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2-6 C
HAPTER
2: N
ETWORK
L
AYER
C
ONCEPTS
Mapping Ethernet and ATM Addresses
Each device connected to an Ethernet port has one or
more MAC addresses.
Each ATM device has a number of LAN Emulation
clients, and each
LAN Emulation Client (LEC)
has an
ATM address. An example of this is shown in Figure 2-3.
Figure 2-3
LAN Emulation Clients and Ethernet Hosts
These clients represent (act as a proxy) for devices
connected to the Ethernet ports.
Whenever an Ethernet device wants to communicate
with another device over the ATM network, the LEC
must first discover the ATM address of the LEC that is
acting as a proxy for the destination MAC address.
The LEC must do this for each unicast Ethernet frame
sent. The process is known as
Address Resolution.
Address Resolution
The process by which a LEC associates a LAN
destination address with the ATM address of another
LEC (or the BUS) is known as
Address Resolution
.
Each LEC keeps a LAN Emulation
ARP Table
(which
should not to be confused with the IP ARP Table)
.
The
ARP Table lists the remote destination MAC addresses
and the ATM address of the LEC though which each
destination MAC address can be reached.
Prior to sending a frame with a known destination,
the LEC checks the ARP Table to see if the destination
MAC address of the frame is listed in the ARP Table.
The action the LEC then takes depends on whether
the MAC address is listed in the ARP Table:
■
If the destination MAC address is listed in the
ARP Table:
■
and there is an ATM connection to that LEC, the
frame is sent directly to that LEC.
■
and an ATM connection has not already been
set up, the LEC sets up an ATM connection.
■
If the destination MAC address is not listed in
the ARP Table
, the LEC sends the frame to the
BUS. The BUS then sends the frame to all LECs on
the Emulated LAN.
Sending a frame to every LEC is an inefficient use
of resources, so the LEC also tries to locate the
MAC address for future use.
To discover the correct address, the LEC uses a
process called
LAN Emulation Address Resolution
Protocol (LE_ARP)
.