Re: [LARTC] Problems in Dead Gateway Detection / Failover -

[gypsy <gypsy@iswest.com>]

Manish Kathuria wrote:
--= snip =--
>   However, if there is a problem in the ISP connectivity at any of the
> subsequent hops, there is no dead gateway detection and failover also
> does not take place. I have tested this on various linux kernels from
> 2.4 as well as 2.6 series.
> 
> Somehow I have never faced a similar problem before and things have been
> working perfectly. In real life situation here, the first hop gateway is
> rarely going to be down so dead gateway detection and failover is going
> to be required whenever there is some connectivity problem at any of the
> later hops. So that's where dead gateway detection needs to work.
> 
> What could be the reason ? How can this be resolved ? I would appreciate
> any pointers or suggestions.
> 
> Thanks,
> 
> Manish Kathuria

Manish,

Same here (a long time ago.  I no longer have multiple ISPs).

I don't have any answers for you, but here are a few pointers:

Use arping in a script, pinging the farthest hop that arping can reach
that is of interest.  Whenever arping returns a bad status, run 'ip
route flush cache'.  Put a nice long sleep in the script and run it all
the time.

Perhaps in that same script, 'ping -n1 -I' each WAN interface in turn to
some destination that must always be up but reachable only by/on that
interface.  Run 'ip route flush cache' whenever that ping fails.

You are just trying to detect the up or down status of the link, so
don't flood the connection with arping and ping packets.  Using sleep,
space those pings apart to something sensible.

Although Julian has never confirmed (or denied) this, it was my
experience that only the **__FIRST__** nexhop affected the up or down
status of the connection.  If that succeeded, nothing would flag the
connection as dead.  If you know C, perhaps you can examine Julian's
kernel patch to see if there is any useful information there.  In my
opinion, Julian should document exactly how DGD works.  Perhaps he has
and I just can't find it on his web site, but (when I cared), I was not
able to find anything useful there.

Have you tried to engage Julian in a conversation to resolve this?  He
posts here occasionally but I do not know if he answers questions about
DGD off this list.
--
gypsy
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