While the route selection in VPRN for other BGP owners described in Route selection across EVPN-IFL and other owners in the VPRN service follows similar criteria, the default selection for EVPN-IFF routes in the VPRN route table follow different rules.
By default, EVPN-IFF routes have a VPRN route table preference of 169; therefore, EVPN-IFF routes are preferred over EVPN-IFL, VPN-IP, or IP owners that have a preference of 170.
When two or more EVPN-IFF routes with the same IPv4 or IPv6 prefix and length, but with different route keys are received (for example, two routes with the same prefix and length but with different RDs), BGP hands the EVPN-IFF routes over to the EVPN application for selection. In this case, EVPN orders the routes based on their {R-VPLS Ifindex, RD, Ethernet Tag} and considers the top one for installing in the route table if ecmp is 1. If ecmp is N, the top N routes for the prefix are selected.
Example:
Consider the following two IP-Prefix routes that are received on the same R-VPLS service.
Route 1: (RD=192.0.0.1:30, Ethernet Tag=0, Prefix=10.0.0.0/24, next-hop 192.0.0.1)
Route 2: (RD=192.0.0.2:30, Ethernet Tag=0, Prefix=10.0.0.0/24, next-hop 192.0.0.2)
Because their route key is different (their RDs do not match), EVPN orders them based on R-VPLS Ifindex first, then RD, and then Ethernet Tag. Because they are received on the same R-VPLS, the Ifindex is the same on both. The top route on the priority list is Route 1, based on its lower RD. If the VPRN's ecmp command has a value of 1, only Route 1 is installed in the VPRN's route table.
If the previously described way of selecting EVPN-IFF routes in the VPRN does not satisfy the user requirements, the configure service system bgp evpn ip-prefix-routes iff-bgp-path-selection command enables a BGP-based path selection for EVPN-IFF routes, which is equivalent to the selection followed for EVPN-IFL or IPVPN routes with the following considerations.
All the tie breakers in Route selection across EVPN-IFL and other owners in the VPRN service are valid for EVPN-IFF, except for the lowest owner type tie breaker that does not affect EVNP-IFF routes.
Note: The ignore-nh-metric command does not exist for EVPN-IFF routes.While the tie breakers in Route selection across EVPN-IFL and other owners in the VPRN service are also valid for comparing an EVPN-IFL and an IPVPN route for the same prefix, an EVPN-IFF and IPVPN route are never compared based on those tie breakers. They are only used to compare multiple EVPN-IFF routes for the same prefix, and only when the iff-bgp-path-selection command is configured.
If the iff-bgp-path-selection command is configured, the EVPN-IFF path selection for the N routes that form the ECMP set follow the same rules as in Route selection across EVPN-IFL and other owners in the VPRN service for EVPN-IFL or IPVPN routes.
Figure 1. EVPN-IFF path selection for N routes with iff-bgp-path-selection configured
Upon receiving the same IP prefix 10.0.0.0/24 for the same VPRN in EVPN-IFF routes with different RDs, as shown in Figure 1, the following selection criteria is used.
If the iff-bgp-path-selection command is configured, the selection is based on BGP path selection, and the selected route is the top route, based on the highest Local Preference (LP)(500>300>200).
However, if the iff-bgp-path-selection command is not configured, the bottom route is selected assuming the three routes are received on the same R-VPLS, and based on the lower RD (1.1.1.1:1<1.1.1.5:1<1.1.1.10:1).
Although, by default, EVPN-IFF routes have an RTM preference of 169 and they are preferred over the RTM preference of 170 used for the other BGP route owners, a selection across EVPN-IFF and the other owners may result if the RTM preference is changed and made equal (via import policy or config>router>bgp>preference). Note that the route table preference for EVPN-IFF routes can be changed from the default value 169 if the iff-attribute-uniform-propagation command is enabled and an import policy or the config>router>bgp>preference command is configured to change it.
In case the RTM preference is changed and made equal to the same as for EVPN-IFF routes, if multiple routes with the same key (different RD) are received for EVPN-IFF and another owner in the same VPRN, the selection order is as follows:
BGP (IPv4 or IPv6)
BGP-IPVPN
EVPN-IFF
EVPN-IFL