3Com PathBuilder S200 Series, Owner'S Manual

Page: 52 / 162

48 Bridging
Transparent Bridging for Ethernet LANs
Transparent Bridging for Ethernet LANs
Introduction A transparent bridge, also known as a spanning tree bridge, decides where to relay
Ethernet LAN frames by using the spanning tree protocol to develop and maintain a
loop-free topology.
Using spanning tree, you can add a bridge anywhere in the Ethernet LAN without
creating loops. The network devices are not involved in this decision process, which
is transparent to them.
Learning
A transparent bridge monitors Ethernet LAN traffic, “learns” the source address of
each frame it receives, and maintains a database (also known as the Forwarder) of
source addresses and associated bridge connections. A transparent bridge uses a
timeout process to purge its database of what it considers inactive addresses.
For the PathBuilder S200 series switch to “learn” where MAC stations are located in
relation to themselves, they use a hardware accelerator, a transparent bridging
forwarder, and a transparent bridging forwarding table.
A transparent bridge learns based upon the MAC source address. This address is
placed into the transparent bridging forwarder table, along with the link that the
PathBuilder S200 series switch received the frame on, if the address is not to be
filtered.
When a transparent bridge receives a frame, it checks its database for the frame
address and performs one of the following actions:
• If the frame’s MAC destination address is not found in the TB forwarding
table, then the bridge sends the frame on all bridge connections (except for the
connection on which it arrived).
• If the bridge has learned the destination address, meaning the frame’s MAC
destination address is found in the TB forwarding table, then the frame is sent
out on the learned link, unless there is a filter set on the link.
• The Hardware Accelerator discards local traffic before it reaches the node.
The Forwarder initiates the Learning process of the transparent bridge logic for
frames received from the LAN and WAN ports.
Transparent Bridge
Forwarder
After receiving an Ethernet frame, the Forwarder applies bridging logic, routes the
frame to the appropriate Handler, and sends the frame to the outgoing link, which is a
path to the frame’s MAC destination address.
The Forwarder:
• Initiates WAN frame learning
• Initiates LAN frame learning
• Filters the frame using the filtering facility
Information sent to the Hardware Accelerator includes:
• Source address
• Destination address