Hi, Speaking about general routing (not specially Bird), I would say that what you want would be perfectly managed by the «advertise best external» feature, which makes a router advertise in iBGP its best external paths, even if they are not the overall best path. This way, in conjunction with solution 1, router 2 uses the routes from router 1, BUT all the routes are known on both routers. Features in Cisco/Juniper: https://www.juniper.net/documentation/us/en/software/junos/cli-reference/top... https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/... Unfortunately this doesn't exist in Bird :P But could be nice to implement... Here, we use a solution «0» :D - we don't use advertise-external - we don't prefer a specific transit provider, so the routes with similarly sized AS paths are known from every routers :) But, for selective iBGP session, we use solution 2: - between countries (we would not really like to see a transit in London be used from routers in Paris by example, even if AS path is shorter) - incoming routes from all transit/peering are tagged with two communities, one for the transit (or peering, not the same), one for the site (showing the country and the POP, one digit for each) -at iBGP ingress policy, received iBGP routes tagged with communities [Transit/Peering + any foreign country] see their locapref slightly decreased. About solution 3: sux, forget it (to my mind) :) regards, Olivier Le mar. 6 août 2024 à 13:28, Nico Schottelius via Bird-users < bird-users@network.cz> a écrit :
Good morning,
you might remember my confusion about non-exported routes some threads away [0] and while not completely bird specific, I think it might be a good topic to discuss for most bird users as well:
How to deal with route preference when one has multiple sources?
Problem statement:
- Your ASN receives the same route multiple times from different upstreams - What is the best way of handling multiple routes in regards to a) best practice ("the next engineer understands the solution") b) efficiency ("how fast to failover") c) robustness ("simplicity" and "default behaviour")
The setup will for almost anybody look something like that:
-------- |Your ASN | --------
router1----------------- ASN A: sends route X | iBGP+IGP (babel, OSPF, etc.) - other-routers-of-the-asn | router2 |--------------------- ASN B: sends route X
And let's assume that by default ASN A is the preferred path for route X.
Solution 1: lower bgp_local pref on import from ASN B ----------------------------------------------- - Advantage: - very easy to configure - has the state local: "this upstream is not so interesting" - Disadvantage - router2 does not export route X into the ASN - Only if the ASN A route vanishes, it is being redistributed
From a setup perspective, this looks very logical and coherent, besides the lack of knowledge of all other routers of the routes.
Solution 2: lower bgp_local pref on import from router2 ------------------------------------------------- - Advantage: - route X will be propagated into the ASN - Should have almost zero failover time, only kernel needs to be updated on each router - Disadvantage - All routes from router2 will have a lower preference, if router2 handles other routes that should be treated normally, configuration can get complex
From a setup perspective this looks more messy and complicated, but should achieve faster failover times and seems more transparent.
Solution 3: Elongate the bgp_path ------------------------------------------------- router2 could prepend ASN B some times on import or export to iBGP.
- Advantage: - route would be propagated (actually unclear, as it is not the best route) - Disadvantage - path is modified in an "incorrect" way
Solution 4: other solutions ------------------------------------------------- Maybe something else is actually much smarter? If you solve the problem differently, I'm looking for feedback on that topic.
Best regards,
Nico
[0] https://bird.network.cz/pipermail/bird-users/2024-July/017743.html
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