ip route

Unanswered Question
Nov 2nd, 2009

Can someone shed light on this...



3 routers: A (Corporate), B (Branch), C (Branch)

A - EIGRP w/ redistribute static & connected

B - EIGRP w/ redistribute connected

C - EIGRP w/ redistribute connected

A is connected to B and C through MLPPP. Site A and B have their own ISP. Only A currently has a 'ip route' static route.

--- B --- ISP

ISP --- A |

--- C --- ISP

If a default route is placed at site B how will B be able to communicate to A or C LAN network as static route has administrative distance value 1 and EIGRP is higher?

Do I have to put static routes to site A and C network in site B router? This doesn't seem right. What am I missing here?

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a.cruea1980 Mon, 11/02/2009 - 11:16

There are two things you can do.

You can add a static route and give it a higher value than your EIGRP routes. This will cause all default traffic to go out A, though.

However, when doing routing, the router looks at the best match for a destination. So for example, if your address pool at site 'A' is, then when 'B' or 'C' send traffic to, it'll go to router A, not to the default route ( because more closely matches than

If you do a "sh ip route eigrp", do sites 'B' and 'C' have the proper routes to 'A'?

Edison Ortiz Mon, 11/02/2009 - 11:18

Placing a default route on B won't affect communication to A or C as those locations will advertise specific subnets from their networks.

Remember, a specific route will be preferred over the administrative distance.

Site B will use its default route for any routes not included in the routing table.



rschachtschneider Mon, 11/02/2009 - 11:32

Hate to be a pain but can either of you point to documentation stating this? It makes sense logically but need proof :)

rschachtschneider Tue, 11/03/2009 - 08:04

The document explains which protocol is added to the routing table. I'm trying to find a document that states something to the effect that a default route is chosen last over administrative distance.

If I go off just administrative distance then my scenario won't work as site B has site C network through EIGRP with an administrative distance of 90 but the default route is static which has an administrative distance of 1. With that being said it would choose the default route over the EIGRP.

"Remember, a specific route will be preferred over the administrative distance." Can you point me to where this is described?

Logically speaking you wouldn't want this to happen. I would think the default route would be last resort if not found in routing table.

I guess what I am looking for is a logic diagram as this document


doesn't tell me either.

Jon Marshall Tue, 11/03/2009 - 11:30


Did you read the specific part in the doc concerning prefix matching -


it states -

"Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table"

ie. it is clearly stating that all routes in the example will be installed and then it goes on to explain how the forwarding decision is made and the forwarding decision is always based on the most specific match (unless of course you are using PBR).

The default-route is the least specific route so it will be used when there is no more specific route in the routing table. Doesn't matter about the AD of the route.

Where the AD does matter is if you have 2 routes that are exactly the same prefix/subnet mask, then the router will choose the one with the lower AD.

So in your scenario as long as the EIGRP learned routes are more specific than the default route, and they should be, then the router will use the EIGRP routes if it finds a match.


Edison Ortiz Tue, 11/03/2009 - 11:57

As you are quoting me, I'll add to Jon's reply.

We are basically saying the same thing. A default route is not a specific route - as you stated - is the gateway of last resort.

I said - a specific route will be preferred over the administrative distance.

If you have an EIGRP route of

and a static route of

and you need to forward traffic towards, the EIGRP route will be used - for other routes, the static will be used.

I believe you misunderstood my initial reply.




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