No auto-summary command

[nellocon4]

HI Folks,

Auto-Summary is enabled on a couple of devices in one of our remote networks. I need to turn it off as it is causing issues.

Will it cause a temporary outage if I issue the "no auto-summary" command?

Thanks

[Peter Paluch]

Hello Shane,

I assume you are running EIGRP. Deactivating the auto-summary will lead to resynchronization between adjacent EIGRP speakers (if using a recent IOS version), or to adjacency drop and reestablishment. You may experience short-term outages lasting at most a couple of seconds, not more.

Deactivating the auto-summary should always be considered one of the compulsory steps in every new routing protocol deployment, ideally at the beginning of the deployment so that it is not necessary to correct it later.

Best regards,

Peter

[dslice]

Well, maybe. Auto-summary is off by default in recent versions, so no need to turn it off if it's already in that state. Show Ip protocol will tell you the state of the setting. Also, if all of the interfaces on the router are in the same major net, the auto-summary setting has no effect, so it doesn't matter whether it's on or off.

Sent from Cisco Technical Support iPad App

[Peter Paluch]

Hello Donald,

Auto-summary is off by default in recent versions

Only in very recent versions as far as I know (15.x).

Also, if all of the interfaces on the router are in the same major net,  the auto-summary setting has no effect, so it doesn't matter whether  it's on or off.

That is absolutely true. However, my point here is more of a pragmatical nature - it never hurts to deactivate the auto-summary explicitly. On the other hand, forgetting to do that may have a negative impact.

Best regards,

Peter

[dslice]

No argument that it doesn't hurt to have it off if it has no impact whether its off or on. I just wanted to make sure it doesn't become "just something to do" on every network even when unnecessary. For example, back when peers bounced due to changing the auto-summary value a customer caused major trauma to their network by doing repetitive "no auto-summary" commands on a bunch of routers that only had a single network statement. The auto-summary value was irrelevant but caused many peers to bounce unexpectedly and unnecessarily due to a lack of understanding how it worked.

Sent from Cisco Technical Support iPad App

[Peter Paluch]

Hello Donald,

Agreed, that's a valid point. My idea behind what I wrote is essentially mimicking exactly what Cisco did when it changed the default auto-summary setting to disabled in recent IOS versions: to ascertain that the auto-summary is deactivated before even adding the network commands, to avoid later "traumas" to the network should a need arise to deactivate it after some time.

Best regards,

Peter