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v LE clients that include the proxy function to provide bridging support forward the
broadcast frame on to other LAN segments.
v All end stations receive and process every broadcast frame.
BCM can be enabled on individual ELANs for any of these protocols:
IP
IPX
NetBIOS
When BCM is enabled, a minimal amount of layer 2 and layer 3 information is
decoded for specific types of broadcast frames sent to the BUS. Whenever
possible, BCM transforms broadcast frames into unicast frames, and sends them
only to interested LE clients and end stations. BCM reduces both network traffic and
associated end-station overhead by filtering nuisance broadcast frames. These
functions can improve overall system performance and enable practical deployment
of larger ELANs.
BCM Support for IP
When enabled for IP, BCM scans all IP ARP requests and replies to learn the
location of IP addresses in the IP subnet that contains this ELAN. The objective is
for BCM to take each broadcast ARP request frame and forward it as a unicast
frame directly to the LE client representing the target IP station. Both network traffic
and end-station processing time are reduced when the request is forwarded directly
to the appropriate LE client on the Multicast Send VCC instead of being broadcast
to all LE clients on the Multicast Forward VCCs. When the destination station is
located behind a bridge function, the LAN that the destination station belongs to
also benefits from the reduced broadcast traffic.
BCM Support for IPX
For IPX, BCM limits the scope of advertisements and other broadcast requests. IPX
routers and servers periodically broadcast their known network and service
information. IPX clients send broadcast requests to locate a particular service or
router. Generally, these broadcasts, called Routing Information Protocol (RIP) and
Service Advertising Protocol (SAP) packets, need to be received only by other IPX
routers and servers.
When it is enabled for IPX, BCM dynamically identifies the set of IPX routers and
servers based on advertisement transmissions, and only forwards RIP and SAP
advertisements and other broadcast requests to other IPX routers and servers. A
broadcast frame managed by BCM IPX is sent as a series of unicast frames to the
dynamically-learned set of IPX routers and servers.
When BCM IPX Server Farm Detection is enabled, BCM IPX will detect an IPX
server farm when the number of IPX routers and servers discovered behind a given
LEC exceeds a configurable threshold, the
BCM IPX Server Farm Threshold
. When
a server farm is detected, BCM IPX broadcasts a managed frame to each LEC
representing a server farm, rather than transmitting multiple unicast frames to each
downstream IPX router and server in the server farm. BCM IPX can now
intelligently use the broadcast mechanism in areas of the network where it is
desirable to do so.
Overview of LAN Emulation
268
MRS V3.2 Software User’s Guide