With BCM IPX enabled, any quiet device (that is, a device that does not transmit
IPX advertisements) that needs to receive IPX advertisements has to be configured
as a BCM static target. An example of such a device is a station running software
that discovers the IPX network topology by monitoring IPX advertisements.
If BCM IPX Server Farm detection is enabled and you wish to prevent a particular
LEC from being treated by BCM IPX as a Server Farm, configure a BCM static
target with the LEC’s ATM address and a MAC address of 00.00.00.00.00.00. This
forces BCM IPX to send frames managed by BCM as multiple unicast frames to
each downstream IPX router and server detected behind this LEC, even if the
number of routers and servers detected exceeds the
BCM IPX Server Farm
Threshold
.
BCM Support for NetBIOS
NetBIOS is considered to be a broadcast-abusive protocol and therefore an
excellent candidate for BCM. NetBIOS communication is based on names.
Transmitting stations can learn the MAC address associated with a particular
destination name by broadcasting a query or by having the frame multicasted to the
NetBIOS functional address. In the latter case, every NetBIOS device in the
network must receive the frame and determine whether the destination name on the
frame applies to itself. To make things even worse, NetBIOS devices tend to repeat
transmission of certain types of frames as much as 10 times. Historically, this was
to ensure that all devices receive the frame in cases where the network is heavily
congested.
The BCM strategy is to associate unique NetBIOS names with MAC addresses and
LE clients by learning names from NetBIOS frames sent to the BUS. After a unique
NetBIOS name is learned, subsequent NetBIOS broadcast frames destined for that
name are forwarded to a single LE client as a unicast frame. BCM also filters
certain NetBIOS frames that are broadcast repeatedly.
BCM provides support for NetBIOS Namesharing. That is, BCM NetBIOS handles
OS/2 LANServer stations with multiple LAN adapters sharing the same NetBIOS
name.
BCM Support for Source Route Bridging
Source Route Management (SRM) is an additional BCM feature that can be
configured for 802.5 ELANs. When enabled, this feature will further process frames
managed by BCM IP or BCM NetBIOS and, whenever possible, transform All
Routes Explorer (ARE) or Spanning Tree Explorer (STE) frames into Specifically
Routed Frames (SRF). Once a frame is transformed into an SRF, the frame no
longer needs to be transmitted onto each ring in the bridged network.
The Token-Ring topology behind each LE client is learned by recording the routing
information field (RIF) of frames received by the BUS. Because SRM dynamically
learns Token-Ring topology information, an aging mechanism is used to remove
information that has not been refreshed recently.
To decide whether to enable BCM or SRM (or both), you should compare the net
system-wide benefit with the inevitable reduction in the rate at which packets are
forwarded when BCM or SRM is enabled.
Overview of LAN Emulation
Chapter 20. Overview of LAN Emulation 269
