BUS processing requirements are increased and multicast frames must be
transmitted twice (once on each Multicast Forward VCC). In general, IBUS
operation is recommended; however, this option should be disabled in
configurations that have source route bridges that join the ELAN as non-proxies.
Establishing Data Direct VCCs
Data Direct VCCs connect two LE clients, and are used to exchange unicast frames
without involving the BUS. The LE client uses the address resolution procedures to
determine the ATMaddress associated with the required LAN destination. If the LE
client already has a Data Direct VCC to the ATMaddress (perhaps for another LAN
destination represented by the target LE client), unicast data frames are
subsequently transmitted on the existing VCC; otherwise, the LE client invokes the
signaling protocol to establish a new VCC.
The LE client maintains an LE_ARP cache containing LAN destination-to-ATM
address mappings. Entries in this cache are aged and must be periodically
refreshed. The entries are refreshed when a data frame is received from the LAN
destination. The LE client also attempts to refresh entries in the absence of data
traffic.
Utilization of Data Direct VCCs is also monitored and the VCCs are released if
there is no traffic for the VCC time-out period, which is configurable.Additionally,
Data Direct VCCs are released in a least-recently used manner when establishment
of a new Data Direct VCC fails due to insufficient resource availability.
Overview of Extensions for LAN Emulation
IBM has made value-add extensions to ATMForum LAN Emulation available on the
router. These extensions offerimproved performance, reliability, security, and
manageability:
Broadcast Manager (BCM)
This function can improve overall network performance by reducing ELAN
broadcasts.
Redundancy
The redundancy mechanism improves reliability by allowing backup servers
to take over if failures occur at primary servers.
Security
Security is improved by letting the LECS control ELAN memberships.
BUS Monitor
This function enhances manageability by identifying the top users of the
BUS.
The following sections describe each of these extensions.
Broadcast Manager
Broadcast Manager (BCM) is an extension to LAN emulation that consists of IBM
enhancement of the LAN emulation BUS. Without BCM, the following events occur:
vA multicast frame sent to the BUS is forwarded to all LE clients on the ELAN.
Overview of LAN Emulation
Chapter20. Overview of LAN Emulation 267