[quagga-dev 6284] Re: [PATCH 20/21] Only announce connected routes if link is detected

Joakim Tjernlund joakim.tjernlund at transmode.se
Fri Dec 5 10:59:37 GMT 2008


On Fri, 2008-12-05 at 10:16 +0100, Simon Bryden wrote:
> > You may have a point. Curious, how do you use the "link down ==
> > host address unreachable"? To be really effective it should change the
> > route table when it detects link down is this way.
> >
> >  Jocke
> 
> There's no magic - when troubleshooting a problem, you ping an
> interface and if it doesn't repond - assuming that you know that the
> router is reachable, you know that the interface is down. And, as you
> say (and which is what started this thread), the routing protocols
> will remove the attached subnet from their tables.

Oh, I figured you had worked it into the routing protocol or something.

> 
> Of course, the subnet may still be reachable via another route, and
> other hosts on the subnet may still be reachable, but our interface
> address will always be unreachable.
> 
> If we continue, as you propose, to advertise the down interface as a
> host route, we may never see the problem.

The interface isn't really down, its not RUNNING and it will respond
to a ping from a neighbour in any case.

>  Every single host could
> still be reachable, even though someone knocked the cable out of the
> router. We would be using our backup path without realising it. Of
> course any serious network will have some other means of detecting
> this, but then there are many networks out there that use these
> 'hands-on' troubleshooting techniques.

I don't think that this somewhat questionable troubleshooting technique
should be preferred over maintaining a connection to the router.

One could add an option to keep the old way though, just in case. I am
not keen on doing that work though.

 Jocke



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