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 B-MAC and B-VLAN ID. The primary B-VLAN ID is first installed for each B-MAC, followed by the secondary B-VLAN ID for each B-MAC, as long as the ECMP max-path value is not reached. On the node with the lowest B-MAC, 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 B-MAC, each node in the vIST cluster selects the same B-MAC 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.