BGP selection

[Ondrej Zajicek]

On Tue, Apr 26, 2022 at 10:57:16AM +0800, Alexandre Corso wrote:
> 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).

Hello

Generally, lengths of cluster_lists are not that important, the IGP cost
difference (67955 vs 68045) has higher priority.

The main question is why they have different IGP cost if both have the
same bgp_next_hop (if 185.x.x.x is the same for both routes). That may be
a bug. Do you have some filters? Do you have more routes for 185.x.x.x?
Do you use BGP import table?

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."