ECMP within IS-IS routes

Multiple BEBs can announce the same route, either because the Layer 2 LAN connects to multiple BEBs for redundancy, or because segments of the LAN are Layer 2 bridged. In Layer 2, if the device has to tie-break between multiple sources, the tie-breaking is based on cost and hop count.

In Layer 3, hop count is not used for tie-breaking. Instead, the device uses the following precedence rules to tie-break. In the following order, the device prefers:

1. Routes that do not include nodes with the overload bit set.

   When a router node runs out of system resources (memory or CPU), it alerts the other routers in the network by setting the overload bit in its link-state packets (LSPs). When this bit is set, the node is not used for transit traffic but only for traffic packets destined to the node's directly connected networks and IP prefixes.

2. Local routes over remote routes.

   If a route is learned locally, for example, through inter-VRF route leaking, it is most preferred.

3. Routes with the lowest route preference.

   By default, IS-IS routes within the same VSN are added to the LSDB with a default preference of 7. Inter-VSN routes are added to the LSDB with a route preference of 200. You can however, change the route preference using IS-IS accept policies.

4. Metric type internal (type 1) over metric type external (type 2).

5. Routes with the lowest SPBM cost.

6. Routes with the lowest prefix cost.

   If the metric type is internal, then the tie-break is on SPB cost first, and then on the prefix cost. Otherwise the tie-break is only on the prefix cost.

   You can either change this using a route-map on the remote advertising node with the redistribute command, or using a route-map on the local node with the IS-IS accept policy.

7. Routes within a VSN with a lower Layer 3 VSN I-SID.

   The device considers the Global Routing Table (GRT) to have an I-SID equal to zero.

When you use multiple B-VLANs in the SPBM core, multiple paths exist to reach a particular SPBM node, one on each B-VLAN; therefore, any IP prefix or IPv6 prefix that the device receives from a BEB results in multiple ECMP paths. These paths may or may not be physically diverse. SPBM supports up to two B-VLANs; a primary B-VLAN and a secondary B-VLAN.

If more ECMP paths are available than the configured number of paths, then the device adds the routes using the following order: The device selects all routes from the primary B-VLAN and orders the routes learned through that B-VLAN from lowest system ID to the highest IS-IS system ID, then the device moves on to select all routes from the secondary B-VLAN, ordering those routes from lowest IS-IS system ID to the highest IS-IS system ID until you reach the number of equal paths configured.

For example, consider an SPB core configured with two B-VLANs (primary B-VLAN 1000 and secondary B-VLAN 2000), and the device learns routes from two BEBs called BEB-A (with a lower IS-IS system ID) and BEB-B (with a higher IS-IS system ID, then the order in which the next-hop is chosen for those routes are as follows.

If a route is learned only from BEB-A with the maximum number of allowed ECMP paths configured as 8 (default), then the order in which the next-hop is chosen for that route is:

1. BEB-A B-VLAN 1000

2. BEB-A B-VLAN 2000

If routes are learned from both BEB-A and BEB-B with maximum number of allowed ECMP paths configured as 8 (default), then the order in which the next-hop is chosen for those routes are:

1. BEB-A B-VLAN 1000

2. BEB-B B-VLAN 1000

3. BEB-A B-VLAN 2000

4. BEB-B B-VLAN 2000

If ECMP is disabled, the maximum number of allowed ECMP paths is 1 and the device adds the route from the lowest system ID with the primary B-VLAN. In this example, the device adds BEB-A B-VLAN 1000.

Note

• ECMP is supported for IPv6 Shortcut routes.

• To add IS-IS equal cost paths in the routing table, you must enable IPv6 ECMP feature globally.