Loop Guard

dppagc

Loop guard prevents the port from transition to forward state if it does not receive BPDU. The assumption is that the link is working fine but the BPDU is not received.

But what happens if there is an actual problem with the link. Wont the traffic be blackholed if there was instead something wrong?

clarson

[FONT=&amp]Lo[/FONT]Loop Guard monitors non-Designated ports and prevents them from becoming Designated ports because of a unidirectional link failure, and is considered the opposite of Root Guard. The goal is similar to UDLD, but STP BPDUs are used to detect unidirectional links. This feature should be enabled across the entire switched network. When enabled, Spanning-Tree does not send BPDUs on Root or Alternate ports, when in PVST+ or Rapid-PVST+ mode. When in MST mode, BPDUs are not sent on nonboundary ports only if the interface is blocked by Loop Guard in all MST instances. On a boundary port, Loop Guard blocks the interface in all MST instances. Loop Guard only operates on interfaces that are considered P2P by Spanning-Tree.

Normally, an Alternate blocking port will receive BPDUs generated by the connected Designated port on the other end of the link (unless Bridge Assurance in configured, in which case all ports send BPDUs regardless of role or state). If the Alternate blocking port stops receiving BPDUs due to a unidirectional link, it will normally enter the forwarding state, which will cause a loop. Loop Guard prevents this by transitioning the blocking port into a Loop Inconsistent state when BPDUs are no longer received.


Loop Guard operates on a per-VLAN basis. For example, if there is an inconsistency with a single VLAN on a trunk port, the other VLANs will still process traffic normally while only the single VLAN is placed into Loop Inconsistent on the interface.



Loop Guard and Root Guard are mutually-exclusive and cannot be enabled at the same time on an individual interface. Loop Guard is intended to complement UDLD.[FONT=&amp]op Guard monitors non-Designated ports and prevents them from becoming [/FONT]

dppagc

So in short loop guard works together with UDLD to prevent unidirectional link failure