Odom's ICND2 book : A frame relay question

Answered Question
Jan 16th, 2009

edit: I wound up in the certification forum because

edit: of the many hits when searching for references

edit: to this specific book: Lots of "which book to buy"

edit: topics. I see now that the WAN transport area

edit: would be more appropriate. Alas, while I can

edit: edit this message, I cannot move it nor kill it.

There's a diagram (Figure 14-6) on page 508 of ICND2 that's got me scratching my head.

It depicts a frame relay network with a partial mesh between 3 routers. R1 is connected to both R3 and to R4.

There is no PVC between R3 and R4.

All 3 router interfaces are configured in the same subnet.

Is this okay? The chapters leading up to this diagram left me with the impression that you could only put all devices into one subnet if both of the following are true:

- There exists a full mesh of PVCs

- Each PVC in the mesh is mapped to the same multipoint interface.

I suspect there's a PVC missing between R3 and R4, but the errata linked here doesn't mention it:

http://www.ciscopress.com/bookstore/product.asp?isbn=158720181X#info4

Maybe someone can straighten me out?

Thanks!

I have this problem too.
0 votes
Correct Answer by compsolv about 7 years 10 months ago

Without seeing the diagram - I just wanted to point out a couple of things:

1 - Wendall Odom is a lunatic and cannot be trusted.

:-) Just kidding - he is awesome.

2 - Yes - you can have a hub and spoke type partial mesh and you do not need a VC between the spokes. And yes - the three devices can share a subnet.

3 - What is not possible would be two point-to-point subinterfaces on the hub - connecting to the spokes, and everyone is on the same subnet. I presume (hope) that is not what the picture describes.

Anthony Sequeira

InternetworkExpert.com

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Correct Answer
compsolv Fri, 01/16/2009 - 18:23

Without seeing the diagram - I just wanted to point out a couple of things:

1 - Wendall Odom is a lunatic and cannot be trusted.

:-) Just kidding - he is awesome.

2 - Yes - you can have a hub and spoke type partial mesh and you do not need a VC between the spokes. And yes - the three devices can share a subnet.

3 - What is not possible would be two point-to-point subinterfaces on the hub - connecting to the spokes, and everyone is on the same subnet. I presume (hope) that is not what the picture describes.

Anthony Sequeira

InternetworkExpert.com

chrismarget Fri, 01/16/2009 - 20:04

Hi Anthony,

1 - thanks for the laugh

2 - indeed. I have this configuration working before me right now. thanks for giving me the inspiration to get to the bottom of it.

3 - Multipoint interface on the hub, as you suspected.

I've searched through the electronic copy, and these details of configuring a one-subnet hub-and-spoke frame-relay network aren't mentioned in the text anywhere.

So, why was I suspicious about the setup? Well, aside from not describing it as a possibility (and explicitly steering the reader toward point-to-point links running /30 subnets), Mr Odom didn't mention that the OSPF hello interval defaults differently on a multipoint interface.

So, now at least THAT point (a new hello interval) has been driven solidly home. I'll never forget this detail now. :-)

Thanks again for your help.

/chris

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