Improving IS-IS Convergence Times

Problem

You want to speed convergence of IS-IS routes in case a path fails with no hardware indication, which might happen on an Ethernet network.

Solution

Have IS-IS perform the SPF calculation more often:

[edit protocols isis] aviva@RouterG# set spf-delay 100

Discussion

One of the most important factors that affects the convergence of IS-IS routes is how often the software performs the SPF calculation. By default, IS-IS performs the SPF calculation 200 milliseconds after a topology change is detected. The command in this recipe halves the time to 100 milliseconds:

[edit protocols isis] aviva@RouterG# set spf-delay 100

Another way to improve link failure detection times is to use the Bidirectional Forwarding Protocol ( BFD), which provides a mechanism to detect communication failures with a forwarding-plane next hop. BFD is a simple hello protocol. A pair of systems exchange BFD packets periodically, and if a system stops receiving the packets for long enough, some component in that particular bidirectional path to the neighboring system is assumed to have failed. If you want to shorten the IS-IS link failure detection time to about 1 second, set the BFD packet exchange interval to 333 milliseconds:

[edit protocols isis] aviva@RouterG# set interface fe-0/0/1 bfd-liveness-detection minimum-interval 333

By default, BFD multiplies the packet-exchange interval by three to determine the link detection failure time. Configuring an interval of 333 milliseconds results in a failure time of just under 1 second.

You also need to configure BFD using the same command on the interface at the other end of the link. Use the show bfd session command to see BFD information:

aviva@RouterG> show bfd session detail Address State Interface Detect Time Interval Multiplier 10.0.1.1 Up fe-0/0/1.0 0.999 0.333 3 Client ISIS L2, TX interval 0.300, RX interval 0.300, multiplier 3 Session up time 00:00:37 Local diagnostic None, remote diagnostic None Remote heard, hears us 1 sessions, 1 clients Cumulative transmit rate 3.0 pps, cumulative receive rate 3.0 pps

If the link goes down, the BFD session fails and you no longer see the link:

aviva@RouterG> show bfd session detail Transmit Address State Interface Detect Time Interval Multiplier 0 sessions, 0 clients Cumulative transmit rate 0.0 pps, cumulative receive rate 0.0 pps