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

[Israel G. Lugo]

Hello,


On 29-09-2015 23:54, Ondrej Zajicek wrote:
> 
> You are right with your analysis of the issue. I agree that in your case
> it does not make sense, but unfortunately, the behavior is IMHO more or
> less correct with regard to RFC 2328. I am not sure how it should be
> modified to be consistent and to make sense in your setting. /32 local
> definitely should be propagated (at least by default). Perhaps ignoring
> zero metric /32 from triggering summarization? Or ignoring any local stub
> network? Or some more general configurable limit for summarization (like
> minimal cost)?

Thank you for being open to the issue.

I think the first option (ignoring zero metric /32 from triggering
summarization) would make sense, and would solve this problem in a safe way.

If summarization is already taking place for another reason, the zero
metric /32 should still be summarized; it would just not be enough to
trigger the summarization.

For example, if we have the following option:

  network { 10.0.0.0/16; };

With the following routes:

  10.0.0.0/24 via 192.0.2.1 on eth0  IA (150/10)
  10.0.1.0/24 via 192.0.2.1 on eth0  IA (150/10)
  10.0.20.1/32 dev eth0  I (150/0)

We should still summarize and announce just the 10.0.0.0/16:

  xnetwork 10.0.0.0/16 metric 10

But if the other routes go away for some reason, leaving just the local
stub /32, then it will not trigger a summarization, and we announce as-is:

  stubnet 10.0.20.1/32 metric 0

If that seems good to you, I think it would be great.

Perhaps it would even make sense to ignore any local stub network,
however I'm not sure if that might affect some use case we're not
thinking of.


> Perhaps you could try to use stubnet option with hidden + summary
> suboptions to hide these /32 routes to not trigger the summary networks.
> 

Thanks for the suggestion. I tried with:

  stubnet A.B.C.253/32 { hidden; summary; };

And with or without:

  networks { A.B.C.0/24 };

There was no change. However, I did manage to get it working like this:

  networks {
    A.B.C.253/32 hidden;
    A.B.C.0/24;
  };

It's not ideal, since I have to list and hide all the local IPs, and it
doesn't announce the /32 in case of link down, but it's a good working
compromise.

Regards,

--
Israel G. Lugo
Núcleo de Redes e Comunicações
Direção de Serviços de Informática
Instituto Superior Técnico

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