ECMP Impact on IS-IS Route Selection for Inter-VRF Routes with vIST

This section illustrates the impact ECMP can have on a configuration that implements user-defined VRFs in a vIST cluster and how to avoid incorrect route selection.

Understanding the Configuration

Imagine the following configuration:

IS-IS sorts paths for the same route by source-BEB BMAC and B-VLAN ID. The primary B-VLAN ID is first installed for each BMAC, followed by the secondary B-VLAN ID for each BMAC, as long as the ECMP max-path value is not reached. On the node with the lowest BMAC, the first path listed is its own local inter-vrf route, while on the other node, the MIM path across the vIST is listed first.

If you disable ECMP, all but the first path is removed. Because IS-IS orders the paths by BMAC, each node in the vIST cluster selects the same BMAC as the nexthop. This configuration leads one of the nodes to select itself, the local inter-vrf route, while the other node selects the MIM path across the vIST to get to the inter-vrf route. This situation results in an incorrect route selection.

Avoiding Incorrect Route Selection

To avoid this situation, create a policy to prevent IS-IS from determining that the MIM path across the vIST and the local inter-VRF route are ECMP paths. Configure the local inter-VRF path as the preferred path, and the vIST path as the backup. The following list identifies way that you can accomplish this: