Hello/Hold Timer
Load isis.cfg
Change the R1-R2 link to be p2p. Configure the Hello/Hold timers to be 2/6 seconds.
Change the hello/hold timer on R3 so that its Gi2.23 interface uses a hold timer of 3 seconds when it is elected DIS on the LAN.
Answer
Verification
On p2p circuits, debug isis adj-packets does not appear to list the hold time. Instead we can check the neighbor table to verify the holdtime oscillates between 5-6 seconds.
On LAN circuits, we can use the debug command (debug isis adj-packets). The hold time is specified as ht().
On a LAN circuit, the DIS will lower its hello/hold to three times lower than the default. This allows the DIS re-election to happen more quickly if the DIS is lost. Because we want R3 to use a hold period of 3 seconds, we must set the default to 9 seconds. We do this by setting the hello to 3 and the hold multiplier to 3. Since R3 is the DIS, it indicates to its neighbors that its hold time is 3 seconds, and sends hellos at a 1 second interval.
In ISIS, the hello/hold period does not need to match between routers. This gives you more flexibility than with OSPF. In ISIS the hello is asymmetrical, so the routers just report their hold time in the Hello. A router sets the hold time for an adjacency to the hold time received from the neighbor, not the locally configured hold time.
Also note that the hello-multiplier command was not strictly necessary above. This is because the default value is 3.
Last updated
