Hello Juan,
The Loopguard is intended to prevent loops ocurrs in a L2 switching from indirect link failure using Redundant Links.
Consider you have two switches , Switch 1, and Switch 2. and these Switches are interconnecte with 2 redundant Links, One of these Links should be in a blocking state while the other is in forwarding state in order to for STP prevent Loops in such example, one of the Ports on the non-root bridge should always remain in blocking state.
The Blocked port in the non-root bridge relies in continously recieving BPDUs from the root bridge to remain in Blocking state, When an indirect link failure occurs, this Port will no longer recieve BPDUs from the root bridge , allowing it to eventually transit into forwarding state and a possibility of creating a layer-2 bridging Loop.
The Sppaning Tree Loopguard feature, is intended to prevent such loops from occuring by preventing Blocked ports from being transit into forwarding state when an indirect link failure occurs by creating additional checks.
So, Where We should apply Spanning-tree (Loopguard feature)? The Most Obvious answer is in Blocking ports, However, the Most appropriate Ports should be enabled for STP Loopguard is in the Root Ports and Alt Blocking Ports.
HTH
Mohamed
