How to route this topology using static summary routes?

[turbo_engine26]

Hello,

I'd like someone to share with me his/her thoughts regarding this static routing topology. As you see, the topology uses different subnets from major networks in a discontigous way.  One of CCIE books said, "With static routing, there should be no problem configuring static routes for discontigous topology like this". However, i am facing reachability problems. Creating static routes using the actual subnet masks of the remote networks (no summary) is not a problem and it's working fine but the challenge here is the summary routes because i want to reduce the route table size.  How can i use summary routes in this topology?

I uploaded the topology in a JPG file format as well as the route table entries file in each router.

Please advise

Thanks

[Peter Paluch]

Hello,

I do not personally believe that your network, regarding its current addressing, is a good candidate for using summarized routes. Network summaries are natural in cases where you are first assigned an address block and you subnet it according to your needs but to the outside world, you still advertise only that block. Many textbooks like to explain summarization the other way around: given a set of networks, find the most efficient summary. While a good exercise, it is by far not a best-case approach as done in real life.

In your topology, I see the following possibilities for simplifying the static routing:

• Have RTA, RTD and RTF use just a default route towards RTB, RTC and RTE, respectively.
• Have RTB and RTE point to RTA and RTF for their local networks, and for all other networks, RTB and RTE can use a default route towards RTC
• RTC has to enumerate all networks. Except 10.1.2.0/23 via RTE, I see no other possibility of effectively summarizing remote networks without creating reachability issues.

Is that your own scenario, or is the topology taken from some textbook?

Best regards,

Peter

[turbo_engine26]

This is not my own scenario, this is taken from an exercise in a CCIE book. The question in this exercise says the following:

"Configure static routes for each router of the network shown in this topology. Write the routes so that every subnet of the internet has an individual entry."

Please note that the chapter before this exercise didn't mention any default routes topic so it is not obvious to configure something that you didn't learn in the first place. Default routes is covered in later chapter.

So, i assumed that the question meant to use summary routes. By the way, the question didn't say to use summary routes but the meaning of it says so since summary routes is covered in the chapter.

Thanks for your help.

[Peter Paluch]

Hello,

By the way, the question didn't say to use summary routes but the  meaning of it says so since summary routes is covered in the chapter.

Personally, I see it differently. The assignment says:

Write the routes so that every subnet of the internet has an individual entry.

To me this seems that on all routers, you are supposed to statically configure each remote network individually - every subnet should have its individual entry, not share a common entry (which is exactly the case with summarization). And by the way, a default route is by its very nature a summary route, is it not? But yeah, it is somewhat extremal. In CCIE labs, usually, you are prohibited to use default routes unless explicitly allowed.

Best regards,

Peter

[turbo_engine26]

Hmmm, i understood the term "Individual entry" as individual major network entry for every subnet.

Ok, now exercise 2 says to rewrite the static routes created to use the minimum possbile route entries. (Hint: RTA will have only two static entries)

What is your opinion in this?

Thanks by the way for solving the question in my first thread regarding classful and classless routing.

[turbo_engine26]

Any new suggestions?!

[Jon Marshall]

I completely agree with Peter on this one. The network example you have posted is not ideal for using summary addressing, static routes for each subnet yes, but not summary addressing.

As Peter said, summary addressing in the real world is done by allocating a supernet to a site/area and then splitting it up rather than allocating subnets and then trying to summarise.

Jon