EIGRP Multiple-AS Redistribution Question

kfarrington · ‎12-03-2009

Hi Guys,

I have the following network topology (please see attached)

Now, if the prefix on the WAN on site 1 192.168.1.x is advertised to the core on site1, the core switch has two EIGRP ASs and redistributes this from AS100 to AS101. (one-way) A couple of questions if I may.

1. If there was a transmission delay on the Site1 to Site2 link, and the route update was passed to site2 via site3 and site4, the prefix would go into the RT under the AS101. Then the Site2 router received an update from site1 over the direct link. What would happen. Assuming that's the metric for the prefix is less on the direct link, would it purge the route from AS101 topo table in the RT now with the route from AS100 with the lower metric?

2. When routes are redist between EIGRP ASs, does the metric get zeroed, or carried into the new AS?

I seem to remember that there have been discussions before and maybe a code change changed the behavior? Ie, the site2 router would just use the update that it received first, or the site2 router would use the update with the lowest EIGRP AS number or, the site2 router would use the route with the lowest metric (assuming AD, prefix-length are the same)

Can someone confirm this for me please?

Many thx,

Ken

andrew.prince · ‎12-03-2009

1. If there was a transmission delay on the Site1 to Site2 link, and the route update was passed to site2 via site3 and site4, the prefix would go into the RT under the AS101. Then the Site2 router received an update from site1 over the direct link. What would happen. Assuming that's the metric for the prefix is less on the direct link, would it purge the route from AS101 topo table in the RT now with the route from AS100 with the lower metric?:-

The EIGRP would just re-calculate the best route and change the routing/topology table accordingly placing the best next hop as the sucessor.

2. When routes are redist between EIGRP ***, does the metric get zeroed, or carried into the new AS?:-

The advertising router will pass it's reported distance on to the router receving the update, the receiving router will take this and add it to the cost to reach the next hop to calculate the metric

You need to remeber things like Sucessor and Feasbile Sucessor:-

Sucessor:- is the best path to the destination that meets the requirements (least distance & loop free)

Feasible Sucessor:- is the best alternate path that mees the requirment (loop free)

HTH>