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Level 10
Level 10

Core issue

There are more VLANs running the Spanning Tree Protocol (STP) than the switch can support.


Some switch devices must automatically change VLAN Trunking Protocol (VTP) device modes upon receipt of a VLAN configuration containing more than a set number of VLANs. The SW_VLAN-6-VTP_MODE_CHANGE: VLAN manager changing device mode from CLIENT to TRANSPARENT message indicates that such a spontaneous conversion has occurred.

Catalyst Layer 2 (L2) and Layer 3 (L3) fixed configuration switches support a different maximum number of instances of STP when using Per-VLAN Spanning-Tree (PVST)+. For example, the Catalyst 2940 supports four instances of STP in PVST+ mode, while the Catalyst 3750 supports 128 instances of STP in PVST+ mode.  If more than the maximum number of VLANs is defined in the VTP, the remaining VLANs operate with STP disabled.

If more than the maximum number of instances of STP is already in use, you can disable STP on one of the VLANs and enable it on the VLAN where you want it to run. Issue the no spanning-tree vlan vlan-id global configuration command to disable STP on a specific VLAN, and  issue the spanning-tree vlan vlan-id global configuration command to enable STP on the desired VLAN.

Switches that do not run STP still forward the Bridge Protocol Data Units (BPDUs) that they receive so that the other switches on the VLAN that have a running STP instance can break loops. Therefore, STP must run on enough switches to break all the loops in the network. For example, at least one switch on each loop in the VLAN must run STP. It is not necessary to run STP on all switches in the VLAN. However, if you run STP only on a minimal set of switches, a change to the network could introduce a loop into the network and could result in a broadcast storm.

To resolve the issue, reduce the number of VLANs configured to a number supported by the switch.

For more information, refer to Spanning Tree Protocol Problems and Related Design Considerations.

Cisco Employee
Cisco Employee

"SW_VLAN-6-VTP_MODE_CHANGE: VLAN manager changing device mode from CLIENT to TRANSPARENT" syslg is seen when you have vtp server configured with number of vlans where clients have exceded supported max vlan limit.

One example would be 3750x configured with more than 255 vlans and 2960s configured as vlan client. In this case as max supported vlan limit on 2960s when it receives vtp updates for more than 255 vlans, this itself changes to transparent mode.

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