When RPVST+ is running in the default configuration on a link where there is a VLAN ID mismatch, PVST blocks the link, resulting in traffic on the mismatched VLANs being 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:
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.
RPVST+ behavior
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) |
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[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. |
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