ATM Configuration and Troubleshooting Guide

Configuring LAN Emulation Interfaces
Summary of LAN Emulation
Chapter 444
3. Data transfer. When the LEC receives a network-layer packet from
a higher layer protocol to send to some destination LEC, the source
LEC initially does not know the ATM address of the destination LEC
but knows only its IP address. The source LEC first sends an ARP
request to the BUS to obtain the destination LEC’s MAC address.
The BUS forwards this ARP request to all known end systems on the
ELAN. It then returns the reply containing the destination MAC
address to the source LEC. The LEC then sends an LE_ARP request
to the LES, to resolve the destination MAC address to an ATM
address.
While waiting for the LES to respond, the LEC forwards the unicast
packet to the BUS and the BUS forwards this packet to all LECs on
the ELAN. This avoids data loss and prevents latency due to
connection setup (the LE_ARP process) that might not be acceptable
to some applications.
When the LE_ARP response is received, the LEC establishes a Data
Direct connection to the destination LEC. This connection is used for
subsequent data transfers. Before the LEC begins to use this
connection, it first sends a “flush” packet via the BUS to the
destination LEC. When the destination LEC acknowledges receipt of
this packet, signifying that the BUS path is empty, the LEC then
begins to use the Data Direct connection for data transfer. This
process ensures that the network protocol’s frames arrive in the
correct order.
If no response to the LE_ARP request is received, the LEC continues
sending data via the BUS, while continuing to send LE_ARPs to the
LES, until a response is received and a Data Direct connection to the
destination LEC is established.
If the LEC already has a Data Direct connection established to a
MAC address it wishes to reach, it need not go through the LE_ARP
process again. Instead, it continues to use the current connection.
This is possible because each LEC maintains a cache of
MAC-address-to-ATM-address mappings that it receives in response
to LE_ARP requests it has sent. Entries in this cache are aged out
over time, and Data Direct connections are also cleared if they
remain inactive for a certain time.