Specifications

Filtering and Forwarding

9-2 Ethernet Bridge Operation

When the bridge is ﬁrst powered up, its SAT is empty:

Assume Station A wants to transmit a frame to Station B. The bridge

receives the frame and checks the CRC (Cyclic Redundancy Check) of the

frame. The bridge then looks at the source address of the frame and puts

that address in the source address table of Network 1 as shown below:

The bridge then checks the destination of the frame to see if it is located on

Network 1. Since Station B has not transmitted anything yet, the bridge

has no idea where it is located so the bridge forwards the frame to

Network 2.

Station B now sends its response to Station A. The bridge receives the

frame and checks the CRC. If the CRC is good, the bridge looks at the

source address and sees that the address is B. The Station B address is

placed in the SAT as shown below:

Next, the bridge checks to see if the destination address (Station A) is

listed in the SAT. In this case, Station A is listed as being on Network 1 so

the bridge blocks the frame from Network 2.

Network 1 Network 2