BGP best path algorithm in RR environment

[Mazur, Dariusz]

Hello Bird Users,
Have a question about bgp best path algorithm  in Route Refletector  environment and possible fix:


Topology ( we use only iBGP with multiply RR):
r01a.tor ----- r01.leaf.105---r01.spine----r01.leaf108-----r09a.tor

I announce the same block 172.232.0.0/19 from both tors

r01.leaf108 receives 172.232.0.0/19 from r01.spine and r09a.tor, and treat them as equal path (multipath).

All BGP attributes are equal except of  BGP originator id and cluster-list

r01.leaf108.ord02.fab> show route for 172.232.0.0/19 all
172.232.0.0/19
                                unicast [192.168.226.56__r01.spine101 2023-09-06] * (100) [AS4250527481?]
                via 192.168.226.56 on vlan.101
                Type: BGP univ
                BGP.origin: Incomplete
                BGP.as_path: 4250527481
                BGP.next_hop: 192.168.226.56
                BGP.med: 0
                BGP.local_pref: 400
                BGP.atomic_aggr:
                BGP.aggregator: 23.192.121.225 AS4250527481
                BGP.community: (63949,1000) (63949,1002) (63949,1004) (63949,1005) (65110,31107) (65310,31107) (65518,31107)
                BGP.originator_id: 23.205.212.134
                BGP.cluster_list: 23.205.212.8 23.205.212.112

                     unicast [192.168.198.35__r09b.tor108 2023-09-06] (100) [AS4250827489?]
                via 192.168.198.35 on vlan.218
                Type: BGP univ
                BGP.origin: Incomplete
                BGP.as_path: 4250827489
                BGP.next_hop: 192.168.198.35
                BGP.med: 0
                BGP.local_pref: 400
                BGP.atomic_aggr:
                BGP.aggregator: 23.213.15.233 AS4250827489
                BGP.community: (63949,1000) (63949,1002) (63949,1004) (63949,1005) (65110,31107) (65310,31107) (65518,31107)

Looks like Bird does not check  cluster_list length to determine better route, is it intentional? Can you suggest any possible fix to prefer route from r09a.tor using cluster-list or anything what is local for router (local pref is not option because all routers are in the same asn and use iBGP)

Thanks,
Dariusz



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