[quagga-users 12456] Re: Automatically influence the Linux Routing Metric of Directluy-Connected routes?

Hirsch, Stephen Stephen.Hirsch at oriongroup.co.nz
Thu Sep 8 04:04:49 IST 2011

I had a  similar problem with two Quagga routers on separate switches on a 
common LAN trying to access a main and backup firewall interface that could 
appear on either LAN segment. If the link between the two LAN segments 
failed, the two routers would both still think they had a valid route to, and would 
both continue to advertise a route to the LAN even though each router could 
only access part of the LAN. 

For traffic from either router destined for interfaces (other than the other router) 
on the LAN, if the destination interface was on the other side of the split in the 
LAN, it would get dropped into a black hole. The two routers knew they couldn't 
see each other since they couldn't see each other's Hello's but that didn't help 
with routing to other interfaces on the LAN. 

Routing protocols do not handle discontinuous LANS very well in some 

I solved my problem by writing a simple script which runs as a daemon on each 
router and periodically pings the firewall interface to make sure it can still see it. 
If it can't it raises the route cost to the local LAN which forces all the inbound 
LAN traffic to the other router which hopefully has the active firewall interface 
on it. 

You might be able to do something similar. 


-----Original Message-----
From: quagga-users-bounces at lists.quagga.net [mailto:quagga-users-bounces at lists.quagga.net] On Behalf Of Daniel Ng
Sent: Thursday, 8 September 2011 1:24 p.m.
To: quagga-users at lists.quagga.net
Subject: [quagga-users 12452] Automatically influence the Linux Routing Metric of Directluy-Connected routes?


Consider the following scenario:

Router A --- LAN with multiple switches --- Quagga Router 1
                           |                    |
                           |                    |
                           |                 PPP Link
                           |                    |
                    Quagga Router 2 ------------           

-Each of the Quagga Routers are connected to different LAN switches (1 LAN
switch each)
-Quagga Routers are also connected via a WAN PPP Link
-One of the interal LAN switches fails such that Quagga Router 1 can only acess
Router A via Quagga Router 2 (via the PPP Link). 
--The failed internal LAN switch is *not* directly connected to either of the
Quagga Routers

In this scenario, Quagga Router 1 still has a healthy connection to its
directly-connected LAN switch. Therefore, when it receives packets destined for
Router A's LAN interface, it will still try to forward these packets out to this

Unfortunately this packet has no chance of getting to Router A because one of
the upstream internal LAN switches has failed, as described above.

Say we are running OSPF. Quagga Router 1 will have stopped receiving Hello
Packets from Router A, due to the internal LAN switch failure. Can Quagga
somehow influence the routing table of Quagga Router 1 so that it will know to
forward the above packets to Quagga Router 2 instead? Furthermore, when the
internal LAN switch recovers from its failure and OSPFd on Quagga Router 1
starts receiving Hellow Packets from Router A again, can Quagga revert this
routing table change?


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