The solution provided with MPOA eliminates the bottleneck and overhead created by the classical routers by setting up a VC connection shortcut between two end-stations once a traffic flow is detected
between them. In other words, the routing function and the physical path are separated from each other to take advantage of the efficiency of ATM-based transport.
MPOA allows seamless networking between ATM and non-ATM subnetworks. The authors of this book believe that MPOA will allow ATM to make significant inroads into the traditional LAN world. The key
factor in this progress is the fact that MPOA eliminates the problems associated with the classical routers such as excessive latency and bottleneck for traffic flow by separating switching from routing.
The power of MPOA comes from the fact that it not only replaces classical routers but provides significant improvements over them.
MPOA also addresses another problem associated with CLIP which is the lack of multicast capability. It also extends the LANE’s multicast capability beyond the ELAN boundaries. MPOA provides multicast
capability at Layer-3.
MPOA is a combination of LANE, NHRP, Multicast Address Resolution Server MARS and Multicast Server MCS. MPOA relies on LANE for Layer-2 bridging between ATM-based LAN segments and
traditional LAN segments based on Ethernet or Token Ring within an ELAN logical subnetwork. It relies on NHRP for Layer-3 routing between the logical subnetworks.
MPOA also provides a very efficient VLAN platform on an ATM network which may cover large geographical distances.
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Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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A. MPOA Architecture
Similar to LANE and CLIP, MPOA also uses the client-server model as shown in Figure 9-12. As we will describe later in this section, the components of MPOA architecture assume a client or server role. An Edge Device or
MPOA Host assumes the role of MPOA Client MPC. A Router assumes a role of a MPOA Server MPS.
An MPOA Host is an end-station directly attached to the ATM network and contains one or more LANE Clients LECs to communicate using LANE services within ELAN domains. An MPOA Host also contains one or more
MPOA Clients to communicate across subnet boundaries using Layer-3 networking protocols i.e., IP protocol.
An Edge Device contains all the functionalties of an MPOA Host. However, it serves as a bridging device between the ATM Network and non-ATM LAN clusters i.e., Ethernet and Token Ring LAN clusters. Hence, it extends the
functionality of MPOA to legacy LANs as part of the ELAN concept.
A Router within the MPOA context contains one or more LECs, one or more MPSs, one or more Next Hop Servers NHS, zero or more Next Hop Clients NHC and zero or more MPCs. Such a router is attached to an ATM
network and is a member of one or more ELAN domains in that ATM network.
The main activity between an MPC and MPS is the resolution of a Layer-3 address i.e., IP address into an ATM address for a destination in another subnetwork. Once the ATM address of the destination is identified, the MPC
sets a shortcut VCC to the destination and forwards the subsequent packets over the shortcut connection, thus bypassing the router path. If the destination resides in the same ELAN then the address resolution takes place at the
Layer-2 level by the LANE component of MPOA.
Both MPS and MPC entities maintain caches to reduce overhead associated with the address resolution. Once an entry is made into routingforwarding tables, this entry is kept for some time for future references. Thus, the
subsequent address resolutions are performed through the cached entries locally. Aged entries are periodically removed from the cache tables to make room for new entries.
An MPC observes its outgoing packet traffic to detect flows going to a router with MPS capability which can benefit from a shortcut VCC connection. Once the determination is made, MPC initiates an NHRP-based query to
get the ATM address of the destination to set up the shortcut VCC. For its incoming packet traffic, MPC performs appropriate DLL encapsulation before forwarding them to either a bridge port if it is an edge device or to higher
layers of the internal protocol stack if it is a host device.
An MPS is part of a router and it includes its own local NHS and routing functions to respond to MPOA queries for address resolutions from MPCS. An MPS typically converts these MPOA queries into NHRP queries if the address
B. MPOA Operations
MPOA operations involve 5 basic operations: configuration, discovery, target resolution, connection management and data transfer as summarized in Table 9-2.
The configuration operation involves each MPOA device retrieving its configuration information typically from a LANE LECS server as they become on-line.
The discovery operation allows each MPOA device MPC and MPS to recognize other on-line MPOA devices. This discovery process is a dynamic process since the configuration might be changing over time as new MPOA
devices become on-line and some on-line MPOA devices become off-line.
The target resolution operation is based on an extended version of NHRP Resolution Request protocol to identify the ATM address of the destination endpoint. The main purpose of the target resolution operation is to switch from
the default router-based data transfer path to the ATM-based shortcut connection. The target resolution is triggered by the detection of data flow to a router destination. An Ingress MPC discovers the MAC addresses of MPSs which
belong to the same ELAN through the LE_ARP responses it receives from these MPSs. The MPC stores these MAC addresses for flow detection purposes. As it forwards data packets it compares the MAC addresses in the
packets against the stored MAC addresses. When a match occurs, the MPC initiates a target resolution operation. A typical target resolution operation flow is shown in Figure 9-11 over the router path.
Data transfer in an MPOA environment occurs either over the default router path or over the shortcut VCC. The degree of shortcut VCCs is the measure of efficient unicast data transfer in an MPOA environment. The shortcut
VCCs are established via target resolution or caching mechanism. When a default router path is used, the MPOA edge device behaves like a Layer-2 bridge. When the shortcut path is used, the MPOA edge device behaves like a
Layer-3 forwarder.
Table 9-2MPOA operations.
Configuration • MPCs and MPSs obtain their configuration data from LANE
LECS as a default option. Other methods are also possible. Discovery
• MPCs and MPSs discovering each other through LANE LE_ARP protocol dynamically.
Target Resolution • Finding the ATM address of an destination with particular
networking protocol address i.e., IP address. Connection Management
• Creating, maintaining, and terminating control and data VCCs. Data Transfer
• Forwarding of internetworking data over the default routed path. • Forwarding of internetworking data over the short-cut path.
The connection management is responsible for establishing control and data VCCs as the MPOA components find each other through the discovery operation. These connections are used to carry control information and data flows
between the MPOA components. The control flows are very important for the proper operation of MPOA. These control and data flows are shown in Figure 9-13. The control and data flows use LLCSNAP RCF 1483
