IP Summary-address eigrp

Answered Question
Sep 24th, 2007
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Hi Experts,

Anyone can explain what is "ip summary-address eigrp" used for on an interfaces?? What is the scenario likely to be?


Thanks!


Correct Answer by Jon Marshall about 9 years 7 months ago

Hi Cindy


It is used to advertise a summary route down the link. An example might help.


Lets say you have an access-layer switch on a floor connecting to a distribution switch. Lets also say that the link between the access-layer switch and the distribution switch is a L3 link so the distribution and access-layer switch are EIGRP neighbours.


In the access-layer you have the following vlans


192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

192.168.4.0/24


Rather than adverise out each individual network you can just advertise a summary route 192.168.1.0 225.255.252.0 using the "ip summary-address eigrp" command. You would configure this under the interface configuration on the access-layer switch that connects to the distribution switch.


Main advantages


1) Smaller routing tables used less resources

2) Summary routes "hide" individual link flaps ie. if one of your vlan interfaces goes down this does not have to be propogated throughout your EIGRP domain.


HTH


Jon

HTH


Jon

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Correct Answer
Jon Marshall Mon, 09/24/2007 - 12:20
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Hi Cindy


It is used to advertise a summary route down the link. An example might help.


Lets say you have an access-layer switch on a floor connecting to a distribution switch. Lets also say that the link between the access-layer switch and the distribution switch is a L3 link so the distribution and access-layer switch are EIGRP neighbours.


In the access-layer you have the following vlans


192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

192.168.4.0/24


Rather than adverise out each individual network you can just advertise a summary route 192.168.1.0 225.255.252.0 using the "ip summary-address eigrp" command. You would configure this under the interface configuration on the access-layer switch that connects to the distribution switch.


Main advantages


1) Smaller routing tables used less resources

2) Summary routes "hide" individual link flaps ie. if one of your vlan interfaces goes down this does not have to be propogated throughout your EIGRP domain.


HTH


Jon

HTH


Jon

cindylee27 Mon, 09/24/2007 - 12:29
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cool, Jon..got you there on this command..:)


But another question to follow not related to this eigrp thinggy, the example you given saying that the link between access-layer switch and dist switch can be in L3 link, how can that be created in terms of configuration?


Thanks again..

Jon Marshall Mon, 09/24/2007 - 12:40
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Cindy


Glad to have helped.


You would have a /30 subnet eg 192.168.5.0 255.255.255.252


You can use use an individual link or a L3 etherchannel link between the 2 switches.


On both switches you make the port a routed port ie.


on the distribution switch


interface gi0/1

no switchport

ip address 192.168.5.1 255.255.255.252

no shut


on the access-layer switch


interface gi0/1

no switchport

ip address 192.168.5.2 255.255.255.252

no shut


These switches are now connected via L3 ie no spanning tree. If you run EIGRP on bith switches they will exchange routes and form adjacencies.


Main Advantages


1) No spanning tree.

2) if you have dual distribution switches you can connect the access-layer to both switches and have 2 equal cost paths from the access-layer.

3) Very quick failover if you loose one of the links - ie. a packet at most is lost.


Main disadvantage


You cannot span a vlan across access-layer switches.


HTH


Jon



cindylee27 Mon, 09/24/2007 - 12:43
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Thanks Jon..:) post rated..


Got all my queries answered..Am glad that you have helped me too..:)

Kevin Dorrell Mon, 09/24/2007 - 13:39
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Sorry Jon, just a small correction for the sake of the record:


If you are summarising


 192.168.1.0/24

 192.168.2.0/24

 192.168.3.0/24

 192.168.4.0/24


the best you can do is ip summary-address eigrp 192.168.0.0 255.255.248.0


I think you meant to say if you are summarising:


 192.168.0.0/24

 192.168.1.0/24

 192.168.2.0/24

 192.168.3.0/24


then you can summarise as ip summary-address eigrp 192.168.0.0 255.255.252.0


Kevin Dorrell

Luxembourg


Jon Marshall Mon, 09/24/2007 - 14:14
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Oops, my mistake. Thanks for catching that one Kevin.


Jon

vishwancc Tue, 09/25/2007 - 00:01
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Hi,


Like to add to all,

Eigrp will only summarize route to network to which it is connected,what i am saying here is


suppose there are 3 network 10.0.0.0,172.16.1.0,172.16.2.0

and for routers connected to 172.16.1.0 and 172.16.2.0 network there is no route summarization command on the router ,due to this both the route will be learned by the router in network 10.x.x.x and the end router which is connnected to ISP or other network will not summarize network 172.16.x.x

but with send route to subnet 172.16.1.0 and 172.16.2.0 since these network are not directly connected to this router and to cover this you use ip summary-address eigrp on the end router in the 10.x.x.x network

ex.

ip summary-address eigrp 10 172.16.0.0 255.255.0.0

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