Switch2's Dilemma: Which VLAN?

Switch2's Dilemma: Which VLAN?

[View full size image]

Switch1 now has an address table for each VLAN. Its VLAN1 address table says to forward the frame for Barney (0200.2222.2222) out port Fa23. However, when switch2 receives the frame, it is confused. Is this frame part of VLAN1 or VLAN2? Should switch2 look in both address tables, hoping to find 0200.2222.2222? Should switch2 assume that the frame belongs in a particular VLAN? Because switch2 supports two VLANs, it simply needs more instructions.

To solve the problem, the switches use something called VLAN trunking. To perform VLAN trunking, before sending the frame over the Ethernet cable to the other switch, switch1 adds another header to the frame. That extra header identifies the frame as part of VLAN1. Switch2 expects frames coming in over port Fa24 to have that extra header, so it can now process the frame. Figure 7-9 shows the change in logic.