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Configuring 2 T1's to same network with a 3rd "DR" network T1 as "backup"

Posted on 2006-11-27
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Last Modified: 2010-04-17
OK Cisco Guru's..Moderatly difficult config help is needed. Scenario:  I have a rather large frame-relay network.  35 sites connecting back to a central Datacenter. Branch sites connect via 2 T1 circuits each (1 ATT, 1 MCI and a single Cisco 2651xm) to T3 ports from each (MCI and ATT) on a single Cisco 7206 at the datacenter. We have now added a DR site with another DS3 and 4k ports at each remote site.  Each site has to connect through the central datacenter to connect to each other and to the internet.  Currently I am doing equal cost load balancing with EIGRP and I wish to keep EIGRP.  I do not want any traffic over the DR network unless all other T1 paths are down. I beleive I can accomplish this by working with the bandwidth and delay statements. Any help would be appreciated.
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Question by:dadennis
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pjtemplin earned 100 total points
ID: 18027078
Lie to the router and tell each backup interface that its 'bandwidth N' is perhaps 1/100th of its true bandwidth.  EIGRP should then route away from the backup links.
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by:mikebernhardt
mikebernhardt earned 100 total points
ID: 18029804
There are 2 methods, each have benefits and downsides.

The first is to just modify the bandwidth on the backup links to be less than the primary link. To be safe, also define the bandwidth for the primary link if you haven't already, i.e. 1544 for the primary link and 1000 for the backup link. The up and downside with this method is that if a single route goes away on the primary link, the spokes will use the DR path to get to it if it's available there. You may or may not want this behavior. To get around that, put an outbound distribute list on the hub side (both primary and DR) to all spokes, allowing only the default route out. This also cuts down a LOT on EIGRP queries and calculations. Making the spokes EIGRP stubs will accomplish the same thing.

The other method is to use the backup-interface command on the DR interface. This will keep the interface down until the primary link is physically down, then bring it up. You set timers to manage this. The downside here is that if the link is up but there are no routes, you are lost. The upside is that you never have to worry about traffic going over the DR link when there's another way.

All in all though, the first method is better, and much faster also.
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