On Thu, Sep 10, 2015 at 10:40:22AM +0100, Adam Priestley wrote:
Hello list,
I am trying to understand where I might be going wrong in my implementation or understanding of the following setup.
The documentation states that "BGP depends on existing IGP routing table with AS-internal routes to determine immediate next hops for routes and to know their internal distances to boundary routers for the purpose of BGP route selection". However: When inspecting the available routes from one of my bird routers elsewhere on the network, I see one for which the next-hop OSPF cost is 20, and one where the cost is 30. Both routes still show the same MED, the same localpref, same as-path etc, but the route with the higher OSPF cost is always being installed.
The only reason I can see for this route to be preferred is that the originating router has the lower router-id, but this check comes after the supposed IGP distance check in the route selection rules. So am I completely misunderstanding how this process works, or have I just done something wrong? Any advice would be greatly appreciated.
Hello For BGP recursive routes, the (X/Y) in show route is priority / IGP metric. '?' means that IGP metric for some reason cannot be determined. This baffled me as otherwise the BGP routes use proper next hops from OSPF routes. I examined it and for some reason[*] BIRD does not accept OSPF type-2 external metric as IGP metric for the purpose of recursive route comparison. There are two solutions, either export that static routes to OSPF as type-1 external routes instead of type-2 external routes, or set the ipg_metric route attribute explicitly in OSPF import filters (e.g. 'igp_metric = ospf_metric1' would use ospf_metric1 always even for type-2 external route). [*] Probably because type-2 external routes have both type-2 metric and regular metric, both have 24-bit range and it would be problematic to compress that into 32-bit igp metric range. -- Elen sila lumenn' omentielvo Ondrej 'Santiago' Zajicek (email: santiago@crfreenet.org) OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net) "To err is human -- to blame it on a computer is even more so."