KB.15.15

Default: Disabled

Example 101 RPVST+ behavior

Table 11 RPVST+ behavior with ignore-pvid-inconsistency enabled

RPVST+ behavior with ignore-pvid-inconsistency

enabledSwitch “B” Peer port on VLAN YSwitch “A” Port on VLAN X

Forward

Untagged on VLAN 10Untagged on VLAN 10

Forward

1, 2

Untagged on VLAN 20Untagged on VLAN 10

DropTagged on VLAN XUntagged on VLAN X

Drop (traffic from both VLANs)Tagged on VLAN YUntagged on VLAN X

Forward

Tagged on VLAN XTagged on VLAN X

Drop (traffic from both VLANs)Tagged on VLAN YTagged on VLAN X

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

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.

Configuring STP loop guard

Spanning tree is used to ensure a loop-free topology over the LAN. Occasionally a hardware or

software failure can cause STP to fail, creating STP/ forwarding loops that can cause network

failures where unidirectional links are used. The non-designated port transitions in a faulty manner

because the port is no longer receiving STP BPDUs.

STP Loop Guard causes the non-designated port to go into the STP loop inconsistent state instead

of the forwarding state. In the loop-inconsistent state, the port prevents data traffic through the link,

therefore avoiding the loop creation. When BPDUs again are received on the inconsistent port, it

resumes normal STP operation automatically.

Syntax:

[no] spanning-tree port-list loop-guard

Enables STP Loop Guard on a particular port or ports. STP Loop Guard is best

applied on blocking or forwarding ports.

The no form of the command disables STP Loop Guard.

Default: Disabled

Figure 21 Loop creation with transmission failure

140 Rapid per-VLAN spanning tree (RPVST+) operation