How to configure EIGRP Summarization

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(system) #2

Hello Rene,

I have a question regarding ip summary-address command in above lesson.
By applying ip aummary-address command in fastethrnet 2/0 interface,
how come heart router know about the summary address.
As my understanding ip summary address is per interface basis.
I should apply ip summary-address in fastethernet 1/0 interface.
Would you correct me and help me to understand better.


(Rene Molenaar) #3

Hi Toral,

I just changed the picture because the interface number didn’t match. You configure a summary outbound on an interface facing towards your EIGRP neighbor.


(system) #4

Simple but effective workout to understand EIGRP summerizing

(Rene Molenaar) #5

Thanks Django!

(system) #6

thanks for the nice lesson, keep up the good work

(Oscar C) #7

Rene, why is it /23 mask? when I see the first statement showing /24.

Spade(config)#<strong>interface fastEthernet 2/0</strong>
Spade(config-if)#<strong>ip summary-address eigrp 1</strong>

1st. statement.

Hearts#<strong>show ip route eigrp</strong> is subnetted, 2 subnets
D <strong></strong> [90/30720] via, 00:02:28, FastEthernet0/0
D <strong></strong> [90/30720] via, 00:02:28, FastEthernet0/0

Is it to cover ?


(Rene Molenaar) #8

Hi Oscar,

That’s right, covers both and


(Diana M) #9

Hello Rene,

Can you show us more examples using summarization? it’s an easy topic but I would like to see another scenarios :slight_smile:


(Rene Molenaar) #10

Hi Diana,

Sure, anything particular you would like to see?


(Tooryalai N) #11

Hey Rene,

I am alittle confused with summarization over here. So taking your example over here, when we want to summarize subnets we used /23 subnet mask that is shared by all the subnets that we want to summarize and use it on interface level. But in your next lesson “no auto summarization” when we disable the “no auto summarization” we use wild card mask under eigrp configuration, what is the difference, why didn’t we use wild card mask over here ?



(Rene Molenaar) #12

Hi Tooryalai,

This can be confusing yes. Cisco isn’t very consistent with their usage of wildcards, subnet masks and CIDR notation. Sometimes it doesn’t make much sense :slight_smile:


(Amit P) #13

Hi Rene,

I have a doubt , how does summarization prevents the SIA issue ?
or in other words how does the neighbor knows not to send the query packet to this neighbor which is summarizing addresses to me !


(Andrew P) #14

Only on the router where the summarization is configured does the Query process stop–not on the router that originates the Query. So for example, if you have three routers A, B, and C with a topology of A—B---C. B has created a summary, say Suppose Router A has a route that goes active. Router A will still send the Query message to B, but B will see that is contained within its own configured summary of This will cause Router B to stop propagation of the Query to router C. Additionally, Router B will reply immediately back to A’s Query saying “Unreachable”

(Ryan M) #15


Has anyone tested this in GNS3?
Can’t seem to get it working

interface Loopback0
 ip address
interface Loopback1
 ip address
interface FastEthernet0/0
 ip address
 ip summary-address eigrp 1 5
 duplex auto
 speed auto

router eigrp 1
 no auto-summary


interface FastEthernet0/0
 ip address
 duplex auto
 speed auto

router eigrp 1
 no auto-summary

R2#show ip route is subnetted, 1 subnets
C is directly connected, FastEthernet0/0

(Lazaros Agapides) #16

Hello Ryan

I notice that in your show IP route command, no EIRGP routes show up. This tells me that for some reason, no EIGRP neighbours have been created. You should at least see the and subnets separately show up. Check the neighbour table to see that they have successfully created an adjacancy. Before you configure any EIGRP summarization, try to get EIGRP alone to function correctly. Then you can add the summarization commands.

I hope this has been helpful.


(Hector M) #17

If the network is summarized, should not have an A.D of 5 in the neighbor routing table???

(Andrew P) #18

No, it should not. The Administrative Distance of 5 for an EIGRP summary route is local only on the router that does the summarizing. When that summary is then advertised to a neighbor, the neighbor installs the summary with the regular AD of 90.

(Hector M) #19

Hi Andrew,

Then, based on the exercise, it is in R1 where the route should appear with A.D. of 5.

R1#show ip route eigrp is variably subnetted, 3 subnets, 2 masks
D is a summary, 00:01:38, Null0

where we can check the A.D.?? Since it does not appear.

And thanks for ur reply :slight_smile:

(Andrew P) #20

To check the AD of the route, issue:
#show ip route