WAN Interfaces

[daniel.bowen]

Could somebody please take the time to explain to me exactly how to point to multipoint interfaces work in a WAN environment?

Is it similar to how sub-interfaces work in an ATM circuit network, with each sub interface representing a VC to specific destination?

Thanks,

[ruwhite]

Somewhat--from the hub side, it looks like a broadcast network. Broadcasts are replicated down each circuit so each remote receives a copy. From the remote side, it looks like a point-to-point. Broadcasts are received only by the hub router.

Russ

[ashok_boin]

Hi,

Point to Multi point interfaces as we take in Frame relay, each subinterface represents a separate VC. The HUB router can communicate with different destinations with a unique DLCI for each. But in Frame Relay is NBMA network, so no real

broadcasts are supported.

Regards...

--Ashok.

[t.graham]

Point to multipoint interfaces "work" by performing pseudo-broadcasts ... the broadcast packet is replicated onto each DLCI. I am not sure this is the question you are trying to ask, however.

A Point to multipoint (P2M)interface is a single subinterface which has multiple PVCs assigned to it, with each PVC going to another router. A point-to-multipoint interface would most likely be used in the "hub" of a hub and spoke configuration. The P2M interface represents a single logical network (single subnet).

Say you have a hub site with two spokes. The hub is running a P2M sub-intf and the spokes just use the physical interface. There would be two DLCIs on the P2M at the hub and one each on the spokes. All three interfaces would have to be on the same logical network (same IP subnet).

Compare this to if you have the same configuration but choose to use two P2P subinterfaces on the hub. In this case the IP subnet on the link between the Hub and Remote "A" would have to be a different network from the link from the hub to Remote "B".

The ATM example you cite is more like a P2P than a P2M subinterface.

[bhedlund]

Read the chapter on Frame-Relay in the book "Cisco Certification; Bridges, Routers and Switches for CCIE's" by Bruce Caslow. In this MUST HAVE book, Bruce Caslow explains this topic better than anyone else has before.

Regards,

Brad