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This chapter describes how to use Multiprotocol over ATM (MPOA) and includes the following section:
The concept of virtual router, as shown in Figure 34, allows you to implement a conventional edge router function using MPOA servers, MPOA clients, and an ATM backbone network.
Figure 34. MPOA Virtual Router
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MPOA uses networking technologies, such as bridging, LAN Emulation, and Next Hop Resolution Protocol, to implement the virtual router concept. As shown in Figure 35, the virtual router model has:
while a conventional edge router model has:
Figure 35. Comparison of Virtual Router and Edge Router Models
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MPOA implements a virtual router with client/server protocols. MPOA clients (MPCs) issue requests to MPOA servers (MPSs). MPSs perform route calculations, while MPCs act as distributed intelligent adapters performing high-speed forwarding and the ATM network provides backplane throughput. MPSs are located with router functions and a NHRP server, while MPCs reside in MPOA hosts or MPOA edge devices, as shown in Figure 36. The functions performed by a MPC in a MPOA host are very similar to those performed by a MPC in a MPOA edge device: establishing shortcut VCCs and forwarding intersubnet traffic over these VCCs to improve system performance. All MPOA devices include a LAN Emulation Client (LEC) that provides default path interconnection.
Figure 36. MPOA Components
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MPOA relies on LAN Emulation for three important functions:
Auto-configuration allows MPOA configuration parameters to be stored and distributed from the LAN Emulation Configuration Server, or LECS. MPOA devices can obtain their configuration information from the LECS while they are being initialized, so that individual device configuration is reduced. See the chapter entitled "Configuring and Monitoring LAN Emulation Services" in the 8371 Interface Configuration and Software User's Guide for additional information about configuring LECS.
MPOA devices dynamically learn about neighbor components through the discovery protocol. MPOA devices attach special TLVs to LAN Emulation control messages and then inspect received TLVs to identify MAC addresses associated with other MPOA devices. Refer to the chapter entitled "Overview of LAN Emulation" in the 8371 Interface Configuration and Software User's Guide for additional information about LAN Emulation TLVs.
MPOA clients bridge intrasubnet traffic over ELANs. Since most MPOA edge devices include LAN switching hardware capabilities, intrasubnet traffic is handled with end-to-end switching. This use of bridging, coupled with dynamic device discovery, enables the MPC to be independent of router topology while maintaining the change management benefits provided by VLANs. For example, a station can be moved from a segment behind one MPC to a segment behind another MPC without any reconfiguration.
MPOA clients are responsible for initiating shortcut establishment. The MPC discovers the MAC addresses of the MPS routers and the corresponding ATM addresses. MPC then monitors traffic flow to these MAC addresses, and when the flow exceeds a configured threshold, MPC initiates shortcut establishment by sending a MPOA resolution request to the associated MPS.
The MPOA implementation supports shortcuts for IP and IPX traffic.