Eigrp Auto-Summary Question

[cindylee27]

Hi Experts,

I am trying to figure out what routes will this auto-summary command does for the network below?

network 122.120.0.0 0.7.255.255

auto-summary

If i have a 122.121.3.0/24 network at this particular Core Switch 4506, will that be any impacts on the routing ?

Thanks!

[Edison Ortiz]

With auto-summary turned on, the 4506 will advertise that network as 122.0.0.0/8 (class A network)

If you have a neighbor router with interface IP address 122.122.3.1 and that router advertises that interface via EIGRP, your 4506 won't accept that route due to split horizon rule on EIGRP protocol.

Split horizon prohibits a router from advertising a route back out the interface from which it was learned.

HTH,

[cindylee27]

Edison,

Yup, you are right. It actually gives me the problem. But lets say I really have a subnet in 4506 which 122.122.3.0/24 that i need to cater to, and I also have 122.122.2.0/24 at diff location that connects through the WAN router.. How can I resolve this issue?

I tried " no ip classless" it still throw the routes to Class A which is 122.0.0.0/8.

and when i did "sh ip route" it gives me

routes to "NULL0".

Thanks again.

[Edison Ortiz]

What's wrong with typing

no auto-summary

under the EIGRP process ?

That's actually best practice EIGRP configuration in most networks.

[Richard Burts · Accepted solution]

Cindy

with these statements in the config:

network 122.120.0.0 0.7.255.255

auto-summary

the router/layer 3 switch will look for any interfaces in the address range 122.120.0.0 through 122.127.255.255 and include those interfaces in the EIGRP process.

The EIGRP process will then consider that it is in class A network 122.0.0.0. It will advertise all subnets that it may learn of the class A network out interfaces in that network and it will advertise a summary address of 122.0.0.0/8 out any interface that is not in that class A network.

Perhaps there is some confusion about what the mask in the network statement means and what auto summary means. The mask in the network statement is only used to describe a range of addresses that will be included in the EIGRP routing protocol. The mask in the network statement has nothing to do with whether (and how) EIGRP will summarize routes.

By default EIGRP summarizes on classful network boundaries. So any interface within the class A network 122.0.0.0 will advertise subnets of that network. And any interface that is not in the network 122.0.0.0 will just advertise a summary route.

If this does not satisfy your question then perhaps you can clarify it a bit? Information about the topology would be helpful (how many interfaces in what networks).

[edit]

having just seen your response to Edison I believe that we need more information about what the network topology is. Given what you have described so far I do not see why it is summarizing. But your description seems to indicate that it is summarizing. Are there interfaces in networks other than 122.0.0.0? Are there any interfaces with ip summary-address eigrp configured?

HTH

Rick

[cindylee27]

Thanks Rick.

Well, clarify here abit.. I have all the interfaces at Core Switch 4506 which is located at location B which are pointing to 122.120.3.0/24 TO 122.120.10.0/24 but only 1 interface 122.121.12.0/24 subnet created at core switch 4506.

But this subnet is belongs to another location which is at location A as this subnet is being migrated to location B as this is purely management request to only move this subnet across instead of changing the subnet to 122.120.12.0/24 which I believe will solve this problem.

And the standard config is always "auto-summary" across all the routers so, am thinking to stick to this OR i have a reason to let the management know that due to this setup, "No auto-summary" is required and from there on, what are the things that I need to take care of?

I believe with "no auto-summary" , I got to put in all the network subnets that i have newly created at the core switch right? or i understand wrongly?

P/S: By the way, no IP summary-address eigrp configured at the Router/Core Switch at location B.

Thanks...

[Edison Ortiz]

If you only have one interface on Core Switch 4506, then the following commands will do the job:

router eigrp x

network 122.121.12.0 0.0.0.255

no auto-summary

If this subnet belongs to another location, you can't advertise it and I don't understand how can you workaround that issue. You must renumber one of the subnets.

auto-summary is the default option on EIGRP, but it's highly recommended to disable it. By disabling it, EIGRP operates as classless protocol.

You don't have to enter all network subnets with no auto-summary. All you have to list (in the network statement) the interfaces you want to advertise. You can use subnet mask to group contiguous subnets.

router eigrp x

network 122.120.12.0 0.0.1.255

no auto-summary

Commands above will enable EIGRP on interfaces 122.120.12.0/24 and 122.120.13.0/24

[cindylee27]

Thanks Edison,

Are you saying I can do this?

Router Eigrp 10

network 122.120.0.0.0.7.255

auto-summary

Router eigrp 11

network 122.120.12.0 0.0.1.255

no auto-summary

IS this also mean that the WAN router that connects to this Core switch will need to have 2 EIGRP config as above? so that they can communicate with each other?

Thanks..

[Edison Ortiz]

The networks I listed before were just an example.

There is no need to have dual EIGRP process.

Can you post the output from typing

sh int | i Internet

at the 4506 switch.

Can you also clarify about having a subnet created on switch that was also create on a remote switch ?

[cindylee27]

Edison,

as below:

Internet address is 122.122.1.3/24

Internet address is 122.122.2.252/24

Internet address is 122.122.3.252/24

Internet address is 122.121.24.252/24

Internet address is 122.122.36.252/24

Internet address is 122.122.37.252/24

Internet address is 122.122.255.245/32

Reason why is the Wintel team doesn;t wants to change the subnet due to complication on the clustering.. They wanna try to stick with the same subnet so no changes will be done on the wintel side.

[Edison Ortiz]

On this device, if you want to enable EIGRP on all its interfaces, the correct commands are:

router eigrp x

network 122.122.0.0 0.0.255.255

no auto-summary

With the 'no auto-summary' this device will advertise the routes as:

122.122.1.0/24

122.122.2.0/24

122.122.3.0/24

122.121.24.0/24

122.122.36.0/24

122.122.37.0/24

122.122.255.245/32

With auto-summary turned on, it will advertise a single route

122.0.0.0/8

and we don't want that.

How many other EIGRP devices you have in your network ? Can we see the show int | i Internet from those devices as well ?

[cindylee27]

Edison..

Sorry for the late reply.

Understood..

There are couple of them in fact...I will go ahead to put " no auto-summary" then for all the devices..

Thanks guys! :)