I am analysing Cisco Switches Loop Guard Feature. It states that it prevents any Blocked Port to become Forwarding state when the port is not receiving any BPDU.
It seems this violates the STP rule. If Blocked state is not coming up when the BPDU is absent, then how STP converge and forward the traffic.
Shall anybody explain this.
Hello R.B. Kumar,
the context is that of a point to point trunk link.
Here with PVST the port has to receive BPDUs for all the Vlans for which it has lost the DP election on segment
The feature is good because:
you can face an event where only on Vlan X you are missing the BPDUs from other side
instead of moving the port in forwarding state for Vlan X it is better to place it in inconsistent state.
Actually the feature works at the Vlan level and has the ability to put in inconsistent state (not blocked) only one or some specific Vlans
Also if BPDUs are received again the port is automatically placed again in STP blocking state without config change.
And your understanding about unidirectional link detection is correct and STP loop guard can be a better way to deal with this issue then UDLD when using Rapid STP because it is too fast for UDLD timers.
Avoiding to form a loop on a specific vlan is important because it is enough to cause a network halt.
when all Vlans don't receive BPDUs or the link goes down it is clearly a different event.
We use STP loop guard also in combination with Rapid STP and with broadcast storm-control and we have found it to work well.
Hope to help