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[a]
Untagged on VLAN 10 Untagged on VLAN 20 Forward[a][b]
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 Forward[a]
Tagged on VLAN X Tagged on VLAN Y Drop (traffic from both VLANs)

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

[b] If both sides (ports) of the link are untagged to different VLANs, but the VLAN on the switch on one end of the link is not RPVST+-enabled, untagged RPVST+ frames received on that switch port (where RPVST+ is disabled) would be forwarded to any other ports belonging to the inbound VLAN.