CCNP - Auto-Summary

Author

Auto-Summary

firechicken

2001-10-13, 6:43 pm

Hi everyone.

I've failed to try and find a good explanation of the auto-summary command. Specifically, I can't figure out why someone would want to turn it off when using a classless routing protocol.

Thanks for any help!

depamo

2001-10-13, 9:25 pm

In networks that you are configuring everything, sometimes you don't want the router to do things automatically. One of the problems that I ran into was that I was using 172.16.X.X as my interconnecting addresses with 192.168.X.X as my host user networks on my private network. Under circumstances where networks are discontiguous (network is separated by another network that is not contigous) you need to disable auto-summary.

Since a router has a network with 172.16.X.X it will announce a summary for 172.16 if you don't turn it off. The effect of this is that a summary route is sent around that will cause traffic to take the wrong route under certain circumstances. There is a point where summary routers are more benificial, such as in the core but beyond that, access and certain distribution routers should not be summarizing routes.

You can use it at certin places in the network but my advice is that you should hard-code your summary routes by hand to know exactly what is going on all the time. Its just one of those things that come up as more trouble to maintain and troubleshoot then hardcoding it by hand. Anyhow, your networks shouldn't be moving around too much so this is a value that won't be chaninging dynamically very often anyhow.

whytokayok

2001-10-15, 12:53 pm

depamo hit on the head. Unless you own all those addresses you should turn of auto-summary.

Retired-Mod

2001-10-16, 3:40 am

You're right to question why Cisco left this a default on command with EIGRP....baffles me!

Retired

firechicken

2001-10-16, 9:17 am

Thanks for that egg-sellent egg-splanation!

Sponsored Links