ATM Configuration and Troubleshooting Guide

Configuring LAN Emulation Interfaces
Summary of LAN Emulation
Chapter 4 47
An Overview of Routing In MPOA, routing is distributed among the
edge devices and the MPOA hosts in the MPOA system. MPCs examine
the destination address of each outbound packet. If the packet is to be
routed, the MPC forwards the packet to a router via the MPC’s internal
LEC interface and then starts monitoring the flow of packets. By doing
the monitoring, the MPC determines when a shortcut route would be
better for a specific data flow to use. A shortcut route is an established
path—shorter than the default route—for a specific flow of data.
The MPCs resolve the destination network layer address to the correct
ATM address, with the help of the MPS on the router. The ATM address
can be either the address of the destination MPOA host or the address of
the appropriate edge device the packets should be sent to. Note that, if
the local MPS does not know the destination’s correct ATM address, it
can use NHRP to ask the other MPSs or routers if they know the correct
address.
A Closer Look at Routing Each time an Ingress MPC receives a
packet to forward, it looks to see if the packet’s destination MAC address
corresponds to the MAC address of an MPS. If it does not, the MPC
forwards the packet via the MPC’s internal LEC interface, along the
default route—from the edge device or MPOA host where the MPC is,
through one or more intermediate routers, and then to the destination
edge device or host.
If the packet’s destination MAC address does correspond to the MAC
address of an MPS, the MPC then checks its Ingress Cache to see if it has
previously forwarded packets to that same internetwork (for example,
IP) address. If it has not, the MPC creates an Ingress Cache Entry for
that internetwork address. An Ingress Cache Entry shows the
shortcut VCC to be used for a specific data flow and how the packets are
to be encapsulated. The Ingress Cache is a collection of Ingress Cache
Entries in an MPC. The MPC then adds the packet to the threshold for
that address. The threshold is the maximum number of packets that
can be forwarded—over the default route—to a specific address during a
fixed amount of time. When an address’ threshold is exceeded, the MPC
begins the process of setting up a shortcut. The MPC sends an MPOA
resolution request to the Ingress MPS, to get the destination’s ATM
address that corresponds to the packet’s destination internetwork
address. When the MPC gets the ATM address, it establishes a shortcut
route to the destination. The shortcut is actually an ATM SVC that is