BGP best path algorithm in RR environment

[Alexander Zubkov]

Hi Dariusz,

Will ASPATH prepends work for you?
Or this feature might help you:
https://bird.network.cz/?get_doc&v=20&f=bird-6.html#bgp-aigp

Regards,
Alexander Zubkov
Qrator Labs

On Wed, Sep 13, 2023 at 11:43 AM Mazur, Dariusz via Bird-users
<bird-users at network.cz> wrote:
>
> Hello Bird Users,
>
> Have a question about bgp best path algorithm  in Route Refletector  environment and possible fix:
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> Topology ( we use only iBGP with multiply RR):
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> r01a.tor ----- r01.leaf.105---r01.spine----r01.leaf108-----r09a.tor
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> I announce the same block 172.232.0.0/19 from both tors
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> r01.leaf108 receives 172.232.0.0/19 from r01.spine and r09a.tor, and treat them as equal path (multipath).
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> All BGP attributes are equal except of  BGP originator id and cluster-list
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> r01.leaf108.ord02.fab> show route for 172.232.0.0/19 all
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> 172.232.0.0/19
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>                                 unicast [192.168.226.56__r01.spine101 2023-09-06] * (100) [AS4250527481?]
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>                 via 192.168.226.56 on vlan.101
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>                 Type: BGP univ
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>                 BGP.origin: Incomplete
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>                 BGP.as_path: 4250527481
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>                 BGP.next_hop: 192.168.226.56
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>                 BGP.med: 0
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>                 BGP.local_pref: 400
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>                 BGP.atomic_aggr:
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>                 BGP.aggregator: 23.192.121.225 AS4250527481
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>                 BGP.community: (63949,1000) (63949,1002) (63949,1004) (63949,1005) (65110,31107) (65310,31107) (65518,31107)
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>                 BGP.originator_id: 23.205.212.134
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>                 BGP.cluster_list: 23.205.212.8 23.205.212.112
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>                      unicast [192.168.198.35__r09b.tor108 2023-09-06] (100) [AS4250827489?]
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>                 via 192.168.198.35 on vlan.218
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>                 Type: BGP univ
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>                 BGP.origin: Incomplete
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>                 BGP.as_path: 4250827489
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>                 BGP.next_hop: 192.168.198.35
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>                 BGP.med: 0
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>                 BGP.local_pref: 400
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>                 BGP.atomic_aggr:
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>                 BGP.aggregator: 23.213.15.233 AS4250827489
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>                 BGP.community: (63949,1000) (63949,1002) (63949,1004) (63949,1005) (65110,31107) (65310,31107) (65518,31107)
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> 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)
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> Thanks,
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> Dariusz
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