BGP selection

26 Apr 2022

      Hello,

I found a strange behaviour on my BIRD route reflector:

BIRD version: 2.0.7
Topology:
------ vrr01_ny2_nyc
RR1                        ||
------ vrr01_pa4_par ---- CORE1

I have one route A.A.A.A received via two other Router reflector (vrr01_ny2_nyc / vrr01_pa4_par)
The origin of the route is 185.x.x.x

$ birdc show route A.A.A.A/32 table ipv4_rr all
BIRD 2.0.7 ready.
Table ipv4_rr:
A.A.A.A/32    unicast [ipv4_vrr01_ny2_nyc 00:54:45.006 from 100.100.254.133] * (100/67955) [AS65030?]
                via 100.99.0.120 on eth2.666
                Type: BGP univ
                BGP.origin: Incomplete
                BGP.as_path: 64655 65030
                BGP.next_hop: 185.x.x.x
                BGP.local_pref: 100
                BGP.community: (35280,843) (35280,844) (35280,1010) (35280,2060) (35280,3070)
                BGP.originator_id: 185.x.x.x
                BGP.cluster_list: 0.0.0.21 0.0.0.11
                    unicast [ipv4_vrr01_pa4_par 2022-04-14 from 100.100.254.134] (100/68045) [AS65030?]
                via 100.99.0.120 on eth2.666
                Type: BGP univ
                BGP.origin: Incomplete
                BGP.as_path: 64655 65030
                BGP.next_hop: 185.x.x.x
                BGP.local_pref: 100
                BGP.community: (35280,843) (35280,844) (35280,1010) (35280,2060) (35280,3070)
                BGP.originator_id: 185.x.x.x
                BGP.cluster_list: 0.0.0.11

I check the metric to 185.x.x.x and I see 67955, but on my second route (learned via vrr01_pa4_par), I see another value (68045)

$ birdc show route 185.x.x.x
BIRD 2.0.7 ready.
Table master4:
185.x.x.x /32    unicast [ospfv2 23:56:36.331] I (150/67955) [185.x.x.x]
                via 100.99.0.120 on eth2.666

Can you help me to understand the behaviour?
I don’t understand why the second route is not preferred as the cluster_list is smaller (probably the BGP algorithm in BIRD).

More information
$ birdc show route 100.100.254.133/32
BIRD 2.0.7 ready.
Table master4:
100.100.254.133/32   unicast [ospfv2 2022-04-20] I (150/133525) [100.100.254.133]
                via 100.99.0.120 on eth2.666

$ birdc show route 100.100.254.134/32
BIRD 2.0.7 ready.
Table master4:
100.100.254.134/32   unicast [ospfv2 2022-04-20] I (150/132615) [100.100.254.134]
                via 100.99.0.120 on eth2.666

Part of the configuration (same for the 2 peers)

protocol bgp ipv4_vrr01_ny2_nyc from IPV4_RR_PEER {
    description "IPv4 vrr01.ny2.nyc";
    neighbor 100.100.254.133;
    ipv4 {
        import filter bgp_in_rr_v4;
        export filter bgp_full_view_v4;
    };
}

protocol bgp ipv4_vrr01_pa4_par from IPV4_RR_PEER {
    description "IPv4 vrr01.pa4.par";
    neighbor 100.100.254.134;
    ipv4 {
        import filter bgp_in_rr_v4;
        export filter bgp_full_view_v4;
    };
}

Does somebody have the full BGP algorithm?

Alexandre Corso
AS35280