IS-IS adjacency-related problems normally are caused by link failures and
configuration errors. On Cisco routers, inspecting the output of the
show interface command can easily identify link failures.
Also, because
IS-IS routing is not required to establish IP connectivity to directly attached
routers, it is easy to discern whether the problem is media-related or specific
to the IS-IS configuration.
The show clns neighbors
command is usually the starting point for troubleshooting IS-IS adjacency
problems. The output of this command should list all neighbors expected to be
adjacent to the router being investigated. The command show clns
is-neighbors provides similar output, but it is intended to list
only neighbor routers or IS-IS adjacencies; whereas show clns
neighbors lists all types of adjacencies, both for IS-IS and ES-IS.
Using the output from the show clns neighbor and
show clns neighbor detail commands, IS-IS adjacencies can
be examined.
Problems
with IS-IS adjacency formation can be registered by the presence of fewer
neighbors than expected or a situation in which the status of an expected
adjacency is not up. Another symptom could be that the neighbor is known
through ES-IS protocol instead of IS-IS.
The router RT1 has properly
formed adjacencies with its directly connected neighbors, RT2 and RT5. All ISO
devices run the ES-IS protocol to facilitate mutual discovery and communication
between end systems and routers in the CLNS environment. End systems and
routers exchange end-system hellos (ESHs) and intermediate-system hellos (ISHs)
within the ES-IS framework. Connected routers also receive each other’s ISHs
and form ES-IS adjacencies. Therefore, it is possible that ES-IS adjacencies
might still be formed between two routers even if there are problems with the
IS-IS adjacency.
Figure
also shows
output from show clns neighbors, but shows problems with
the adjacencies formed with RT2 and RT5. In this example, the IS-IS adjacency
with RT2 is in INIT state instead of UP. The protocol is correctly shown as
IS-IS. The adjacency with RT5 shows UP, however the protocol is ES-IS instead
of IS-IS. As explained previously, the ES-IS protocol runs independently of
IS-IS; therefore, the ES-IS adjacency formed between RT1 and RT5 has nothing to
do with IS-IS.
These routers cannot form an IS-IS adjacency with each
other, apparently because of a problem in the configuration or the IS-IS
environment. Most adjacency problems related to the IS-IS environment can be
debugged with the debug isis adj-packets command. The
output of this command can be daunting if the router under inspection has a lot
of neighbors because the display shows all the hellos transmitted and received
by the local routers.