OSPF: directly attached summarized networks announced with metric 0 when link goes down

Christian Tacke Christian.Tacke+bird.network.cz at cosmokey.com
Thu Oct 1 11:27:51 CEST 2015


On Wed, Sep 30, 2015 at 19:18:38 +0100, Israel G. Lugo wrote:
> Have you considered using a loopback IP for accessing the A machine?
> That's how I would do it. I mean, place the IP on interface "lo" as a
> /32. Then you let OSPF announce it and everyone can access the machine
> from everywhere, using the most efficient route. That is of course
> assuming you 1) have additional IPs to assign and 2) can change the
> clients to use the new IP.

Yes, I have considered this. But since the ISP network is
just a plain ethernet /29 without ospf/bgp, I would need to
apply proxyarp or something else. All of that feels more
complex than my "stubnet IP/32 ..." line in bird's configs.

> > One brainstorm idea is a "stubnet filter". Normal filters
> > could be used. Routes get run through it before being
> > announced. So for the original problem, one could do "if
> > it's a /32 then reject;" or "if it's a /32 then set metric
> > to 10000" (so that the summary network would also be 10000
> > and better routes would be used). It wouldn't directly help
> > my problem, because I would want to create an additional
> > route.
> So what you are proposing is basically to create an additional filter
> hook, inside the OSPF protocol. Something on the path between the OSPF
> protocol's route table and the LSAs being sent. Correct?


> Perhaps that may be useful. I do worry, however, that it may start to
> become too complex, or allow diverging too much from "standard" OSPF.

Thanks for thinking more about it.  That's what a
brainstorm idea is for: Someone making thoughts about it.
And you're very likely right: It might add a lot of
configuration complexity that will confuse people.
I wonder, whether some of this could be handled in a
normal import filter instead?




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