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| 5.8 |  | Troubleshooting IS-IS |
| 5.8.5 |  | Route advertisement problems |

Most route advertisement problems can be narrowed down to configuration problems at the source or LSP propagation issues.

Because IS-IS is a link-state protocol, IS-IS routers depend on LSP flooding to obtain topology and routing information. During stable conditions, each IS-IS router in an area will have the same Level 1 link-state database, which contains the LSPs generated by every router in the area.

Dijkstra’s algorithm is run over the LS database to obtain the best path to every advertised route. If a route is missing in a section of the area, it is because the routers in that section did not receive the original LSP, or the LSP was received corrupted, therefore, was purged.

An even simpler reason could be that the route was not even put into the LSP at the source. The outputs of debug isis update-packets and debug isis snp-packets could help decipher this sort of problem as it is related to LSP flooding or issues with link-state database synchronization.

The highlighted portion of the debug isis update-packets command shows RT1 flooding its LSP and also receiving an LSP from RT2. Because the adjacency was brought up, the output also shows the one-time exchange of CSNPs on point-to-point links between RT1 and RT2.

The highlighted portion of the debug isis snp-packets command indicates receipt of CSNP by RT5 from the DIS (RT1). By comparing the contents of the CSNP with the local Level 1 database, RT5 determines that no changes occurred in all known LSPs.

There can be many potential causes for problems in which routes are not reaching remote locations in the network including adjacency problems, Layer 1/2 problems, IS-IS misconfiguration, and other issues.