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New Member

deciding when a gateway is not available

I have a Cisco IOS router that has ethernet connectivity to two low-end DSL routers. I would like to setup some kind of faul tolerance here but I am not clear on the procedure that the Cisco will use to determine if its primary default gateway is not working. My question is the following?

If the primary DSL router loses its DSL link will it send some kind of ICMP packet to the Cisco when it cannot forward the Cisco's packets to the Internet? If the DSL router just drops the packets will the Cisco eventually "get the picture" and use another gateway?

If none of the above are true, what about a more primitive method of disconnecting the ethernet on the DSL router? Will the Cisco then realize that it cannot use this gateway?



New Member

Re: deciding when a gateway is not available

Quick questions:

1. What do you use as DSL routers in this network?

2. How are they connected to the Cisco router? (Directly connected using 2 ethernet interfaces on the router, or through a hub or switch?)

I would presume that you've configured static default routes on the Cisco router pointing to the two DSL routers. If so, you would definitely have two default routes on your routing table. Cisco then would automatically load-balance between these two links.

I can think of a few solutions:

A. If the DSL routers (wishfull thinking) can support a dynamic routing protocol, you could source a default route from them. The Cisco box would then should learn 2 default routes and would automatically load-balance between them. If a link should fail on say, DSL-1, the locally-sourced default route on DSL-1 would dissappear from the routing table. Give the routing protocol a few seconds to converge and the default route to DSL-1 on your Cisco box would dissappear as well, and will leave you just 1 default route to DSL-2.

B. If the DSL routers are connected directly on the router using the local ethernet ports, you can pull the plug on the misbehaving DSL router and traffic would still flow to the second DSL-router. You can't do this "primitive" method if the devices are connected through a hub or switch (or more appropriately, when the DSL routers are sharing a single ethernet interface on the Cisco device).

New Member

Re: deciding when a gateway is not available

I have two segments on the network. I have a segment where the internal FastEthernet0 of the 1710 and all the client boxes are connected. I have a second hub where I connect the outside Ethernet0 of the 1710 and the 2 DSL router ethernets. My plans is to have the Ethernet0 of the 1710 with two IPs, one for each public subnet from each provider (using secondary IP command on Cisco box). I have two default routes but one I have made it a higher cost route. The idea is that the 1710 will use its primary IP and primary DSL as long as its available. If not, the second default route will kick in and the second IP and router will be used.

For now, I would like to achive the following:

1) Simple failover, no load balancing yet

2) Hopefully I won't have to pull any cables. I was hoping that when the first DSL fails it would send "destination unreachbles" to the Cisco which will then use the second default route.

Is this overly simplistic and/or optimistic?


New Member

Re: deciding when a gateway is not available

It may seem "simple", but unfortunately what you're asking is quite impossible given the existing equipment set-up.

There's simply no way for the 1710 to find out that the DSL line have gone down on any of the DSL routers if the latter doesn't support any dynamic routing protocols. You mentioned that the failed DSL router would send an ICMP destination unreachable, however, this is just for informational purposes for hosts. The IOS would just simply pass on this information to the source host that is sending out packets to the unreachable network.

If you want a solution to this, the simplest way is to have two different ethernet ports on your router connected directly to the DSL modem (not DSL router). This way, even if you have two statically defined default-routes, if any of the two directly connected DSL lines go down, the default-route to the failed link would dissappear from the routing table. Hence, all traffic is redirected to the 2nd DSL line using the other default-route.

When both links are up, you'll be enjoying load-balancing by default. (You can disable load-balancing by configuring one default-route with a higher administrative distance than the other. This is also known as a floating static)

Hope this helps.

New Member

Re: deciding when a gateway is not available

I did some tests and your are right. When I pull the DSL routers DSL link the Cisco still keeps the "down" DSL router as a default route. It seems like I am going to have to try to get the ISPs to configure their routers to broadcast a default route but I don't think they will oblige. If not I will try to take over admin of the DSL routers.

Thanks for all your help.