information. If any indication of high CPU
utilization caused by the SPF process is determined, the
show isis spf-log command can determine SPF-related
events that might cause the high CPU utilization. The output
of the show isis spf-log command lists SPF process runs
by time, trigger, and duration. LSPs are refreshed every 15
minutes, triggering periodic SPF calculations. Such events are
labeled periodic in the show isis spf-log output. It
also shows that at 03:25:25, the process was run over an
insignificant period of time because of receipt of a new LSP
from RT5. Figure lists and describes common SPF triggers. The
following is a list of IS-IS debugging commands that can also
be used to narrow SPF-related problems: - debug isis
spf-events
- debug isis spf-triggers
- debug isis spf-statistics
- debug isis
update-packets
- debug isis adj-packets
Use caution when enabling debugging in a
situation in which the route processor CPU already is
overtaxed. Output of the debug isis spf-events command
displays the events following an interface shut down to
simulate a link flap. Lines indicating LSPs flagged for
recalculation as a result of this event are highlighted. Also,
events, flagging computation of the Level 1 and Level 2 SPF
trees are highlighted. Most route-flapping problems can be
traced to unstable links in the network. In more complicated
scenarios, however, the flaps could be caused by LSP corruption
storms or even a routing loop. This might be the case when
there are no unstable links in the network, but CPU utilization
is high, indicating continuous running of the SPF process. The
show isis spf-log command can provide an indication of
which LSPs are changing the most frequently and are triggering
the SPF calculations. Similar clues can be gleaned by enabling
debugging of the update process with debug isis
update-packets. This should be done with care so that the
CPU is not overloaded. The logs also can be observed for LSP
error loggings, which could give an indication of packet
corruption by a culprit device. Zeroing in on any continuously
changing LSPs is critical for determining and addressing the
problem.