Workings of auto-summary in EIGRP?

Unanswered Question
Nov 1st, 2007

We are using 172.20.0.0 internally with /25 mask for local user subnets. This network is spread around 40 locations. Normally we have 'no auto-summary' under our router command like:

router eigrp 1

network 172.20.0.0

no auto-summary

So if I wanted to know if a particular subnet was in use, ie, 'sh ip route 172.20.100.0', I would see that in a routing table, and felt confident that there truly was a 172.20.100.0 subnet.

Someone inadvertently configured a router with 'auto-summary', and I was trying to troubleshoot a problem on 172.20.100.0. I shut down the interface that is configured for 172.20.100.0, yet it still showed up in the routing table. I track down where that route was advertised, and I found it was coming from a router that has the 'auto-summary' and the 'show ip route 172.20.100.0' on that router showed it coming from interface null-0.

The reason I don't like to use auto-summary is because of just that - I don't get a true picture of what subnets are actually real - everything gets summarized into the major network.

Is that the way auto-summary is supposed to work? If one uses the 'auto-summary on all routers, how does one tell if a particular subnet is in actual use of not?

I have this problem too.
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luismarinaray Thu, 11/01/2007 - 18:40

Hi,

I think that depends on what is your network design, if you have designed your network carefully with consecutive networks numbers you could know how is auto-summary working.

Autosummary allow to reduce the number of networks advertised, so if your routers have one network for each node, and one central hub , you could announce your network just with one network number.

You should check every router and your IP addressing plan first.

Richard Burts Fri, 11/02/2007 - 11:06

Jim

In your description you tell us that your network uses 172.20.0.0 and do not mention any other networks. If this is true (that there are no other networks than 172.20.0.0) then it makes no difference whether no auto-summary is configured or not - you will get the exact same results as long as the network is based on a single major network (a class B network in your case).

auto-summary only makes a difference when a router has an interface in one network and has another interface(s) in another network. If all interfaces are in the same network then EIGRP advertises all subnets out all interfaces. If the router has interfaces in two networks (say for example that your LAN interfaces were in 172.20.0.0 and you put your serial interfaces on 10.0.0.0) the the router would not advertise subnets of 172.20.0.0 over the serial 10.0.0.0 interfaces but would advertise a summary route.

In the situation that you describe that you found a router with an entry for 172.20.100.0 to null 0 then the logical explanations would be that either there is a summary address configured on that router for 172.20.100.0 or that someone configured a static route for 172.20.100.0. Or is it possible that the route that you were looking at was really for 172.20.0.0 and not for 172.20.100.0?

HTH

Rick

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