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Frame Relay Question

I am using the Trainsignal DVD to study for the CCNA certification test. I just worked through the frame relay lab. I have three routers connected to a fourth router that is configured as a frame relay switch. The lab was quite interesting. One thing that I need some clarification on is the use of subinterfaces in frame relay.

The instuctor had us use a physical interface on the hub router, a multipoint subinterface on one of the spokes, and a point-to-point subinterface on the second spoke. Each serial interface had an ip address on the same subnet. He explained that all the different interfaces were not require for the lab, but that he wanted us to experience configuring each type. I presumed that the use of the various types of interface are proper to more advance study. None the less it would be nice to understand their context.

Could one configure three routers in a hub and spoke topology using all physical interfaces with frame relay maps? If so, when would subinterfaces be required? I understand that a multiport subinterfaces are required to connect to multiple networks. I did some reading on the web and found that subinterfaces are used to deal with a split-horizon problem. I did not understand what that meant. could someone explain that?

Thanks

Reply to
tman
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The short is answer is absolutely, yes you can use regular serial interfaces with maps. This works fine, but the 'split horizon' issue you reference will be an issue. The issue is that a router will typically not advertise a route back out the same interface it just heard it from. Sub-interfaces may share the same physical port, but they are indeed separate logical interfaces, and therefore help mitigate the issue with split horizon.

As an example, your hub router (lets call it A), has two maps to routers B and C which each own a /24. If you are not using sub- interfaces, router A will hear about the two /24s from routers B and C, but will not propagate them out to one another. In other words, router B will advertise say 192.168.1.0/24 to router A, but router A will then not send that on to router C. Similarly, router C's network will not make it around to router B. This issue could be solved with using separate VCs on separate serial interfaces, but that can be costly in hardware. Therefore, sub-interfaces are a great alternative.

Here are a few links:

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Reply to
Trendkill

Thanks for the explanation. Let's see if I can understand it. Two possibilities present them selves to my dim understanding of the issue:

  1. A subinterface on router A will accept an advertisement from router B and send it on to router C because a subinterface does not observer split-horizon.
  2. You have to configure two subinterfaces on router A with 2 DLCIs mapped to the serial ports on router B and router C so that when router A receives an advertisement on one subinterface from router B it sends it out to router C on the other subinterface thus observing split-horizon?

Thanks for the links, I will study them.

Thanks for your help/

Reply to
tman

I would be careful using the word 'observing', as I'm not sure exactly what you mean. That being said, I believe your understanding of the concept is correct. The alternative option is to leave it as a single interface, but disable split-horizon. I think all routing protocols support the ability to disable split-horizon, but you should check. However, sub interfaces is definitely the more eloquent solution.

Reply to
Trendkill

Do you have to set two subinterfaces on the hub or just one?

Thanks

Reply to
tman

Depends whether or not you are doing point to point or point to multipoint. If its point to point, you need multiple dlci's on a single sub interface. If its point to point, its one dlci per sub- interface. In short, you can do multiple sub-ints or a single sub-int with multiple dlci's. See the link below, and it shows you configuration examples:

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Reply to
Trendkill

quoted text -

Correction above, point to multipoint would require multiple dlcis on same subint. Point to points would require multiple sub-ints.

Reply to
Trendkill

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