Transparent Bridging and MAC Address Filtering
By Stephen Bucaro
A network bridge is a device that connects nodes on multiple network segments.
In Ethernet bridging is sometimes called transparent bridging because bridges
presence and operation are transparent to network hosts. A bridge works at the
data link layer (Layer 2) of the OSI model.
To determine if it needs to send a data frame to a different segment, it reads the frame's
destination MAC address. If it determines that the destination node is on another segment
on the network, it forwards (retransmits) the frame to that segment. If the destination
address belongs to the same segment as the source address, the bridge filters (discards) the frame.
A bridge uses a forwarding database (also known as a forwarding table or filtering database)
to send frames across network segments. Initially the forwarding database is empty.
As nodes transmit data through the bridge, the bridge establishes a filtering database
of MAC addresses and their locations on the network. To translate between two segments,
a bridge reads a frame's destination MAC address and decides to either forward or filter.
If an address entry is not found in the forwarding database, the frame is forwarded to all
segments except the segment with the source address. By means of these broadcast frames,
the destination segment will respond and a forwarding database entry will be created.
A bridge and a switch are very similar, the terms bridge, switch, and layer 2 switch often
being used interchangeably. The only real difference is, if it has a small number of ports
it might be referred to as a bridge, if it has a a larger number of ports it would be referred
to as a switch.
A bridge and a router my physically appear the same, but bridges look at the MAC address
to decide which network segment to send a frame to, while a router looks inside each
packet to identify the source and target IP addresses and chooses the best path for the
packet to travel. So routers are much more sophisticated than a bridge or a switch.
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