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Unusual Network Activity
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”.
Switch “Y”
Switch “X”
Link supporting VLAN_1
and VLAN_2
VLAN Port Assignment
Port VLAN_1 VLAN_2
X-3 Untagged agged
Port X-3
Port Y- 7
VLAN Port Assignment
Port VLAN_1 VLAN_2
Y-7 Untagged agged
T
T
Figure C-3. 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 Series 5300XL switch
operates 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 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.
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 Series 5300XL switch has 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 con-
nected 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.
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