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

[Mazur, Dariusz]

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
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