Configuring Principles for Using Equal-Cost IS-IS Routes

If multiple equal-cost IS-IS routes are available on a network, configure the equal-cost IS-IS routes to work in load-balancing mode to increase the bandwidth usage of each link, or configure preference values for the equal-cost IS-IS routes to facilitate traffic management.

Context

If there are redundant IS-IS links, multiple routes may have an equal cost. Choose either of the following methods to use these equal-cost IS-IS routes:

Configure load balancing for equal-cost IS-IS routes so that traffic will be evenly balanced among these links.

This mechanism increases the link bandwidth usage and prevents network congestion caused by link overload. However, this mechanism may make traffic management more difficult because traffic will be randomly forwarded.
Configure preference values for equal-cost IS-IS routes so that only the route with the highest preference will be used and the others function as backups.

This configuration facilitates traffic management and improves the network reliability, without the need to change original configurations.

Procedure

Configure equal-cost IS-IS routes to work in load-balancing mode.
1. Access the system view.
  system-view
2. Access the IS-IS process view.
  isis [ process-id ]
3. Set the maximum number of load-balancing equal-cost IS-IS routes.
  maximum load-balancing number
  When the number of equal-cost routes is greater than number specified in the maximum load-balancing command, valid routes are selected for load balancing based on the following criteria:
  1. Route preference: Routes with lower preferences are selected for load balancing.
  2. Interface index: If routes have the same priorities, routes with higher interface index values are selected for load balancing.
  3. Next hop IP address: If routes have the same priorities and interface index values, routes with larger IP address are selected for load balancing.
Configure preference values for equal-cost IS-IS routes.
1. Access the system view.
  system-view
2. Access the IS-IS process view.
  isis [ process-id ]
3. Set a preference value for an equal-cost IS-IS route.
  nexthop ip-address weight value
  
  A larger value of the value parameter indicates a higher preference. The default value of the preference is 255.