-log      IS-IS LSP log
  mesh-groups  IS-IS mesh groups
  mpls         IS-IS MPLS
  neighbors    IS-IS neighbors
  rib          ISIS local RIB information
  route        IS-IS level-1 routing table
  spf-log      IS-IS SPF log
  topology     IS-IS paths to Intermediate Systems

Among these options are several with useful information, most notably database, neighbors, route, and topology.

The output of the topology command is shown, notice that it includes both Level 1 and Level 2 routers, which both of the referenced routers are members of, because they were created with the default options on router isis. Router2 sees that the router Router1 is found on interface Fa0/0. So, with the topology command, you can see neighbors and further, whereas the neighbors command would only show adjacent routers.

Router2#show isis topology
IS-IS paths to level-1 routers
System Id            Metric     Next-Hop             Interface   SNPA
Router2              --
Router1              10         Router1              Fa0/0       0017.e0c9.b7b0
IS-IS paths to level-2 routers
System Id            Metric     Next-Hop             Interface   SNPA
Router2              --
Router1              10         Router1              Fa0/0       0017.e0c9.b7b0

Looking at debug options

Finally, you look at the debug options, which are similar to the Open Shortest Path First (OSPF) debug command. Depending on the types of problems you are experiencing, you may want to focus on different parts of the IS-IS protocol, such as adj-packets, protocol-errors, or spf-statistics.

Router2#debug isis ?
  adj-packets      IS-IS Adjacency related packets
  authentication   IS-IS packet authentication
  checksum-errors  IS-IS LSP checksum errors
  local-updates    IS-IS local update packets
  mpls             IS-IS MPLS
  nsf              IS-IS Non-Stop Forwarding
  protocol-errors  IS-IS LSP protocol errors
  rib              IS-IS IP Routing Information Base
  snp-packets      IS-IS CSNP/PSNP packets
  spf-events       IS-IS Shortest Path First Events
  spf-statistics   IS-IS SPF Timing and Statistic Data
  spf-triggers     IS-IS SPF triggering events
  update-packets   IS-IS Update related packets