ATM NBMA

The software supports nonbroadcast multiaccess (NBMA) networks, which interconnect more than two routers and have no broadcast capabilities.

Note: The E120 and E320 routers do not support ATM NBMA in the current release.

An ATM NBMA network can be thought of as an interface stack with a single IP interface at the top, eventually fanning out to multiple independent PVCs. See Figure 2.

Figure 2: NBMA Interface Stack

NBMA Interface Stack

Unlike standard point-to-point ATM interfaces and broadcast-oriented Ethernet interfaces, NBMA interfaces form a point-to-multipoint connection. For example, you can use NBMA to connect a router to multiple stations.

An NBMA interface consists of a single ATM 1483 subinterface that has two or more VCs. You can add circuits to an existing ATM 1483 subinterface at any time. New circuits become usable after they have valid ARP table entries. NBMA circuits support only IP directly over ATM 1483.

The software restricts NBMA interfaces so that all circuits reside on the same physical interface. An NBMA interface can use as many PVCs as are available on a physical port.

ARP Table

To maintain the Address Resolution Protocol (ARP) table, you can use either static mapping via the CLI or Inverse ARP (InARP). InARP provides a way of determining the IP address of the device at the far end of a circuit. For NBMA interfaces, InARP enables automatic creation of ARP table entries for each circuit on the interface.

You must enable InARP when you create a PVC by using the atm pvc command. After you configure InARP, a protocol mapping between an ATM PVC and a network address is learned dynamically as a result of the exchange of InARP packets.

Static Map Versus Inverse ARP

If the device at the other end of a circuit does not support InARP, static mapping is required for that circuit. One of these two methods must be used to generate an ARP table entry for each circuit of the NBMA interface.

InARP and static mapping are complementary within an NBMA subinterface, but are not compatible with regard to individual circuits. If InARP is configured on a circuit, the corresponding virtual circuit descriptor (VCD) cannot be present in a static map applied to that interface.

Aging

ARP table entries, with the exception of those declared static, are aged out based on an aging interval defined on a subinterface basis. For the purposes of aging, entries produced via a static map are treated as static ARP table entries. InARP-generated entries are also treated as static; however, the InARP state machine automatically removes entries that cannot be successfully refreshed after three successive failed InARP requests.

Removing Circuits

If a circuit is removed, it is also removed from the ARP table, but not from the static map. If the circuit is reconfigured, a new ARP table entry is generated from the existing map entry. If the circuit uses InARP, the ARP table entry is immediately removed on removal of the circuit.

If a subinterface is removed, all associated circuits and their associated ARP table entries are removed.