Enterasys Networks 802.1Q - Manual (Page 67)

Example 2, VLANs Across Multiple Switches
Examples 4-7
2. Switch 4 receives the frame from Bridge 1 and immediately classifies it as belonging to the Red
VLAN. After the frame is classified, Switch 4 checks the Destination Address and, upon
discovering that it is a Broadcast Destination Address, forwards the frame out all ports in the
Red VLAN Forwarding List excluding Port 1, which received the frame. In this example, it is
only Port 4.
Switch 4 updates its Source Address Table in FID 2 if it didn’t already have a dynamic entry for
MAC address “Y” in FID 2. Because Switch 4 received the frame on Port 1, it does not forward
the frame out that port, but does forward the frame to Port 4.
The frame is transmitted to Switch 2 with a VLAN Tag Header inserted in the frame. The VLAN
Tag Header indicates that the frame is classified as belonging to the Red VLAN.
Figure 4-5
shows the path taken to this point to reach Switch 2.
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.
Figure 4-5 Transmitting to Switch 4
Floor 2
22631_15
Floor 3
Floor 4
Bridge 1 Bridge 2
Bridge 3 Bridge 4
Redco
Redco
Blue Industries
Blue Industries
Red VLAN Red VLAN
Red VLAN Red VLAN
Blue VLAN Blue VLAN
Blue VLAN Blue VLAN
User A
File Server
4
2
1
2
3
4
1
2
3