split horizon and summary addresses

daghighi.navid · ‎06-11-2007

Hi,

The cisco doc says that when split horizon is enabled (which is the default), then RIP will NOT send any ad about summary addresses.

What is the reason why this behavior has been established ? I really don't see what we want to avoid by not sending these ad. If I manually configure an interface sumarry address, I know where the subnet is, and I know there will be no loop. But I still don't want to disable split horizon, which would be too dangerous to do.

Thanks,

Navid

Richard Burts · ‎06-11-2007

Navid

Perhaps you can provide some specifics about what this doc is and exactly what it says so that we can respond to it in a better way. In my experience of RIP it does send summary advertisements out interfaces that have networks on them that are different from the network being summarized. And RIP does not send the summary advertisement out the interfaces that have the subnets of the network being advertised because on interfaces on that network RIP needs to see individual subnets and not the summary. Is this consistent with what you have read or is it different?

HTH

Rick

HTH

Rick

daghighi.navid · ‎06-11-2007

Hi Rick,

Here is the exact sentence :

"If split horizon is enabled, neither autosummary nor interface summary addresses (those configured with the ip summary-address rip command) are advertised. "

on this page :

http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_command_reference_chapter09186a00800ca697.html

Thanks,

Navid

Richard Burts · ‎06-11-2007

Navid

One of the basic assumptions in classful protocols like RIP is that within a particular network (for example within 10.0.0.0/8) all subnets will be contiguous and that all routers will have knowledge of what subnets exist (and which subnets do not exist). The implementation of split horizon is associated with RIP as a primary protection against the formation of loops. The 2 principles work together within the protocol. Advertising the summary out an interface within that network is in a sense a violation of split horizon which says that you do not advertise information out an interface about things that you learned on that interface. Note that this discussion primarily applies to RIPv2.

If you configure a manual summary address on an interface of some subnets within that major network and want it to be advertised out that interface, then you will need to disable split horizon.

HTH

Rick

HTH

Rick