Frame-Relay multipoint

Unanswered Question
Jun 1st, 2007


Currently I'm studying for the CCNA ICND exam and i have this question:

When i have a full-mesh topology(lets say its a Diamond shape)and instead of configuring many point-to-point sub-interfaces on each router i want to configure just one multipoint on each router.So as i understand, on every router i write something like this:

>interface serial 0

>encapsulation frame-relay

>no shut

>interface serial 0.100 multipoint

>ip address [some address and mask]

>frame-relay interface-dlci 102

>frame-relay interface-dlci 103

>frame-relay interface-dlci 104


As i understand i just configured the router to use "like 3 point-to-point" interfaces(which is what multipoint is all about).

the DLCI numbers are not the same on every router(so the next will be like 201 301 401)..

so my question is that if all this is configured right how come when i configured it on my Boson Net-Sim router simulator, after executing >show frame-relay pvc it showed that only one pvc is active,it also showed a lot of other DLCI's that i didnt even configured...

please help me this is killing me..

I have this problem too.
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Richard Burts Fri, 06/01/2007 - 10:54


The best guess that I can give for the answer to your question of why did the simulator not do as you expected with your configuration is that either you made a mistake in your configuration that you have not yet recognized or that you did something that the simulator did not expect. Unfortunately simulators are not real equipment and do not run the real operating system. They are programmed to react in certain ways when certain things are configured. When something is configured that they did not expect then their response becomes unpredictable.

If 1 PVC shows as active then I would assume that only 1 of the PVCs that you configured does match what the simulator is expecting. Other PVCs that show up that you did not expect may be provided for other exercises or something like that.

I would also comment on your statement that configuring the multipoint is "like 3 point-to-point" interfaces. The multipoint interface may connect to 3 neighbors like 3 point to points would, but in several respects it is quite different. In the multipoint all 4 interfaces will be in the same subnet, while with point to point connections each point to point would be a separate subnet. With multipoint interfaces where all devices are in a common subnet we tend to make some assumptions, such as the assumption that every device in the subnet can get directly to every other device in the subnet. But in Frame Relay networks that assumption may not be true.



milkdroogy Fri, 06/01/2007 - 11:16


Thank you for the answer.

But assuming that i would run this config on a real equipment - is this the right way to configure multipoint?(the basics ofcourse)

Richard Burts Fri, 06/01/2007 - 11:25


There are a couple of ways to configure multipoint Frame Relay. One way is basically as you did it and the other is to configure the IP address on the physical interface rather than a multipoint subinterface. Unless DLCIs as specifically assigned to subinterfaces they default to assignment on the physical interface which automatically is a multipoint interface. I am not sure that either of these approaches is any more "right" than the other.

There is an issue in the way that you configured the multipoint subinterface. There will be 3 neighbors over the Frame Relay and each neighbor will have a unique IP address within the subnet. But there is nothing that tells your router which DLCI goes to which IP address (or tells the router when it wants to get to a particular IP address which DLCI to use to get there). So you will need to either get inverse ARP to work or you will need to configure frame relay maps on the subinterface. I believe that frame relay maps are preferred and if you have configured the frame relay map there is really not a need to configure the interface-dlci command.



milkdroogy Fri, 06/01/2007 - 11:36


1.I thought that Inverse ARP is enabled by default...

2.How can i not-use the interface-dlci command? suppose i get into serial 0 interface - i need that command to assign that interface the dlci's i want.

3.So when i use multipoints like in my example i have to use "manual-mapping" instead of inverse ARP?

Richard Burts Fri, 06/01/2007 - 13:12


I thought that I remembered that Inverse ARP was enabled on physical interfaces but not on multipoint subinterfaces. Looking at documentation it does seem to be enabled on both physical and multipoint subinterfaces. It has been a long time since I configured multipoint Frame Relay and either the behavior changed or my memory played a trick on me.



Mikkel Troest Sat, 06/02/2007 - 15:23


The interface gets its DLCIs through LMI, so you don't normally need to specify them with the interface-dlci command.

An exception is if you want to be able to ping your own interface address - then you'd need to specify which dlci to use - just pick one of them. (IARP can't tell you the mapping of your own ip address to a dlci).

Another exception is if you want to apply different traffic shaping to the different DLCIs. In that case you would have to do something like:


frame-relay interface dlci 101

class classA

frame-relay interface dlci 102

class classB


- but that's beyond the scope of CCNA

:O) Mikkel

Mikkel Troest Sat, 06/02/2007 - 16:49


interface-dlci is NOT necessary to ping your own ip. I mixed stuff up in my memory... You'd just need to manually specify a map between interface ip address and any one dlci on that interface.


:O) Mikkel

milkdroogy Sun, 06/03/2007 - 11:22


Why do i need to map it manually if i have Inverse-ARP to do it for me automatically?

Richard Burts Sun, 06/03/2007 - 12:55


Mikkel is talking about a particular aspect of multipoint Frame Relay interfaces. And he is correct that if you want to be able to ping your own interface, then you must manually configure a Frame Relay mapping of your own address to one of the DLCIs on the interface.

Inverse ARP will map the remote IP addresses to the correct DLCI. But it will not map your own address. If you want to ping your own address then your address must be mapped to a DLCI. (Interestingly it does not matter which DLCI you choose - any one of them will work.)



milkdroogy Sun, 06/03/2007 - 13:05


What do you mean by pinging my own address? and why will anyone want to do that?


Richard Burts Sun, 06/03/2007 - 13:50


Not your loopback address but your frame relay interface address. In your original example:

>interface serial 0.100 multipoint

>ip address [some address and mask]

it would be some address. As to why you would want to ping it, I can only say that people do sometimes ping their own interface address for one reason or another. And people sometimes get concerned when they have multipoint Frame Relay and realize that they can ping their neighbor just fine but can not ping their own interface. But this is the default behavior and does not indicate a problem.

If you do not care about the ability to ping your own frame relay interface address then do not worry about creating a map for it. But if you do want the ability to ping your own frame relay interface address then you must manually create a map entry for it.



devang_etcom Fri, 06/01/2007 - 11:16

I think in boson simulator you have the predefine numbers for the PVC and you have to use that number during the configuration...

devang_etcom Fri, 06/01/2007 - 11:20

try "show frame-relay map" to verify your mapping of DLCI to Remote IP...


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