ZyXEL Communications ZyWALL 300 User Manual

Chapter 10 Interface

ZyWALL USG 300 User’s Guide

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

0A:0A:0A:0A:0A:0A

2

Table 55   Example: Bridge Table After Computer B Responds to Computer A

0A:0A:0A:0A:0A:0A

2

0B:0B:0B:0B:0B:0B

4