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MPOA Overview

MPOA Overview

This chapter describes the Multiprotocol over ATM (MPOA) feature, which is supported in Cisco  IOS Release  11.3(4)WA4(6) and later.

MPOA enables the fast routing of internetwork-layer packets across a nonbroadcast multi-access (NBMA) network. MPOA replaces multi-hop routing with point-to-point routing using a direct virtual channel connection (VCC) between ingress and egress edge devices or hosts. An ingress edge device or host is defined as the point at which an inbound flow enters the MPOA system; an egress edge device or host is defined as the point at which an outbound flow exits the MPOA system.

The following components are required for an MPOA network:

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 document refer to both a router and the RSM/VIP2 with an ATM interface.

MPOA provides the following benefits:


The MPOA feature is supported on the following platforms running Cisco IOS software Release  11.3(4)WA4(6) or later:

How MPOA Works

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 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. (See Figure 37.)

Figure 37: MPOA Message Flow Between MPCs and MPSs

The sequence of events shown in Figure 37 is 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

Table 15 lists and defines the MPOA terms used in Figure 37.

Table 15: MPOA Terms
MPOA Term Definition

MPOA resolution request

A request from an MPC to resolve a destination protocol address to an ATM address to establish a shortcut VCC to the egress device.

NHRP resolution request

An MPOA resolution request which has been converted to an NHRP resolution request.

MPOA cache-imposition request

A request from an egress MPS to an egress MPC providing the MAC rewrite information for a destination protocol address.

MPOA cache-imposition reply

A reply from an egress MPC acknowledging an MPOA cache-imposition request.

NHRP resolution reply

An NHRP resolution reply that eventually will be converted to an MPOA resolution reply.

MPOA resolution reply

A reply from the ingress MPS resolving a protocol address to an ATM address.

Shortcut VCC

The path between MPCs over which Layer-3 packets are sent.

Traffic Flow

Figure 37 shows how MPOA messages flow from Host A to Host B. In this figure, an MPC (MPC-A) residing on a host or edge device detects a packet flow to a destination IP address (Host B) and sends an MPOA resolution request. An MPS (MPS-C) residing on a router converts the MPOA resolution request to an NHRP resolution request and passes it to the neighboring MPS/NHS (MPS-D) on the routed path. When the NHRP resolution request reaches the egress point, the MPS (MPS-D) on that router sends an MPOA cache-imposition request to MPC-B. MPC-B acknowledges the request with a cache-imposition reply and adds a tag that allows the originator of the MPOA resolution request to receive the ATM address of MPC-B. As a result, the shortcut VCC between the edge MPCs (MPC-A and MPC-B) is set up.

When traffic flows from Host A to Host B, MPC-A is the ingress MPC and MPC-B is the egress MPC. The ingress MPC contains a cache entry for Host B with the ATM address of the egress MPC. The ingress MPC switches packets destined to Host B on the shortcut VCC with the appropriate tag received in the MPOA resolution reply. Packets traversing through the shortcut VCC do not have any DLL headers. The egress MPC contains a cache entry that associates the IP address of Host  B and the ATM address of the ingress MPC to a DLL header. When the egress MPC switches an IP packet through a shortcut path to Host B, it appears to have come from the egress router.

Interaction with LANE

An MPOA functional network must have at least one MPS, one or more MPCs, and zero or more intermediate routers implementing NHRP servers. The MPSs and MPCs use LANE control frames to discover each other's presence in the LANE network.

Caution For MPOA to work properly, you must first create an ELAN identifier for each ELAN. Use the lane config database or the lane server-bus ATM LANE commands to create ELAN identifiers. These commands are described in the Catalyst  5000 Series Command Reference publication.

An MPC/MPS can serve as one or more LAN Emulation Clients (LECs). The LEC can be associated with any MPC/MPS in the router or Catalyst 5000 series switch. A LEC can be attached both an MPC and an MPS simultaneously.

Figure 38 shows the relationships between MPC/MPS and LECs.

Figure 38:
MPC-LEC and MPS-LEC Relationships

Configuring an MPC/MPS

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

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 Figure 38 , 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.

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