Route summarization

Mikdilly

If you had the following subnets configured on a router, how would the router advertise the summary route?

Fa1/1 172.16.4.0 /26
Fa1/2 172.16.5.0 /26
Fa2/1 172.16.6.0 /24
Fa2/2 172.16.7.0 /24

Would it be 172.16.4.0 /22?

Netstudent

Yes that would work because that covers 4.1 - 7.254

But note that you have /26 masks on those other subnets. So fa1/1 would be 172.16.4.1 -172.16.4.62

This is not the complete class C range of 172.16.4.1 - 172.16.4.254. Having said that, you want to make sure you are not overlapping subnets wioth your summary address. Be mindful what range the summary address includes. This could very well work in a given situation and it could not.

shednik

I came up with that answer as well, although i thought what netstudent said as well

how else could you answer that as in a more specific summary route, not sure if you can just asking

Netstudent

shednik wrote:

I came up with that answer as well, although i thought what netstudent said as well

how else could you answer that as in a more specific summary route, not sure if you can just asking

You could summarize the last 2 subnets with a 255.255.254.0 mask. That would cover 6.1 - 7.254. But that would leave out the others which aren't really contiguos enough to summarize in the first place. I mean his summary address is correct, but I'm not sure about a tighter fit that covers all ranges in the requirement.

Mikdilly

Thanks for the replies, just trying to clear this up, would the 172.16.4.0 /22 be the summarized route that say eigrp would automatically advertise? If so, is there a show command that would return the summarized route, something like show ip protocol?

Netstudent

I believe you would need the "ip summary-address eigrp as# <network> <mask>" commmand on the interface that would be advertising the aggregate route.

networker050184

EIGRP automaticaly summerizes at the classful boundry, so no it would not automatically summerize that range. You can use the sho ip eigrp topology command to see all the routes in the eigrp topology table or sho ip route eigrp to show the eigrp routes that have been placed into the routing table. As Netstudent stated you must use the interface level summerization command to advertise that route. Be careful when you do your summerization as to not include a range that is too large. Its not really a problem in a lab enviroment, but in a real network could cause you some routing issues.

Mikdilly

networker050184 wrote:

EIGRP automaticaly summerizes at the classful boundry, so no it would not automatically summerize that range. You can use the sho ip eigrp topology command to see all the routes in the eigrp topology table or sho ip route eigrp to show the eigrp routes that have been placed into the routing table. As Netstudent stated you must use the interface level summerization command to advertise that route. Be careful when you do your summerization as to not include a range that is too large. Its not really a problem in a lab enviroment, but in a real network could cause you some routing issues.

Thanks, that helps a lot, does using ip summary-address replace(disable) the automatic summarization at the class boundary?

Netstudent

No, the no auto-summary command under router configuration mode will do that. EIGRP will auto-summarize by default.

IE.
en
conf t
router eigrp 200
no auto-sum

networker050184

Its a good idea to always disable auto summary on eigrp routers unless you have a very well designed network. The auto summary can cause some issues that can be noticed easily in a small lab but can be a little harder to find in a large network. I always turn it off in labs or production.