LAN Emulation
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Note that each ATM end-system can simultaneously be members of several
Emulated LANs. In these situations the End System would contain several
LECs, one for each Emulated LAN using a unique MAC address to join
different ELANs.
LEC software bridges data frames between Ethernet ports on the same
ELAN. In addition, bridge software will work with the LEC software to
allow a LEC to respond to LANE Address Resolution Protocol (ARP)
requests for end-stations behind the Ethernet interfaces and to detect
changes in the topology of any bridged network that the LEC is a member.
Packets destined to cross VLANs are filtered or discarded by the bridge. A
router should, therefore, be used to communicate between VLANs.
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).
