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Troubleshooting
Unusual Network Activity
Troubleshooting
STP Blocks a Link in a VLAN Even Though There Are No Redundant
Links in that VLAN. In 802.1Q-compliant switches such as the Switch
4000M and Switch 2424M, STP blocks redundant physical links even if they
are in separate VLANs. A solution is to use only one, multiple-VLAN link
between the devices. Also, if ports are available, you can improve the band-
width in this situation by using a port trunk. See “STP Operation with 802.1Q
VLANs” on page 6-44.
VLAN-Related Problems
Monitor Port. When using the monitor port in a multiple VLAN environ-
ment, it can be useful to know how broadcast, multicast, and unicast traffic
is tagged. The following table describes the tagging to expect.
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”.
Within Same
Tagged VLAN
as Monitor Port
Within Same
Untagged VLAN
as Monitor Port
Outside of
Tagged Monitor
Port VLAN
Outside of
Untagged Monitor
Port VLAN
Broadcast Tagged Untagged Untagged Untagged
Multicast Tagged Untagged Untagged Untagged
Unicast Flood Tagged Untagged Untagged Untagged
Unicast Not
to Monitor
Port
Untagged Untagged Untagged Untagged
Unicast to
Monitor Port
Tagged Untagged N/A—Dropped N/A—Dropped