ZyXEL Communications 100 Series User Manual

 Chapter 10 Interface

ZyWALL USG 100/200 Series User’s Guide

A bridge creates a connection between two or more network segments at the layer-2 (MAC 
address) level. 

Unlike the device-wide bridge mode in ZyNOS-based ZyWALLs, this ZyWALL can bridge 
traffic between some interfaces while it routes traffic for other interfaces. The bridge 
interfaces also support more functions, like interface bandwidth parameters, DHCP settings, 
and ping check. To use the whole ZyWALL as a transparent bridge, add all of the ZyWALL’s 
interfaces to a bridge interface. 

In the following example, bridge X connects four network segments.

Figure 180   Bridge Example

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 76   Example: Bridge Table After Computer A Sends a Packet to Computer B

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

2

Table 77   Example: Bridge Table After Computer B Responds to Computer A

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

2

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

4