IBM OPTIONS ATM OC-3c, User Manual

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What is LAN Emulation (LANE)? 2-7
LAN Emulation Address Resolution Protocol
(LE_ARP)
An LE_ARP request is sent to the LES to locate the
destination MAC address. The LES in turn sends the
LE_ARP request to all of the LECs in the Emulated LAN.
LECs represent (act as a proxy) for MAC address
devices connected to the Ethernet ports. When a LEC
receives an LE_ARP request it checks whether the
MAC address is on its Switch. It does this by checking
the entries in the Switch database.
If the MAC address belongs to one of the devices
connected to an Ethernet port, the LEC sends an
LE_ARP response to the LEC that sent the original
LE_ARP request.
The LEC that sent the LE_ARP request adds this
information to its ARP Table. The LEC then sets up a
direct connection through the ATM network to the
appropriate LEC, so that subsequent frames are
forwarded more efficiently.
What Happens to Unicast Frames?
The path a unicast frame takes through the ATM
network depends on whether the location of the
destination address is known to the sending LEC.
■
If the location of the destination address is
known
, the LEC sets up a direct connection to the
LEC serving the destination address.
■
If the location of the destination address is
unknown
, a unicast frame is sent to the Broadcast
and Unknown Server (BUS) ; where it is treated in
the same way as a broadcast or multicast frame.
In addition the sending LEC attempts to locate the
LEC serving the destination address. It does this
using the LE_ARP process, described in “LAN
Emulation Address Resolution Protocol (LE_ARP)”.
What Happens to Broadcast and Multicast Frames?
Each Emulated LAN (ELAN) acts as a broadcast
domain. When a broadcast or multicast frame is
passed to the LEC for transmission, the frame is sent
to the
Broadcast and Unknown Server (BUS) .
When the LEC receives a broadcast, multicast, or
unicast frame it checks to see if it originally sent the
frame, and then does the following:
■
If the LEC sent the frame, it discards the frame.
■
If the LEC did not send the frame, the LEC passes
the frame to the Ethernet device so that it can be
forwarded to the appropriate port(s).
Unlike broadcast and multicast frames, the number of
unicast frames that can be sent to the BUS every
second is limited so as not to overload the BUS and
LECs with too much traffic.