EIGRP question

Since 12.0 code, you can add a mask in the network command of eigrp

router eigrp 5
network 172.30.3.0 0.0.0.7
network 10.10.1.0 0.0.0.255
no auto-summary

Why then would you ever need to use ip summary address?

int Ethernet0/1
ip summary-address eigrp 5 10.10.1.0 0.0.0.255

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Comments

networker050184

The network command does not tell the protocol what network to advertise. It only tells the protocol what interfaces to include. The IP and mask on that interface is what is used to form updates.

For example if you have an interface with 10.0.0.1/30 and use a network command of 10.0.0.0/24 EIGRP will run on that interface (and any other interface that has an IP that falls withing that range), but only the /30 will be advertised.

neotesla

Excuse me for barging into the class above mine

... (Just happen to be studying EIGRP tonight). My understanding (and excuse me if I'm wrong) is that in the above example (first four lines of commands) there would be no summarization, and network with summary routes is said to be "more efficient".

(Btw in the ip-summary command, there should be a subnet mask, not a wild card.)

Danielh22185

Both previous comments have great information. Keep in mind summarization is actually a preferred where it can be used. It is preferred in areas it can be used in because it has the ability to lower CPU utilization on routers. EIGRP sends entire table updates which can be detrimental for router exchanges thus why a summarization can mitigate a lot of this information being sent.

It is however not a great idea to summarize routes in areas where you may have a dis-contiguous network (because the router will not have a defined path to a network) or like in areas of Frame Relay where you may have connections to different subnets across sub-interfaces on a common physical interface.

reloaded

Danielh22185 wrote: »

Both previous comments have great information. Keep in mind summarization is actually a preferred where it can be used. It is preferred in areas it can be used in because it has the ability to lower CPU utilization on routers. EIGRP sends entire table updates which can be detrimental for router exchanges thus why a summarization can mitigate a lot of this information being sent.

It is however not a great idea to summarize routes in areas where you may have a dis-contiguous network (because the router will not have a defined path to a network) or like in areas of Frame Relay where you may have connections to different subnets across sub-interfaces on a common physical interface.

Agreed. Don't forget the Admin Distance of 5 for EIGRP summaries. Could make a difference if you've got a large network.

MrBrian

Danielh22185 wrote: »

It is preferred in areas it can be used in because it has the ability to lower CPU utilization on routers. EIGRP sends entire table updates which can be detrimental for router exchanges thus why a summarization can mitigate a lot of this information being sent.

Keep in mind that EIGRP only sends full topology updates when the neighborship comes up, after that it will only send partial updates for changes/additions. Although RIP does send full updates every interval.. EIGRP will just send hello's on a stable network.

I think the biggest benefit of summarization is to keep the routing tables small. As for CPU utilization that's an interesting point.. I guess it would reduce CPU because EIGRP wouldn't need to send constant updates if an aggregated route is flapping, as the summary would be intact.

Can someone else add to this to make sure my thinking is correct? I.e. the benefits of summarization on the CPU?

networker050184

Danielh22185 wrote: »

Both previous comments have great information. Keep in mind summarization is actually a preferred where it can be used. It is preferred in areas it can be used in because it has the ability to lower CPU utilization on routers. EIGRP sends entire table updates which can be detrimental for router exchanges thus why a summarization can mitigate a lot of this information being sent.

This really isn't much of a concern on modern routers. They can handle most anything EIGRP can throw at it assuming you don't have a full table in your IGP.

The real benefit is query scoping.

Danielh22185

Originally Posted by Danielh22185 It is preferred in areas it can be used in because it has the ability to lower CPU utilization on routers. EIGRP sends entire table updates which can be detrimental for router exchanges thus why a summarization can mitigate a lot of this information being sent.

Keep in mind that EIGRP only sends full topology updates when the neighborship comes up, after that it will only send partial updates for changes/additions. Although RIP does send full updates every interval.. EIGRP will just send hello's on a stable network.

I think the biggest benefit of summarization is to keep the routing tables small. As for CPU utilization that's an interesting point.. I guess it would reduce CPU because EIGRP wouldn't need to send constant updates if an aggregated route is flapping, as the summary would be intact.

Can someone else add to this to make sure my thinking is correct? I.e. the benefits of summarization on the CPU?

I was more or less speaking in terms of initial EIGRP computation. If there are good summary routes in places where they can be used effectively the receiving router of the routing information will not be as bogged down compiling the data to establish its own routing / topology tables. Yes, after that EIGRP will only send partial updates.