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Broadcast and Unknown Server (BUS)
The BUS provides broadcasting support for an ELAN. The BUS distributes multicast data, sends or
distributes unicast data, and connects the other LANE components. When the destination address of an
Ethernet or Token Ring frame contains a local broadcast or a multicast address, the LEC forwards the traffic
to the BUS, which forwards it to all the other LECs in the ELAN. At least one combined LES and BUS is
required per ELAN. The BUS has the following functions:
Distributes multicast data•
Sends or distributes unicast data•
Interfaces to LEC and LES•
The LES is the component responsible for resolving MAC addresses to ATM addresses, and the BUS is the
component responsible for servicing multicast, Ethernet, and Token Ring broadcasts. The Cisco LANE
implementation calls for the LES and the BUS to be configured in the same end−station.
ATM Module Subinterfaces
In order to configure ATM components on an ATM LANE module for the Catalyst 5000 or 6000 family of
switches, you need to use subinterfaces. Depending on the level of traffic you expect on your network, you
may want to place different LES/BUS components throughout the framework of your network. To configure a
LES or BUS on the ATM module, you need to complete the following tasks:
Enter Interface Configuration mode.1.
Specify the subinterface for the ELAN.2.
Specify the type of link: Ethernet or Token Ring.3.
Enable the LES and BUS on the ELAN.4.
Repeat this process for each LES/BUS.5.
The BUS must be used to sequence and distribute broadcast data to all the LECs. However, sending a large
volume of broadcast data to all the LECs can severely impact the overall performance of the network. For this
reason, it may be necessary for the BUS to place restrictions on the LANE components to control the
maximum throughput rate for each device. The BUS’s primary function is to provide broadcast management
support for LANs. The BUS must supply the following services:
Distribute unicast and multicast data to all the LECs in the network•
Connect interfaces to the ELAN•
Distribution of unicast and multicast data includes the transmission of data to the LECs in the network.
Whenever possible, the LEC will establish a direct connection to another LEC. If this isn’t possible, then data
the BUS receives is broadcast to each LEC on the ELAN. This option can be enabled and disabled, and you
should carefully consider whether you need this option, because it can eat up costly bandwidth.
Note When interfacing to ELANs, the BUS establishes a bi−directional connection that allows forwarding of
multicast and unicast frames with unknown destinations.
LEC Queries
LECs send queries for configuration information to receive the LES address. The LECS then assigns the
correct LES address for each LEC. The LES also has the ability to establish a connection with the LECS.
A reply to a query can be as simple as providing a single LES address or it can provide more information,
such as:
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