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Good morning at all, I m new in the forum and I will thank everybody can answer me at a question on route summarization.

I don’t understand the difference when to apply the ip summary-address on interface or when to apply at protocol level. If I apply it at interface, after disable auto-summary, (like the example of

Todd Lemmle book at section 7.10)

Core#config t

Core(config)#router eigrp 10



Core(config-router)#no auto-summary

Core(config-router)#interface ethernet 0

Core(config-if)#ip summary-address eigrp 10

(In the book it’s wrote this: The summary route we will advertise to the backbone network is placed on the interface connected to the backbone, not under the routing process. This summary route tells EIGRP to find all networks in the network with a block size of 32 and advertise them as one route out interface E0. This means, basically, that any packet with a destination IP address of through will be forwarded via this summary route).

If I try on Cisco Packet Tracer or in the GNS3 this examples and if I have on the Core router other 2 network card with other 2 router, in all router attached at the Core I find all the route summarized of all router. I assumed that I could find only in the Router linked at ETH0.

Another question: If I add another router in cascade on link of the network and another in cascade with the network

(R0 eth0<---R_CORE--> -->R1-->>R2--->, too this 2 network (.96 --.128) are advertised in all router attached at core, not only at router R0 linked at eth0’s CORE.

Can somebody explain or clarify me why that route are advertised in all router and not only in the router R0 and and why are advertised also .96 and .128? Seem like summary address at protocol level.

Thank you in advance.

Hall of Fame Cisco Employee

Hi Camillo,

As a rule: in distance-vector protocols such as RIP and EIGRP, the summarization is always configured on an interface. The "auto-summary" command in RIP and EIGRP merely activates or deactivates an automatic summarizing process that is nevertheless performed on a per-interface basis. Also, with distance-vector routing protocols, summarization can be configured on any router.

In link-state protocols such as OSPF and IS-IS, the summarization is always configured only on routers that interconnect different areas, and it is configured in the protocol configuration, not on an interface. In general, summarization in link-state protocols is less flexible.

My two cents...

Best regards,



I Peter, Thank for your answer,

Then, if I apply in an interface es: ETH0 the ip summary-address summarization it’s normal to find the same route summarized, in another router linked at another interface of CORE

But why I find also the two network (>R2---> that are excluded by the subnet mask in the command: ip summary-address eigrp 10 ?

Thank you in advance.



because these are other prefixes with a different prefix-length that are advertised by other routers.



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