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
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).
