Loop Guard
A unidirectional failure on a root or alternate port will cause spanning tree to loop.
Loopguard enforces a simple rule.
If a port was receiving BPBUs and suddenly it stops, don’t change the STP.
- This is one of the unidirectional preventatives
- This is only for switch-to-switch ports
… if a port doesn’t get a BPDU, it enters STP loop-inconsistent disabling the port.
Terms
Unidirectional Link
A failure where one side of a fiber pair is broken.
A unidirectional failure will always result in one side not getting information.
- SW1 sends BPDUs
- SW2 never gets BPDUs
Topology
- SW1 is a root bridge
- SW2 is experiencing a UL failure
- SW2 will transition Port 1 to a RP
No BPDU Received
┌────────────────────┐ ┌────────────────────┐
│ ┌─────────────┐│ │ ┌────────────┐ │
│ │ Port ┌────┐ ││ BPDU ────►XXXX │ │┌────┐ Port │ │
│ │ 1 │ TX ├─││────────────── Fiber Cut! ──│─│┤ RX │ 1 │ │
│ SW1 │ └────┘ ││ │ │└────┘ │ SW2 │
│ │ RP ┌────┐ ││ │ │┌────┐ DP │ │
│ │ │ RX ├─││────────────────────────────│─│┤ TX │ │ │
│ │ └────┘ ││ ◄───── BPDU │ │└────┘ │ │
│ └─────────────┘│ │ └────────────┘ │
└────────────────────┘ └────────────────────┘
Config
Default
spanning-tree loopguard default
Per Port
interface 1
spanning-tree guard loop