BGP best path algorithm in RR environment

[Alexander Zubkov]

Hello Dariusz,

On Wed, Sep 13, 2023 at 3:19 PM Mazur, Dariusz <dmazur at akamai.com> wrote:
>
> Hello Aleksander,
> Prepends does not work for me because I would prepending on every leaf so as-path still will be the same from perespective r01.leaf108

If you prepend announces from every leaf to the spine, then
r01.leaf108 in your example will see the announce from the spine with
one extra prepend and from its tor witout extra prepends. Or not?

> Looks like AIGP is not for me too because we don’t have IGP inside network. Every iBGP session is established over directly connected p2p links.

Take a look at the description of the next option, following "aigp". I
don't think IGP dynamic routing is required to use aigp metric in BGP.
The question is if you can setup costs and metrics as you need. I
haven't used this feature myself, so I would not help with specific
configs. But it seems to me that it could be used with BGP sessions
only.

> cost number
> When BGP gateway mode is recursive (mainly multihop IBGP sessions), then the distance to BGP next hop is based on underlying IGP metric. This option specifies the distance to BGP next hop for BGP sessions in direct gateway mode (mainly direct EBGP sessions).

>
> I was thinking to increase BGP preference (not bgp local pref) on leafs to prefer routes from tor and not to compare them with the same routes from spine (I don’t like this workaround)
>
>
> Thanks,
> Dariusz
>
> On 9/13/23, 2:17 PM, "Alexander Zubkov" <green at qrator.net <mailto:green at qrator.net>> wrote:
>
>
> Hi Dariusz,
>
>
> Will ASPATH prepends work for you?
> Or this feature might help you:
> https://urldefense.com/v3/__https://bird.network.cz/?get_doc&v=20&f=bird-6.html <https://urldefense.com/v3/__https://bird.network.cz/?get_doc&v=20&f=bird-6.html>*bgp-aigp__;Iw!!GjvTz_vk!Xxvtj4yzVIakFfyFYALg5qiCWoJ-VfKsSyJy2aYMXvIoY5myyvsy41XQPC862yoN5iLL7GvrlmJ2$
>
>
> Regards,
> Alexander Zubkov
> Qrator Labs
>
>
> On Wed, Sep 13, 2023 at 11:43 AM Mazur, Dariusz via Bird-users
> <bird-users at network.cz <mailto:bird-users at network.cz>> wrote:
> >
> > 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
> >
> >
> >
> >
> >
> >
>
>
>