Frame-relay point-to-point subinterfaces vs Frame-relay point-to-multipoint

Unanswered Question
May 2nd, 2007


i want to know the advantages and disadvantages of chosing either frame-relay point-to-point subinterfaces of frame-relay point-to-multipoint subinterfaces.

I have this problem too.
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mohammedmahmoud Wed, 05/02/2007 - 05:07


Frame Relay supports two types of interfaces: point-to-point and multipoint. The one you choose determines whether you need to use the configuration commands that ensure IP address to data-link connection identifier (DLCI) mappings. After configuring the PVC itself, you must tell the router which PVC to use in order to reach a specific destination. Let's look at these options:

Point-to-point subinterface - With point-to-point subinterfaces, each pair of routers has its own subnet. If you put the PVC on a point-to-point subinterface, the router assumes that there is only one point-to-point PVC configured on the subinterface. Therefore, any IP packets with a destination IP address in the same subnet are forwarded on this VC. This is the simplest way to configure the mapping and is therefore the recommended method. Use the frame-relay interface-dlci command to assign a DLCI to a specified Frame Relay subinterface.

Multipoint networks - Multipoint networks have three or more routers in the same subnet. If you put the PVC in a point-to-multipoint subinterface or in the main interface (which is multipoint by default), you need to either configure a static mapping or enable inverse Address Resolution Protocol (ARP) for dynamic mapping.

After a specific type of subinterface is created, it cannot be changed without a reload. For example, you cannot create a multipoint subinterface Serial0.2, and change it to point-to-point. To change it, delete the existing subinterface and reload the router or create another subinterface. When a subinterface is configured, an interface descriptor block (IDB) is defined by the Cisco IOS? Software. IDBs defined for subinterfaces cannot be changed without a reload. Subinterfaces that are deleted with the no interface command are shown as deleted by issuing the show ip interface brief command.

HTH, please rate if it does help,

Mohammed Mahmoud.

akobwaycct Wed, 05/02/2007 - 06:34

all that u have said i already knew am still not answered,my question is why would one choose frame-relay point-to-point subinterfaces instead of point-to-multipoint.

royalblues Wed, 05/02/2007 - 06:48


Mohammed has given a very good explanation.

The reason to chose one would basically depend on whether you want to have a broadcast type of network or P2P.

The multipoint option does saves you some IP addresses though


Richard Burts Wed, 05/02/2007 - 06:56

I agree that Mohammed has given a good explanation. But if Moakofi wants some reasons to choose one or the other, then I think that these are some of the reasons:

- point to point subinterfaces remove any ambiguity about what is connected on that subinterface and how to reach them. On multipoint subinterfaces there are multiple peers connected and there must be some mechanism to identify how to get to each one. With point to point subinterfaces there is only 1 peer and there is no ambiguity about how to reach it.

- point to point subinterfaces are a better match in a logical sense to the physical implementation of Frame Relay because each PVC is a point to point connection to some peer device.



akobwaycct Wed, 05/02/2007 - 21:33

thanks Rick atleast now u are givin me some reasons not explain what the two are..thanks a lot man


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