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

[Israel G. Lugo]

On 09/30/2015 06:29 PM, Christian Tacke wrote:

> [...]  Now if internal boxes want to reach P.P.P.A
> they will go via the 10.0.0.R into the ISP network and then
> to P.P.P.A instead of 10.0.0.A.  For the way back, A knows
> a shorter way, directly into the internal network. 
> Assymetric routing, not really nice. [...]

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.


> 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.

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

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