3-3Configuring an Emulated LAN3.2.1 LAN Emulation Client (LEC)The LEC is the component in an end system that performs data forwarding,address resolution, and other control functions when communicating withother components within the ELAN. It also provides a MAC level emulatedEthernet interface and appears to higher level software as though a physicalEthernet interface is present. Each LEC must register with both the LES andBUS associated with the ELAN it wishes to join before it may participate inthe ELAN. 4.0 supports a maximum of 16 LECs per adapter card.3.2.2 LAN Emulation Configuration Server (LECS)The LECS is responsible for the initial configuration of LECs. It providesinformation about available ELANs that a LEC may join, together with theaddress of the LES associated with each ELAN. With 4.0, the user may alsouse the LECS to associate multiple LESs with a given ELAN. This featureallows LECs to “failover” to a hierarchy of redundant services if the primaryLES for an ELAN goes down.3.2.3 LAN Emulation Server (LES)The LES implements the control coordination function for the ELAN. The LESprovides the service of registering and resolving MAC addresses to ATMaddresses. A LEC registers its own address with the LES. A LEC also queriesthe LES when the client wishes to resolve a MAC address to an ATM address.The LES either responds directly to the client or forwards the query to otherclients so they may respond. There is only one instance of an active LES perELAN.3.2.4 Broadcast and Unknown Server (BUS)Unlike traditional shared-media LAN architectures such as Ethernet, ATM isconnection based. Therefore, it has no built-in mechanism for handling con-nectionless traffic such as broadcasts, multicasts, and unknown unicasts. Inan emulated LAN, the BUS is responsible for servicing these traffic types byaccepting broadcast, multicast, and unknown unicast packets from the LECsto the broadcast MAC address (FFFFFFFFFFFF) via dedicated point-to-pointconnections, and forwarding the packets to all of the members of the ELANusing a single point-to-multipoint connection. Each LEC is associated withonly one active BUS. 4.0 also supports the use of a colocated BUS (alsoreferred to as an intelligent BUS or a LES/BUS pair) that allows the BUS touse the LES’s registration table to direct unknown unicast traffic.
