you are using PVST+ or Rapid-PVST, if the BPDUs for STP instance vlan21 are not received from the designated port STP loopguard puts the port of the local switch in loop inconsistent state for vlan21 instead of promoting the port to forwarding state.
As soon as BPDUs for vlan21 are received again the port is moved to a normal STP state.
We had a similar trouble between a C6500 with linecard 6708 and a C4500.
The link was tengigabit on fiber.
We solved the issue with an IOS upgrade on the C6500 moving from 12.2(18)SXF to 12.2(33)SXH2 because the previuos release was not good in managing that kind of linecard (we had another issue related to the metropolis ASIC in 6708 and TAC suggested us an IOS upgrade that solved also the stp loopguard issue).
Very Interesting, i was searching for this issue and found that these could not be the reason for this issue as out of many vlan's why only one vlan is having loop problem.
Move back to the SSH and start finding the cause. Interesting i found that the root for this vlan is the edge switch, just change the spanning-tree root priority for this vlan at Core (6500) and it fixed the issue.
This document gives several answers on frequently asked questions for PFRv3 channel state behavior.
Q1: What are all the channel operational states from a BR (border role) perspective and what are the rules/conditions to be in each st...
The need was to reach an host inside a LAN through a VPN connection managed by the LAN gateway (Cisco 1921).
The LAN gateway performs NAT and there was a dedicate nat rule for the host i wanted to reach through VPN.
I couldn't connect to the hos...
We have 3 identical switches configured by someone else and would like to claim some of the Gigabit ports(G1/G2/G3/G4) for use on servers. When we try to change the wiring and configuration, we run in to connectivity issues. Attached is a des...