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C-20
Troubleshooting
Unusual Network Activity
None of the devices assigned to one or more VLANs on an 802.1Q-
compliant switch are being recognized. If multiple VLANs are being used
on ports connecting 802.1Q-compliant devices, inconsistent VLAN IDs may
have been assigned to one or more VLANs. For a given VLAN, the same VLAN
ID must be used on all connected 802.1Q-compliant devices.
Link Configured for Multiple VLANs Does Not Support Traffic for One
or More VLANs. One or more VLANs may not be properly configured as
“Tagged” or “Untagged”. A VLAN assigned to a port connecting two 802.1Q-
compliant devices must be configured the same on both ports. For example,
VLAN_1 and VLAN_2 use the same link between switch “X” and switch “Y”.
Figure C-5. Example of Correct VLAN Port Assignments on a Link
1. If VLAN_1 (VID=1) is configured as “Untagged” on port 3 on switch “X”,
then it must also be configured as “Untagged” on port 7 on switch “Y”.
Make sure that the VLAN ID (VID) is the same on both switches.
2. Similarly, if VLAN_2 (VID=2) is configured as “Tagged on the link port on
switch “A”, then it must also be configured as “Tagged” on the link port
on switch “B”. Make sure that the VLAN ID (VID) is the same on both
switches.
Duplicate MAC Addresses Across VLANs. The switch operates with mul-
tiple forwarding databases. Thus, duplicate MAC addresses occurring on
different VLANs can appear where a device having one MAC address is a
member of more than one 802.1Q VLAN, and the switch port to which the
device is linked is using VLANs (instead of STP or trunking) to establish
redundant links to another switch. If the other device sends traffic over
multiple VLANs, its MAC address will consistently appear in multiple VLANs
on the switch port to which it is linked.
Switch “Y”
Switch “X”
Link supporting VLAN_1
and VLAN_2
VLAN Port Assignment
Port VLAN_1 VLAN_2
X-3 Untagged Tagged
Port X-3
Port Y- 7
VLAN Port Assignment
Port VLAN_1 VLAN_2
Y-7 Untagged Tagged