Cabletron Systems 802.1Q manual Transmitting to Switch

Example 2, VLANs Across Multiple Switches

The VLAN Tag Header is inserted because Switch 4, Port 4 is designated as an 802.1Q Trunk port. In this case, the Port Mode setting for Port 4 is 802.1Q Trunk and the VLAN Frame format for that VLAN is tagged.

Redco					Blue Industries
			2
User A	Bridge 1	1	4	3	Bridge 2
	Red VLAN				Blue VLAN
				4
Floor 4
Floor 3
Blue Industries					Redco
			2		Bridge 4	File Server
	Bridge 3
		1	2 3
	Blue VLAN				Red VLAN
Floor 2
						22631_15

Figure 4-5 Transmitting to Switch 4

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 4-6.

802.1Q VLAN User’s Guide 4-9