Multiprotocol over ATM Overview
This chapter describes the Multiprotocol over ATM (MPOA) feature, which is supported in Cisco IOS Release 11.3 and later releases.
MPOA enables the fast routing of internetwork-layer packets across a nonbroadcast multiaccess (NBMA) network. MPOA replaces multihop 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.
Procedures for configuring MPOA are provided in the following chapters in this publication:
"Configuring the Multiprotocol over ATM Client" chapter
"Configuring the Multiprotocol over ATM Server" chapter
"Configuring Token Ring LAN Emulation for Multiprotocol over ATM" chapter
For a complete description of the commands in this chapter, refer to the the
Cisco IOS Switching Services Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see
Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to
www.cisco.com/go/cfn. An account on Cisco.com is not required.
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 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:
MPOA resolution request
sent from MPC-A to MPS-C
NHRP resolution request
sent from MPS-C to MPS-D
request sent from MPS-D to MPC-B
MPOA cache-imposition reply
sent from MPC-B to MPS-D
NHRP resolution reply sent
from MPS-D to MPS-C
MPOA resolution reply sent
from MPS-C to MPC-A
Shortcut VCC established
The table below lists and defines the MPOA terms used in in the figure.
Table 1 MPOA Terms
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 that has been converted to an NHRP
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
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
The path between MPCs over which Layer 3 packets are sent.
The figure in the "How MPOA Works" section 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
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 command 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.
The figure below shows the relationships between MPC/MPS and LECs.
Figure 2. MPC-LEC and MPS-LEC Relationships
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.
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.
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 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.
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.
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.
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