Reference Guide

Role of VLT backup link in the prevention of loops during VLTi failure
When the VLTi is down, STP may fail to detect any loops in the system. This failure creates a data loop in an L2 network.
As shown, STP is running in all three switches:
In the steady state, VLT Peer 1 is elected as the root bridge. When the VLTi is down, both the VLT nodes become primary. In
this state, VLT Peer 2 sends STP BPDU to TOR assuming that TOR sends BPDU to VLT Peer 1. Due to this, VLT Peer 2 does
not receive BPDU on the VLT port, but receives TOR BPDU from the orphan port. The STP in VLT Peer 2 assumes that there
is no loop in the system and opens up both the VLT and the orphan ports. Opening up both the VLT and orphan ports creates a
data loop and brings down the system.
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Virtual Link Trunking