ISIS Hellos
Last updated
Last updated
Load base.ipv4.and.ipv6.cfg
Configure ISIS on R5, R6 and XR1 as follows:
Use a hello timer of .333/1 on R5 and R6
Use a hello timer of 1/3 on XR1
Enable ISIS for IPv4 on the following interfaces:
R5-R6
R6-XR1
Use NET IDs that are in area 49.0001.
Will all adjacencies come up?
In ISIS, timers can be mismatched between neighbors. The Hello simply contains the local Hold time. This tells the neighbor how long it should wait before declaring the neighbor down (if it doesn’t receive any Hellos). It is up to the neighbor to send Hellos fast enough so that the neighbor won’t declare it down in this time. This is much more flexible than OSPF. This allows you to easily migrate to different timers without bringing down adjacencies. OSPF would require downtime to do that (unless you can do it really quickly before the neighborship is declared down).
In ISIS, IOS-XE also is able to use sub-second hellos. This is done by using the isis hello-interval minimal command which creates a 1 second hold time. The router will calculate the Hello rate by dividing 1 by the hello-multiplier that you specify (which is by default 3). IOS-XR does not support this functionality. But, it can still form an adjacency with an IOS-XE router that is using a 1 second hold time. (Because the hold time never needs to match between neighbors, and 1 second is a valid hold time).
There is another difference with ISIS timers, which is that the DIS on the LAN will set its hello/hold time to one third the rate of the configured value. This allows the routers to elect a new DIS more quickly if the existing DIS goes down. However, this doesn’t appear to be the case for IOS-XE when it is already using a minimal hello-interval. On R5, R6 is the DIS, but the hold time is always above 666ms.
If we change R6 to use a hello/hold of 3/9 seconds we should see an actual hello/hold of 1/3. Below, note that the holdtime never drops below 2, indicating the actual hold is 3 seconds, even though the configured hold on R6 is 9 seconds (3x3).
On XR1, because we set the hold so low (3 seconds), and it is the DIS, we ended up achieving sub-second hellos, even though it cannot be manually configured. XR1 shows a local configuration of 1/3 seconds, but that the LAN IIH is always due within less than 333 ms.
It is also worth mentioning that the default hello multiplier is 3 seconds, so this did not need to be specified in the configuration.
