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Use the isis hello-interval command to set the length of time (in seconds)
between hello packets sent on a specific interface. Configure independently for
level 1 and level 2, except on point-to-point interfaces because only a single type
of hello packet is sent on serial links. For this reason, it is independent of levels
1 and 2. For example, you can specify an optional level for Frame Relay
multiaccess networks.
The hello-interval is equal to the hello multiplier times the hello interval seconds
and is advertised as the holdtime in the hello packets transmitted. The range is
0–65535; the default value is 10 seconds.
NOTE: The hello-interval value must be the same for all routers attached to a common
network. With smaller hello intervals, topological changes are detected faster, but
there is more routing traffic.
Use the isis hello-multiplier command to set a number by which to multiply
the hello interval seconds. This number determines the total holding time
transmitted in the IS-IS hello packet. The default is 3. Use when hello packets
are frequently lost and IS-IS adjacencies are failing unnecessarily.
The advertised hold time in IS-IS hellos is set to the hello-multiplier times the
hello-interval. Neighbors declare an adjacency to this router to be down after
not having received any IS-IS hellos during the advertised hold time.
The hold time (and thus the hello-multiplier and the hello-interval) can be
set on a per interface basis, and can be different between different routers
in one area.
Using a smaller hello-multiplier will give fast convergence, but can result in
more routing instability.
Increment the hello-multiplier to a larger value to help network stability
when needed.
CAUTION: Never configure a hello-multiplier lower than the default.
Holding time Time a neighbor waits for another hello packet before declaring
the neighbor is down. It determines how quickly a failed link or neighbor is
identified so that routes can be recalculated.
Raise the hello multiplier and lower the hello interval simultaneously to make
the hello protocol more reliable without increasing the time required to detect
a link failure.
Example
host1(config-if)#isis hello-interval 6 level-1
host1(config-if)#isis hello-multiplier 10 level-1
Use the no version to restore a default value.
Configuring IS-IS Interface-Specific Parameters
Chapter 6: Configuring IS-IS
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