2811 - 1 Serial - 1 FE - 2 Internet providers

Answered Question
Oct 29th, 2008

Is it possible to have 2 separate internet connections coming into one router, and just route connections through one and not the other unless the one goes down? Would you do this with static routes:

ip route 0.0.0.0 0.0.0.0 serial0

ip route 0.0.0.0 0.0.0.0 fa0/1 254

Would that work?

Thanks!

--John

I have this problem too.
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Correct Answer by Giuseppe Larosa about 8 years 1 month ago

Hello John,

you can do exactly in this way:

the second default route is used only if the first one fails.

You may also use object tracking:

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

in this case you can define other events that will make the router to use the backup default route (for example ip reachability of some specific address)

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Wed, 10/29/2008 - 15:43

Hello John,

you can do exactly in this way:

the second default route is used only if the first one fails.

You may also use object tracking:

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

in this case you can define other events that will make the router to use the backup default route (for example ip reachability of some specific address)

Hope to help

Giuseppe

patrickvanham Thu, 10/30/2008 - 04:57

This line will not work:

ip route 0.0.0.0 0.0.0.0 fa0/1 254

Ethernet is a broadcast interface, you'll need to route to the remote IP address:

ip route 0.0.0.0 0.0.0.0 A.B.C.D 254

Richard Burts Thu, 10/30/2008 - 07:50

Patrick

While I agree with you that routing by next hop address is BETTER than by specifying the outbound interface when routing out a LAN interface such as FastEthernet, it is not correct to say that it will not work. As long as the next hop router has enabled proxy arp then the static route specifying fa0/1 WILL work. It will create more work on the router, but it will work.

HTH

Rick

patrickvanham Thu, 10/30/2008 - 08:06

Thank you for the correction. However, I've been working with a few ISP's so far and not one has proxy arp enabled. Rather it's explicitly disabled. Since the port is going to an ISP, it would be... unusual if proxy arp was enabled, hence I wrote it wouldn't work.

It would have been more correct to say it wouldn't work under most circumstances, considering the remote end.

John Blakley Thu, 10/30/2008 - 08:12

Well, the reason that I wrote it that way was because of the connection that we have. S0 is a frame connection, and FA0/1 is connected to a DSL modem. So I just needed to get it to the next device. Would that still work?

--John

patrickvanham Thu, 10/30/2008 - 08:40

The DSL modem is most likely in bridging mode, so the next hop IP address would be on the ISP side. Is the address you get from the ISP fixed or dynamic? Do you know the IP address on the ISP side the modem connects to?

You can try the ethernet interface and see if it works, although if it is your backup route you may not notice it if the ISP changes anything and it no longer works.

To be certain use the remote end IP address (ISP side) as a next hop.

Richard Burts Thu, 10/30/2008 - 09:24

John

The static route using fa0/1 MIGHT work. As Patrick and I have been discussing you are at the mercy of the ISP (and whether they disable proxy arp). And even if it does work, I would say that it is a less than optimal solution. If the static route names the outbound interface fa0/1 then the router will have to ARP for every outgoing destination address (and maintain a large ARP table to record all the resonses). So you are using more CPU cycles and more memory.

If you know the next hop address (or can figure it out by doing a traceroute through that interface) you can use Patrick's suggestion and code the next hop address in the static route. Or if the interface is learning its address from the provider (address negotiated) using DHCP then there is an option to define the static default route to use the learned next hop address:

ip route 0.0.0.0 0.0.0.0 fa0/1 dhcp

see this link for more information about this feature:

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t8/feature/guide/gtddgtwy.html#wp1046045

HTH

Rick

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