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Example 2, VLANs Across Multiple Switches

12-30 VLAN Operation and Network Applications

3. When Switch 2 receives the tagged frame on its Port 2, it checks the frame’s VLAN Tag Header

and determines that the frame is classified as belonging to the Red VLAN, and that the frame is

a broadcast frame. Switch 2 forwards the frame to all ports in the Red VLAN Forwarding List

excluding Port 2, which received the frame. In this example, the only eligible port is Port 3,

which connects to Bridge 4. Switch 2 checks its Forwarding List, which specifies that the VLAN

frame type for that port is untagged. Switch 2 then updates its Source Address Table in FID 3

for MAC address “Y” if necessary. The untagged frame is then transmitted out Port 3 to

Bridge 4. Bridge 4 forwards the frame out all its ports because it is a broadcast frame, and the

server receives it as shown in Figure 12-17.

Figure 12-17 Transmitting to Bridge 4

4. The File Server responds with a unicast frame to User A. All switches between the File Server

and User A have an entry in their respective Source Address Tables identifying which port to

use for forwarding the frame to User A, MAC address “Y” in FID 3. All switches update their

Source Address Tables for the File Server’s MAC address “Z” as the frame is forwarded through

the switch fabric to User A. The 802.1D switches update their Source Address Tables based on

the source MAC address and receive port and the 802.1Q switches update their databases based

on the source MAC address, VLAN, and receive port.

5. The frame from the File Server is received on Switch 2, and forwarded to Switch 1 as a tagged

frame classified as belonging to the Red VLAN. Switch 1 removes the tag and forwards the

frame to Bridge 1, which in turn forwards the frame out of the port attached to User A. All

subsequent frames between User A and the File Server are forwarded through the switch fabric

in the same manner.

Floor 2

30691_72

Floor 3

Bridge 3

Bridge 4

Redco

Blue Industries

Red VLAN

Blue VLAN

File Server