Bird does not prefer older eBGP route - RFC5004 and "older prefer on"

[Maria Matejka]

Hello!

I suspect that the routes either aren't all external, or are otherwise 
compared different before it comes to breaking ties. Could you please 
share the `show route all` output to see all the relevant BGP attributes?

Maria

On 6/30/23 11:43, Mazur, Dariusz via Bird-users wrote:
>
> Hello Bird users,
>
> Have you ever use RFC 5004 and “older prefer” knob. I am trying to use 
> it but it seems not to work:
>
> *1.Router learns the same route from different ebgp peers, it prefers 
> route from r01a  and this route is exported to BGP peers*
>
> 172.232.160.0/19     unicast [192.168.196.129__r01a.tor106 2023-06-14] 
> * (100) [AS4250627481?]
>
>                 via 192.168.196.129 on vlan.201
>
>                      unicast [192.168.196.131__r01b.tor106 2023-06-14] 
> (100) [AS4250627481?]
>
>                 via 192.168.196.131 on vlan.202
>
>                      unicast [192.168.196.133__r02a.tor106 2023-06-14] 
> (100) [AS4250627482?]
>
>                 via 192.168.196.133 on vlan.203
>
> *2.Once we lose links to r01a  and r01b  route from r02a is preferred 
> and exported to BGP peers. It is expected*
>
> 172.232.160.0/19     unicast [192.168.196.133__r02a.tor106 2023-06-14] 
> * (100) [AS4250627482?]
>
>                 via 192.168.196.133 on vlan.203
>
> *3.When links to r01a  and r01b. are again online route, route from 
> r01a is pricked as primary and exported to BGP. It causes route 
> oscillation*
>
> 172.232.160.0/19     unicast [192.168.196.129__r01a.tor106 
> 09:14:19.982] * (100) [AS4250627481?]
>
>                 via 192.168.196.129 on vlan.201
>
>                              unicast [192.168.196.131__r01b.tor106 
> 09:14:19.896] (100) [AS4250627481?]
>
>                 via 192.168.196.131 on vlan.202
>
>                      unicast [192.168.196.133__r02a.tor106 2023-06-14] 
> (100) [AS4250627482?]
>
>                 via 192.168.196.133 on vlan.203
>
> *4.I believe it is default behavior not to prefer older path. 
> According to documentation RFC 5004 and "prefer older on" should fix 
> my problem, but it does not work.*
>
> Bird doc says:
>
> /prefer older switch/
>
> /Standard route selection algorithm breaks ties by comparing router 
> IDs. This changes the behavior to prefer older routes (when both are 
> external and from different peer). For details, see RFC 5004. Default: 
> off./
>
> *5. According to documentation RFC 5004 and "prefer older on" should 
> fix my problem, but it does not work.*
>
> *a) added "prefer older on", bgp flapped and routes were re-learnt*
>
> 172.232.160.0/19     unicast [192.168.196.129__r01a.tor106 
> 09:22:12.949] * (100) [AS4250627481?]
>
>                 via 192.168.196.129 on vlan.201
>
>                              unicast [192.168.196.131__r01b.tor106 
> 09:22:13.527] (100) [AS4250627481?]
>
>                 via 192.168.196.131 on vlan.202
>
>                      unicast [192.168.196.133__r02a.tor106 
> 09:22:12.683] (100) [AS4250627482?]
>
>                 via 192.168.196.133 on vlan.203
>
> *b) shut links to r01a and r01b*
>
> *172.232.160.0/19     unicast [192.168.196.133__r02a.tor106 
> 09:22:12.683] * (100) [AS4250627482?]*
>
> *                via 192.168.196.133 on vlan.203*
>
> *c) unshut links to r01a and r01b, route from r01a is again  
> preferred, so looks like "older" knob does not work*
>
> 172.232.160.0/19     unicast [192.168.196.129__r01a.tor106 
> 09:27:55.841] * (100) [AS4250627481?]
>
>                 via 192.168.196.129 on vlan.201
>
>                      unicast [192.168.196.131__r01b.tor106 
> 09:27:54.448] (100) [AS4250627481?]
>
>                 via 192.168.196.131 on vlan.202
>
>                      unicast [192.168.196.133__r02a.tor106 
> 09:22:12.683] (100) [AS4250627482?]. *---> this route is older*
>
>                 via 192.168.196.133 on vlan.203
>
> Thanks,
>
> Dariusz
>
-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://trubka.network.cz/pipermail/bird-users/attachments/20230630/5d2af3da/attachment.htm>