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OSPF vs EIGRP for Cisco 26xx failover?

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9 replies

by tomtowle

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I have two Cisco 26xx routers, one at each building, connected with WICs over a T1 to each other and each router over another T1 to a different Internet provider. They both serve static IPs and NATted private groups. Both route those users to the Internet connection and use their respective serial port and associated T1 but I would like to configure a "failover" scenario in which a downed T1 on one side would cause users on that side in that building to choose an alternate path over the extra hop internal T1 to the other router and out that side's Internet T1 connection.

I was told HSRP originally by an engineer that did not understand the situation, later given an OSPF solution I thought would work and now someone has said OSPF is toocomplex and I should use EIGRP. I was told by the OSPF CCIE that Cisco 26xx routers don't have enough memory for BGP4 and my intentions.

Any thoughts or suggestions?

9 replies

by mcoya

In my opinion it is even simpler. With two routers you don't need a complex routeing protocol. Even EIGRP is to complex. I would simply use RIP and if you feel the need throw in a floating static route to be safe. RIP will be all you will need for applications that small.

by tomtowle

This is not enough information. The other suggestions were given with the same promise.

by etherealmind

There are no easy answers to this one. But here is a scenario to try.

Ask both ISPs to originate a BGP default route to your routers. You will need to configure them to speak BGP and for each ISP to allocate you a Private Autonomous System. Do not configure IBGP between the routers but redistribute this default route into a routing protocol like OSPF or EIGRP, use a filter so that no other routes are accidentally propagated.Don't use RIP, its too slow.

You may need to manipulate the costof the redistributed route so that each router then has a default route from the BGP Peer (thus saying the Internet is thataway) but if you disconnect the T1 another default route will appear from the neighboring router to say the Internet is thisaway.

You may also need to check your NAT config to make sure both address ranges are in the NAT outbound translation map.

Either way, this will fix your outbound traffic flows. But remember that it won't fix your inbound traffic flows e.g smtpmail, web server because they will be fixed to the IP address in your DNS record.

HSRP might work in your situation, but you would have to have ethernet connectivity between the routers for HSRP keepalives and so that workstations can all have the same gateway address.

Hope this helps.

Rgds

Taphagreg

by ckilday

Here's a suggestion: backup static routes that point to the other router, with a higher administrative distance applied to them. It's been used to incorporate ISDN as a backup to a dedicated T-1. How it would work is you would add a static route to your configs that would point internet traffic to the ip address of the other router's T-1 line, but with a higher administrative distance. If the primary link dies, the router protocol recognizes the secondary route and pushes traffic that way. When the primary line is fixed, it has a lower administrative distance and takes over. You would have something like this...
Rtr1: Wic IP = 192.168.2.1 255.255.255.252
Rtr1: Internet IP = 24.237.18.5
Rtr2: Wic IP = 192.168.2.2 255.255.255.252
Rtr2: Internet IP = 166.9.83.18
configs
Rtr1: IP route 0.0.0.0 0.0.0.0 24.237.18.5
: IP route 0.0.0.0 0.0.0.0 192.168.2.2 120 (admin. dist.)

Rtr2: IP route 0.0.0.0 0.0.0.0 166.9.83.18
: IP route 0.0.0.0 0.0.0.0 192.168.2.1 120 (admin. dist.)

You could be more granular by limiting it to http traffic using port 80 in your route statement.

As a cautionary note, this is not something I have done or have seen done anywhere, but it may keep you from going the BGP route. The only problem is that to test this configuration, you will need to turn down the primary circuit. Might be best to schedule an after-hours test. Another source of information/suggestions would be the TAC at Cisco. Best wishes,
- ck

by Former member

ckilday's option will work and looks to be the best. BGP is not an option. Just as the CCIE stated, eBGP is way too big for your routers, plus the fact you'll have to get AS numbers. The static IPs are the best solution. I'm fairly certain that the first static route, once it sees the interface down will hop on the second route. Good solution.

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