MPLS/BGP/VRF tie-breaker

[lunarman_98]

hi,

what happens if there are multiple routes with the same route-target are received for a particular FEC? what BGP route selection process (in what order) will a cisco router select (i.e. the best route)?

thanks.

[swaroop.potdar]

I believe your question is , what will happen when BGP receives multiple routes to the same destination with the same route-distinguisher. As route-targets wont affect the choice of routes, they only define colors or membership of the route to a certain VPN.

In this case simply the closest IGP path to the next-hop PE which sent the update would be choosen. If this is equal, then it will follow the regular BGP path selection process, begginning from weight -- till the end of steps.

HTH-Cheers,

Swaroop

[mheusinger · Accepted solution]

Hello,

an example to highlight what will happen. Assume a PE receives the following routes:

65000:1:10.0.0.0/8 RT 65000:100 next hop 1.1.1.1 locPref 100

65000:1:10.0.0.0/8 RT 65000:100 next hop 2.2.2.2 locPref 200

65000:3:10.0.0.0/8 RT 65000:100 next hop 3.3.3.3

First path selection will be performed for identical VPNv4 prefixes. In the example this means

65000:1:10.0.0.0/8 RT 65000:100 next hop 1.1.1.1 locPref 100

>65000:1:10.0.0.0/8 RT 65000:100 next hop 2.2.2.2 locPref 200

>65000:3:10.0.0.0/8 RT 65000:100 next hop 3.3.3.3

Then import will take place, i.e. one path per identical VPNv4 prefix will get copied into the VRF, if a route target matches an import statement. Per default import is set to 1. Assuming VRF RD 6500:123 This results in a VRF specific BGP table:

65000:123:10.0.0.0/8 RT 65000:100 next hop 2.2.2.2 locPref 200

65000:123:10.0.0.0/8 RT 65000:100 next hop 3.3.3.3

Now path selection will install the best route into the VRF routing table, in the example this will be:

B 10.0.0.0/8 next hop 2.2.2.2

If you would like to have load sharing in place, then you need to increase import to (at least) 2 and maximum path for iBGP should be at least 2:

router bgp 65000

address-family ipv4 vrf Example

maximum-path ibgp unequal-cost 2 import 2

Hope this helps! Please rate all posts.

Regards, Martin