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Bridging Functionality
The first LAN network bridges to be developed were Ethernet
transparent bridges. They were created by the Digital Equipment
Corporation, one of the members of the original DIX Ethernet
standards consortium. The transparent bridge standard was later
co-opted by the IEEE as part of the 802.1 standard, which describes
Layer 2 addressing and management concerns. Source route bridges
were developed by IBM. Originally, source route bridging vied with
transparent bridging to be the 802.1-bridging standard. It lost out,
but later emerged as part of the 802.5 Token Ring standard. Source
route bridging is dependent on the utilization of the Routing
Information Field (RIF), which is an optional field in the Token Ring
frame (see Chapter 4, "LAN Internetworking Technologies," for
review). Translating and encapsulation bridges, although they
represent a special type of bridge functionally, can operate as
transparent, source route, or as a hybrid (part transparent and part
source route) bridge. This functionality is largely determined by the
types of Layer 2 transmission media the switch is connecting together.
It should also be noted that hybrid-based translating bridges
represent a significant level of complexity because they interconnect
two bridging protocols that are basically opposed to each other.
Transparent Bridges
The standard transparent bridge consists of two or more LAN media
ports. IEEE 802.1 standard, hardware-based transparent bridges are
plug-and-play and require no configuration. You should recall from
Chapter 1, "Understanding Networking Concepts," that after the
bridge is connected to the network, it "learns" the location of the
end-stations connected to its own segment in respect to the
stations'adjacency to the bridge's ports. The interfaces of the bridge
operate in what is known as "promiscuous mode" this is to say, they
examine each packet transmitted across the wire. During this process,
the bridge reads the Layer 2 source (MAC) address of the packets as
they enter the bridge's interfaces. These addresses are then stored in
a list, along with the port from which the packet was received. This
table is known as the Bridge or Source Address Table (BAT or SAT)
depending on who you are talking to. The bridge " learning" process is
known as the Backward Learning Algorithm (BLA).
NOTE