Allowing traffic on VLAN ID (PVID) mismatched links

When RPVST+ is running in the default configuration on a link where there is a VLAN ID mismatch, PVST blocks the link, causing traffic on the mismatched VLANs to be dropped. However, there can be instances where traffic passing between mismatched VLANs on a link is desirable. When enabled on the switch, the ignore-pvid-inconsistency command allows this behavior. That is, where the ports on both ends of a point-to-point link are untagged members of different VLANs, enabling ignore-pvid-inconsistency enables RPVST+ to process untagged RPVST+ BPDUs belonging to the peer’s untagged VLAN as if it was received on the current device’s untagged VLAN.

Syntax:


no spanning-tree ignore-pvid-inconsistency

Causes the switch to ignore per-VLAN ID inconsistencies when the ports on both ends of a point-to-point link are untagged members of different VLANs, thus allowing RPVST+ to run on the mismatched links. On a given switch, this affects all ports belonging to VLANs on which RPVST+ is enabled.

Default: Disabled

RPVST+ behavior with ignore-pvid-inconsistency enabled

Switch “A” Port on VLAN X

Switch “B” Peer port on VLAN Y

RPVST+ behavior with ignore-pvid-inconsistency enabled

Untagged on VLAN 10

Untagged on VLAN 10

Forward

Untagged on VLAN 10

Untagged on VLAN 20

Forward1,1

Untagged on VLAN X

Tagged on VLAN X

Drop

Untagged on VLAN X

Tagged on VLAN Y

Drop (traffic from both VLANs)

Tagged on VLAN X

Tagged on VLAN X

Forward11

Tagged on VLAN X

Tagged on VLAN Y

Drop (traffic from both VLANs)

1

Forwarding state applies if the link has not been blocked by RPVST+ as a redundant link.

NOTE:

The no spanning-tree ignore-pvid-inconsistency command is ineffective when there is a PVID inconsistency between a VLAN1 and any non-VLAN1 member because VLAN1 uses IEEE BPDUs to form a spanning tree topology.