EIGRP Summary routes

Just working my way through some practice labs and have come unstuck on EIGRP summary routes. Eigrp has a default AD of 90, yet it was my undestanding that summary routes are injected into the table with an AD of 5. I have replicated this in a lab scenario.

I have a route (R1) which is connected to (R2) via s0/3,

on serial 0/3 I have entered the command

IP summary-address eigrp 100 10.0.0.0 255.0.0.0

yet on R2 the route is injected as follows

D 10.0.0.0/8 [90/2681856] via 172.31.0.1, 00:10:49, Serial0/0
|
I was expecting a 5 here !

or is all this study put me way of the mark!

Any pointers

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Comments

EdTheLad

No the summary route will only get an AD of 5 in the local router where the summary was configured.AD isnt propagated.

wildfire

dejavu wasnt this similar to a question you posted at the weekend ed lol?

My brain is mashed,

Ok so then what is the significance of the AD of 5 for summary if indeed it has no local signifacance because its used to advertise out of an interface?
With the command any AD can be put in there 1-255 but it seems to make no bearing on the routing table of the local and neighbouring routers? so why have it at all? wow Im getting more confused by the minute lol

EdTheLad

wildfire wrote:

dejavu wasnt this similar to a question you posted at the weekend ed lol?

My brain is mashed,

Ok so then what is the significance of the AD of 5 for summary if indeed it has no local signifacance because its used to advertise out of an interface?
With the command any AD can be put in there 1-255 but it seems to make no bearing on the routing table of the local and neighbouring routers? so why have it at all? wow Im getting more confused by the minute lol

Nope, i didnt ask this question on the forum,but i did ask myself this question
last week

, and i also wondered why bother since the routing table works on longest match.All i can come up with is this.
Since you explicitly config the summary on this port, a low AD distance is used to protect against any rogue summaries arriving with the same network mask combo on another port.

wildfire

ok thanks Ed that makes sense now, but that means I need to re-read it all again!! that now confuses me with the static route to null 0! I will get there in the end!

PCHoldmann

My understanding is the null interface logic is something like, add a route that will not be used, because a longer match is in the routing table. If part of the summary block is lost, then the traffic dies, instead of getting forwarded on to another router to die, or worse, loop.

Imagine a summary for 192.168.0.0/22 on R1. It gets three /24 routes from R2, and the fourth from R3. A summary static route is created to R2, but all traffic for R3 is routed there because of a longer match. Everything works fine, so long as there is a route from R3. If this route is lost, traffic will be forwarded to R2, and if it has the summary from R1, which it could have, because the local routes have a longer match, it would see the traffic for R3, forward it back to R1, which would dutifully router it back to R2, and so on. With at null interface, the traffic for R3 would die at R1. The redistributed static is used with BGP, and could be used with other protocols, and I assume that the router is performing similar logic in the background for EIGRP.