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Summarization Question

I am the Administrator of a private network of 42 nodes , 100 switches(approx) and 1000 users(approx). When I received the network RIP was running . After I upgrade some of the devices to all Cisco devices I changed RIP to EIGRP which runs fine.

I also change the IP address scheme so I can implement summarization . I am talking with people experts on Cisco Routing and they told me that actually for this kind of network (it is small) and because your devices(Routers) are powerful enough(planning to replace all old models to new models in 2010) Summarization will not make any different. Also they told me to avoid auto-summary under EIGRP routing process.

What's your opinion about the above?

Thanks a lot for your time.

Moses

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Summarization Question

Hello Moses,

It might be true that with today's powerful CPUs and ASICs in routers, summarization will not detectably speed up the routing performance in your network. However, that is not the only reason for deploying the summarization. There are several more benefits to it.

One of them is that your routing tables become more readable for you. While it might not be a huge difference for a router, it might be a difference for you to see just one entry in your routing table instead of, say, 16 or so. Having a simplified routing table also means that if you plan to deploy redistribution, distribute lists or whatever manipulation with your routing entries, you may have less work, your ACLs may be simpler and so on. So that's one reason for summarization - your own convenience.

Another good reason for summarization is that it effectively creates boundaries between failure domains. In a network without summarization, if a single interface is flapping continuously, the router must advertise the change in connectivity again and again, thereby creating churn in the routing tables throughout a network. However, if you deploy summarization, the information about interface flapping will propagate only up to the summarization point and no further. This is another considerable benefit of the summarization.

Spefically with EIGRP, the summarization has another very important aspect: it limits the depth into which the EIGRP Query can be propagated. As you know, the EIGRP employs diffusing computations to find a shortest route to a destination. This diffusing computation grows by sending Queries and shrinks by receiving Replies. However, a diffusing computations is not allowed to finish until a router received Replies for all its Queries - and until the diffusing computation is finished, the router must not modify its present route to that destination. Within large networks with possibly unreliable links, the Replies may get lost or delayed, thereby slowing the convergence or leading to so-called Stuck-In-Active status for a destination (often called SIA). Solving SIAs can be difficult. Summarization effectively creates a boundary for propagating EIGRP queries - a router that knows only a summary route but does not know about its components will immediately reply with "unknown destination" if it receives a query about the summary's component, thereby limiting the propagation of a query. So in EIGRP networks, especially in larger ones, the summarization is an important architectural issue that should be deployed.

In my personal opinion, you should deploy the summarization even if you have a smaller network. You can remove it any time if it does not work well for you, but I see far more benefits to using it.

About the no auto-summary command - yes, absolutely, you should turn off the automatic summarization and instead rely on manually summarized networks. The automatic summarization is basically a remnant of the classful routing protocol behavior and it does not apply very well in today's networks.

Best regards,

Peter

2 REPLIES
Cisco Employee

Re: Summarization Question

Hello Moses,

It might be true that with today's powerful CPUs and ASICs in routers, summarization will not detectably speed up the routing performance in your network. However, that is not the only reason for deploying the summarization. There are several more benefits to it.

One of them is that your routing tables become more readable for you. While it might not be a huge difference for a router, it might be a difference for you to see just one entry in your routing table instead of, say, 16 or so. Having a simplified routing table also means that if you plan to deploy redistribution, distribute lists or whatever manipulation with your routing entries, you may have less work, your ACLs may be simpler and so on. So that's one reason for summarization - your own convenience.

Another good reason for summarization is that it effectively creates boundaries between failure domains. In a network without summarization, if a single interface is flapping continuously, the router must advertise the change in connectivity again and again, thereby creating churn in the routing tables throughout a network. However, if you deploy summarization, the information about interface flapping will propagate only up to the summarization point and no further. This is another considerable benefit of the summarization.

Spefically with EIGRP, the summarization has another very important aspect: it limits the depth into which the EIGRP Query can be propagated. As you know, the EIGRP employs diffusing computations to find a shortest route to a destination. This diffusing computation grows by sending Queries and shrinks by receiving Replies. However, a diffusing computations is not allowed to finish until a router received Replies for all its Queries - and until the diffusing computation is finished, the router must not modify its present route to that destination. Within large networks with possibly unreliable links, the Replies may get lost or delayed, thereby slowing the convergence or leading to so-called Stuck-In-Active status for a destination (often called SIA). Solving SIAs can be difficult. Summarization effectively creates a boundary for propagating EIGRP queries - a router that knows only a summary route but does not know about its components will immediately reply with "unknown destination" if it receives a query about the summary's component, thereby limiting the propagation of a query. So in EIGRP networks, especially in larger ones, the summarization is an important architectural issue that should be deployed.

In my personal opinion, you should deploy the summarization even if you have a smaller network. You can remove it any time if it does not work well for you, but I see far more benefits to using it.

About the no auto-summary command - yes, absolutely, you should turn off the automatic summarization and instead rely on manually summarized networks. The automatic summarization is basically a remnant of the classful routing protocol behavior and it does not apply very well in today's networks.

Best regards,

Peter

New Member

Re: Summarization Question

Dear Peter,

thanks a lot. I think you covered me 100%. Have a nice day

Moses

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