Specifications

LAN Emulation
116
Lan Emulation
Services
The LES, BUS and LECS are collectively referred to as LAN Emulation
Services.
LAN Emulation Server (LES). Each LES provides Address Resolution
Request (ARP) services for one ELAN. When a client joins an ELAN, the
client broadcasts a 48-byte MAC address to the LES. The LES maps the
MAC address to a 20-byte OSI NSAP address to create an ATM address. The
new ATM address is then returned to the client. When a client requests an
ATM address for a specific MAC address, it send an ATM ARP request to the
LES.
Conversely, in an Ethernet LAN based network, ARP and RARP protocols
are used to map a 32-bit IP address to a 48-bit Ethernet address. The ARP
protocol is necessary when there is no pre-established relationship (or table)
correlating an IP and Ethernet address. When a new network client is
attached to a network, the client can issue a RARP broadcast (‘who am I?’) to
a RARP server to find out its IP address. The RARP server responds to the
client broadcast by returning an IP address, if known, that corresponds to the
client MAC address.
The LAN Emulation Server (not a part of the switch) can be summarized as a
facility that provides the registration and resolution of MAC addresses to
ATM addresses. When a client wishes to join an emulated LAN, it does so
through protocol processing with the LES. The LES has a bi-directional
point-to-point VC coming from the LEC and unidirectional point-top-
multipoint VC going to the LEC.
Initially, the LEC sets up the control VCC to the LES. After the VCC is
successfully set up, it sends a join request to the LES to participate in the
ELAN. That is, the LEC sets up a bi-directional point-to-point VCC to the
LES for sending traffic. Next, the LES sets up a unidirectional point-to-
multipoint VCC to distribute the control traffic to the LECs.
When an LEC needs to locate the ATM address for a particular MAC address
it does so by sending an ATM ARP request to the Server. The LES will either
respond directly to the client or forward the query to other clients for a
response.
Broadcast and Unknown Server (BUS). The BUS handles data sent by
an LEC to the broadcast MAC address, all multicast traffic, and initial
unknown unicast frames (those that are sent by a LEC before an ATM
address has been resolved or before a VCC has been established).
A BUS must always exist in the Emulated LAN and all LECs must join its
distribution group.
In an SVC environment, the BUS needs to participate in the LE Address
Resolution Protocol (LE_ARP) to enable a LEC to locate the BUS. The BUS
also handles ATM connections and manages its distribution group.
Once a LEC has successfully joined an ELAN, it will request the LES with
the BUS address to set up a connection to the BUS. If it loses this connection,
it has to rejoin the ELAN. If it receives a packet with a multicast or
broadcast MAC (group) address, the LEC sends the packet to the BUS which
forwards it to all the other LECs in the ELAN.