ATM Configuration and Troubleshooting Guide

Conﬁguring LAN Emulation Interfaces

Summary of LAN Emulation

Chapter 448

established for that speciﬁc data ﬂow. Then, any future packets that are

part of that data ﬂow are sent over the shortcut route instead of the

default route.

If the packet has the destination’s MAC address and a matching Ingress

Cache Entry exists—that is, the MPC has previously forwarded packets

to that MAC address—the MPC then checks its Ingress Cache to see if a

shortcut already exists for that data ﬂow. If a shortcut does exist, the

MPC sends the packet over the shortcut. If a shortcut does not exist, the

MPC adds the packet to the address’s threshold and checks to see if the

threshold has been exceeded (as explained above).

When a packet sent over a shortcut route arrives at the Egress MPC, the

MPC checks its Egress Cache to see if a matching Egress Cache Entry

exists. An Egress Cache Entry describes how a speciﬁc ﬂow’s packets

are to be encapsulated and sent. The collection of these entries in an

MPC is called the Egress Cache. An entry is a match if it contains the

same information as the packet—usually based on two main keys (a

source/destination ATM address pair and an internetwork layer

destination address). An optional tag (part of the ATM Forum

speciﬁcation) can also be a part of the Egress Cache Entry that must

match. If the packet does have a matching Egress Cache Entry, the

packet is encapsulated according to the information in the Egress Cache

Entry and then forwarded. If the MPC does not ﬁnd a matching entry,

the packet is dropped.