Auto-summary behaviour with EIGRP

Hi All,

 


I understand the EIGRP auto-summary feature summarizes a subnet
to it’s classsfull network when crossing a classfull network boundary. However,
I am seeing different results when using the network command to advertise the subnet  vs. redistributing it via connected.

 

For example, see below. I have two routers R2 (auto-summary) and R3 (no auto-summary) connected together running EIGRP 60 between them using the 174.1.200.0/24 subnet. R2 also has a directly connected LAN segment 192.168.0.16/29. 

 

192.168.0.18/29 – R2 - 174.1.200.1 <---EIGRP 60 ----> 174.1.200.2 - R3 

 

When I advertise the 192.168.0.16/29 subnet on R2 via the 'network' command, the subnet appears in R3's routing table as 192.168.0.0/24.. which is expected. However, if I advertise the 192.168.0.16/29 subnet with the 'redistribute connected' command, the subnet appears in R3's routing table as 192.168.0.16/29 and no auto-summarization has occured.

 

I tested exactly the same scenario as above using RIPv2 instead of EIGRP, and auto-summary is in effect when using either the 'network' command, or redistributing the subnet on R2.

 

Can anyone explain this behaviour?  EIGRP appears to handle the auto-summary command slightly differently than RIPv2 when it concerns redistribution.

 

Thanks for your help.

 

Mario.

 

Comments

  • The network command only tells eigrp to scan for participating interfaces which fall into the given segment.. next it is the speficic networks on those interfaces which will get advertised. EIGRP has the distinction between external and internal routes whether RIP doesn't. When you redistribute into the EIGRP protocol it receives external routes which are to be treated differently. This external routes will have a higher administrative distance making them less preferred then the internal routes.. but if there are also internal routes in the EIGRP network which fall in the same external network and which are being auto summarized then no auto summarization of external routes will have the effect that the more specific external routes would be preferred.. so in those cases those external routes will also be auto summarized (that is if auto summarization is enabled) to make EIGRP prefer the internal routes above the external routes.

    Look here: http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml


  • That explains it. :-)

     

    Thanks again.

Sign In or Register to comment.