How to configure EIGRP Summarization

Lessons Discussion
41

Hi Guys,

If you have the summarized route configured on the interface like so:

image

Will it have any effect what so ever unless I advertise this route using the network command, or redistribute command?

Also, if I didn’t use the ip summary-address command on the interface level, and instead used “network 172.16.0.0 0.0.1.255” would this not achieve the exact same thing?

42

Hello Joseph

If you are not advertising the particular routes using the network command, then even if you issue the summary-address on the interface, no EIGRP routes will be advertised. I labbed this up to observe the behaviour, and what I did find was that if you advertise any subset of the summary address using the network command, only the summary configured on the interface will be advertised. For example, I created a loopback with an address of 172.16.0.1/24 and issued the summary address command on the interface like so:

ip summary-address eigrp 1 172.16.0.0 255.255.254.0

And what was advertised was the summary-address of 172.16.0.0/23. So the summary address will be advertised as long as there is a network command that contains a subset range of that defined in the summary address. Otherwise, if that network command is removed, the summary-address command on the interface will essentially do nothing.

Actually, when a router advertises a network using EIGRP, it will always advertise the subnet mask that appears on the interface, and not that configured in the network command. However, you can use EIGRP’s auto-summary feature that will automatically summarize all of the local networks on its interfaces before advertising them. You can find out more about that at the following lesson:

However, keep in mind that the summary-address command shown above is more granular in its configuration, as it can be configured on a per-interface basis, while auto-summary is configured globally on the device.

I hope this has been helpful!

Laz

43

Thanks Laz,

You’re the man :+1:

1 Like
44

Thank you Rene for your amazing way to explain everything in simple but effective way.

2 Likes
45

Perhaps this should be added to the lesson, no?

46

Hello NetworkGuy

I’ll let @ReneMolenaar know of your suggestion, thanks!

Laz

47

Hi,
When is a good case to use summarization ?
What I mean is from my understanding it is easier on the processor to have a single route (summarization) than to have many but in a very small network of say 3 or 4 subnets but would it really make much difference ? especially given the power of todays routers and switches ?
At what stage does it start to become useful ? for example would 20 or more subnets or 50 or more subnets be a good use case ?

Also can I have multiple summarization statements on the same interface ? or how do I summarize multiple subnets out one interface to a neighbour ?
Lets say for example I have a router with many interfaces and I had many subnets within the 10.x.x.x , 172.16.x.x and 192.168.x.x networks and I wanted to summarize all these out one interface to my neighbour ?

Thanks.

48

Hello Sean

Summarization is indeed useful for reducing the size of routing tables, thus making the network easier to manage and reducing the load on the router’s CPU. However, the point at which it becomes useful can vary greatly depending on the specific network architecture, the capabilities of the routers being used, and the overall network load.

In a small network with only a few subnets, the benefits of summarization might not be noticeable. But as the network grows, the benefits become more apparent. There’s no hard and fast rule about how many subnets make summarization worthwhile, but in general, if you’re dealing with dozens of subnets, it’s worth considering.

Remember, system resources of a router are shared with other features, so if you’re already employing other resource-intensive features, summarization will be beneficial regardless of the number of subnets.

As for your second question, yes, you can have multiple summarization statements on the same interface. I tried applying multiple such statements on an interface and was successful. Take a look at the following configuration applied to a particular interface of a router:

R1#show run interface gigabitEthernet 0/1 
Building configuration...

Current configuration : 216 bytes
!
interface GigabitEthernet0/1
 ip address 192.168.12.1 255.255.255.0
 ip summary-address eigrp 1 10.0.0.0 255.0.0.0
 ip summary-address eigrp 1 192.168.0.0 255.255.0.0
 duplex auto
 speed auto
 media-type rj45
end

R1#

I hope this has been helpful!

Laz

1 Like