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Chapter 10 Interface
ZyWALL USG 1000 User’s Guide
204
10.5.1 Bridge Overview
A bridge creates a connection between two or more network segments at the layer-2 (MAC
address) level. In the following example, bridge X connects four network segments.
When the bridge receives a packet, the bridge records the source MAC address and the port on
which it was received in a table. It also looks up the destination MAC address in the table. If
the bridge knows on which port the destination MAC address is located, it sends the packet to
that port. If the destination MAC address is not in the table, the bridge broadcasts the packet
on every port (except the one on which it was received).
In the example above, computer A sends a packet to computer B. Bridge X records the source
address 0A:0A:0A:0A:0A:0A and port 2 in the table. It also looks up 0B:0B:0B:0B:0B:0B in
the table. There is no entry yet, so the bridge broadcasts the packet on ports 1, 3, and 4.
If computer B responds to computer A, bridge X records the source address
0B:0B:0B:0B:0B:0B and port 4 in the table. It also looks up 0A:0A:0A:0A:0A:0A in the table
and sends the packet to port 2 accordingly.
Table 54 Example: Bridge Table After Computer A Sends a Packet to Computer B
MAC ADDRESS PORT
0A:0A:0A:0A:0A:0A 2
Table 55 Example: Bridge Table After Computer B Responds to Computer A
MAC ADDRESS PORT
0A:0A:0A:0A:0A:0A 2
0B:0B:0B:0B:0B:0B 4