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.
Imagine the following configuration:
A vIST cluster exists with multiple VRF contexts.
On both nodes, VRF A redistributes routes into IS-IS as external. VRF B uses an IS-IS accept policy to accept these routes.
Each node learns three paths to the route:
The nodes learn one path using local inter-VRF redistribution.
The nodes learn the other two paths from the IST peer.
The routes are treated as ECMP paths because the preference, metric-type, and metric are equal.
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.
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:
Redistribute the VRF route into IS-IS using the internal metric-type. IS-IS will always select the local inter-VRF route. For more information about the metric type for IS-IS routes, see Fabric Basics and Layer 2 Services.
If an IS-IS internal metric-type is not an option, configure an IS-IS accept policy to change the preference of inter-VRF routes learned from the IST peer. The local inter-VRF route is preferred over the inter-VRF routes learned from the IST peer.