Multiprotocol over ATM Overview

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In an NBMA network, intersubnet routing involves forwarding packets hop-by-hop through intermediate routers. MPOA can increase performance and reduce latencies by identifying the edge devices, establishing a direct VCC between the ingress and egress edge devices, and forwarding Layer 3 packets directly over this shortcut VCC, bypassing the intermediate routers. An MPOA client (MPC) provides the direct VCCs between the edge devices or hosts whenever possible and forwards Layer 3 packets over these shortcut VCCs. The MPCs must be used with MPSs resident on routers.

Figure 1

MPOA Message Flow Between MPCs and MPSs

The sequence of events shown in the figure are summarized as follows:

1. MPOA resolution request sent from MPC-A to MPS-C
2. NHRP resolution request sent from MPS-C to MPS-D
3. MPOA cache-imposition request sent from MPS-D to MPC-B
4. MPOA cache-imposition reply sent from MPC-B to MPS-D
5. NHRP resolution reply sent from MPS-D to MPS-C
6. MPOA resolution reply sent from MPS-C to MPC-A
7. Shortcut VCC established

The table below lists and defines the MPOA terms used in in the figure.

• Traffic Flow
• Interaction with LANE

The following components are required for an MPOA network:

• MPOA Client (MPC)
• MPOA Server (MPS)
• Catalyst 5000 series ATM module
• LAN Emulation (LANE)
• Next Hop Resolution Protocol (NHRP)

An MPC identifies packets sent to an MPS, establishes a shortcut VCC to the egress MPC, and then routes these packets directly over the shortcut VCC. An MPC can be a router or a Catalyst 5000 series ATM module. An MPS can be a router or a Catalyst 5000 series Route Switch Module/Versatile Interface Processor 2 (RSM/VIP2) with an ATM interface.

Note	Since the RSM/VIP2 can also be used as a router, all references to router in this chapter refer to both a router and the RSM/VIP2 with an ATM interface.

MPOA provides the following benefits:

• Eliminates multiple router hops between the source and the destination points of the ATM cloud by establishing shortcuts for IP packets and other protocol packets.
• Frees the router for other tasks by reducing IP traffic.
• Provides backward compatibility as an ATM network by building upon LANE, and can be implemented using both MPOA and LANE-only devices.

To configure an MPC/MPS, perform the following tasks:

• Define a name for the MPC/MPS.
• Attach the MPC/MPS to a major interface. This task serves two purposes:
  • Assigns an ATM address to the MPC/MPS.
  • Identifies an end point for initiating and terminating MPOA virtual circuits.
• Bind the MPC/MPS to multiple LECs.

Multiple MPCs/MPSs can run on the same physical interface, each corresponding to different control ATM address. Once an MPC/MPS is attached to a single interface for its control traffic, it cannot be attached to another interface unless you break the first attachment. The MPC/MPS is attached to subinterface 0 of the interface.

In the figure in the "Interaction with LANE" section, MPC/MPS 1 is attached to interface 1; MPC/MPS 1 can only use interface 1 to set up its control virtual circuits (VCs). MPC/MPS 2 is attached to interface 3; MPC/MPS 2 can only use interface 3 to set up its control VCs.

Note	An MPC/MPS can be attached to a single hardware interface only.

More than one MPC/MPS can be attached to the same interface. MPC/MPS 3 and MPC/MPS 1 are both attached to interface 1, although they get different control addresses. Any LEC running on any subinterface of a hardware interface can be bound to any MPC/MPS. However, once a LEC is bound to a particular MPC/MPS, it cannot be bound to another MPC/MPS.

Note	Once a LEC has been bound to an MPC/MPS, you must unbind the LEC from the first MPC/MPS before binding it to another MPC/MPS. Typically, you will not need to configure more than one MPS in a router.

Ensure that the hardware interface attached to an MPC/MPS is directly reachable through the ATM network by all the LECs that are bound to it.

Note	If any of the LECs reside on a different (unreachable) ATM network from the one to which the hardware interface is connected, MPOA will not operate properly.