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 output of the debug isis update-packets command
shown in Figure
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 output of the
debug isis snp-packets command shown in Figure
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.