HP 200 Series Services And Applications page 56

Bridging Service
Transparent Bridging
Transparent Bridging
Transparent bridges provide network interconnection and/or extension
services to LANs that employ identical protocols at the data link and
physical layers. Transparent bridges place no burden on end nodes; they
take no part in the route discovery or selection process. From the point of
view of an end node, it appears that all nodes are resident on a single
extended network with each node identified by a unique MAC-level address.
Essentially, a transparent bridge provides a relatively uncomplicated relay
function.
The transparent bridge provides three primary services: it learns the
addresses of end nodes on connected networks; it relays frames on the basis
of its acquired knowledge of end-node addresses; and (if the spanning tree
algorithm is enabled), it ensures a loop-free topology throughout the
extended network.
The transparent bridge identifies nodes on a network using the station
address (also known as the MAC address, Ethernet address, and physical
address). The end-node's station addresses is found in the source address of
each frame received by the bridging service. As the bridge receives frames, it
builds and updates a database that lists each source address. This bridging
database is known as a forwarding and filtering table or an address table; it
is comparable to the "routing table" used for the other services that actually
route at the network layer. In the address table, each source address is
accompanied by the circuit group on which the address was observed and
by a timer value that indicates the age of the observation.
The transparent bridge relays frames on the basis of address table entries.
When the bridge receives a frame, it compares the frame's destination
address with addresses found in the address table. If the bridge fails to find a
match, it relays the frame on all circuit groups (except the circuit group on
which the frame was received). This action of relaying a frame on multiple
circuit groups is called flooding.
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