Troubleshooting

Unusual Network Activity

 

 

 

 

Link supporting VLAN_1

 

 

 

 

 

Switch “X”

 

and VLAN_2

 

Switch “Y”

 

 

 

 

 

 

 

 

 

 

 

Port X-3

 

 

Port Y- 7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN Port Assignment

VLAN Port Assignment

Port

VLAN_1 VLAN_2

Port

VLAN_1

VLAN_2

X-3

Untagged Tagged

Y-7

Untagged

Tagged

Figure C-4. 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 switches covered in this guide operate with multiple 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 MSTP 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.

Note that attempting to create redundant paths through the use of VLANs will cause problems with some switches. One symptom is that a duplicate MAC address appears in the Port Address Table of one port, and then later appears on another port. While the switches have multiple forwarding databases, and thus does not have this problem, some switches with a single forwarding database for all VLANs may produce the impression that a connected device is moving among ports because packets with the same MAC address but different VLANs are received on different ports. You can avoid this problem by creating redundant paths using port trunks or spanning tree.

C-22