Dell SmartFabric OS10 User Guide Release 10.5.3

EVPN route selection based on AS path length

In BGP EVPN VXLAN, consider a scenario where there are two sites, Site1 and Site2, with each site containing its own border leaf switch.

When these sites are interconnected, the routes that are learned or configured on each border leaf switch on a site is advertised through BGP EVPN to other border leaf switches and also to other VTEP leaf switches in both the sites.

If the same prefix is learned or configured on all the border leaf switches, the leaf switches corresponding to both the sites have more than one path for the destination prefix. If all the paths for that route are considered as ECMP paths, the traffic may take a suboptimal path through a remote site.

The AS path length is shorter for the route that is received from the local border leaf switch and longer for routes that are received from the remote border leaf switches. The AS path length is considered for best path selection, so that the path through the local border leaf switch is preferred over the path through the remote border leaf switches.

The following topology depicts this scenario:

For a leaf VTEP to communicate to the external network, a default route is configured on the border leaf VTEP and advertised to all the leaf VTEPs through EVPN. This behavior can also be achieved by advertising the routes that are learned from the external network in the border leaf switch to all other leaf VTEPs.

When the routes that are advertised by the border leaf switch reach the leaf VTEPs, the routes have the AS path updated with the AS through which the route is advertised.

In Leaf-1 switch, the routes originating from BL-1 have the following AS paths: AS-100 and AS-101. The routes originating from BL-2 in Site2 have the following AS paths: AS-200, AS-100, and AS-101.

When traffic hits the default route in leaf-1 and if the default route is installed as ECMP, the traffic is forwarded to either BL-1 in the local site or BL-2 in the remote site based on the traffic hashing.

To send traffic to BL-2 in the other site is suboptimal. For optimal forwarding, the default route must be installed in the local site with BL-1 as the only path.

Achieve this behavior considering the AS path length for EVPN route selection. So, in Leaf-1, if there exists two routes for the prefix from BL-1 and BL-2, the route with the shorter AS path length (AS-100, AS-101) is selected as the best route.

The route selection process is applicable for both default routes and all other type5 route prefixes. For all the EVPN type5 routes in Symmetric IRB, route selection is done by default.