STP loop guard
Spanning Tree (STP) is used to ensure a loop-free topology over the LAN. Occasionally a hardware or software failure can cause STP to fail, creating STP/forwarding loops that can cause network failures where unidirectional links are used. The non-designated port transitions in a faulty manner because the port is no longer receiving STP BPDUs.
STP Loop Guard causes the non-designated port to go into the STP loop inconsistent state instead of the forwarding state. In the loop-inconsistent state, the port prevents data traffic and BPDU transmission through the link, therefore avoiding the loop creation. When BPDUs again are received on the inconsistent port, it resumes normal STP operation automatically. STP loop guard is best applied on blocking or forwarding ports.
Syntax:
spanning-tree port-list loop-guardno spanning-tree port-list loop-guardEnables STP loop guard on a particular port or ports. The
no form of the command disables STP loop guard.
Default: Disabled.
Enabling spanning tree loop guard on Port 2 and Viewing the port's status
switch(config)# spanning-tree 2 loop-guard
switch(config)# show spanning-tree
Multiple Spanning Tree (MST) Information
STP Enabled : Yes
Force Version : MSTP-operation
IST Mapped VLANs : 1-4094
Switch MAC Address : 0024a8-d13a40
Switch Priority : 32768
Max Age : 20
Max Hops : 20
Forward Delay : 15
Topology Change Count : 1
Time Since Last Change : 20 mins
CST Root MAC Address : 001083-847000
CST Root Priority : 0
CST Root Path Cost : 60000
CST Root Port : 1
IST Regional Root MAC Address : 0024a8-d13a40
IST Regional Root Priority : 32768
IST Regional Root Path Cost : 0
IST Remaining Hops : 20
Root Guard Ports :
Loop Guard Ports : 2
TCN Guard Ports :
BPDU Protected Ports :
BPDU Filtered Ports :
PVST Protected Ports :
PVST Filtered Ports :
| Prio | Designated Hello
Port Type | Cost rity State | Bridge Time PtP Edge
------ --------- + --------- ---- ------------ + ------------- ---- --- ----
1 100/1000T | 20000 128 Forwarding | 001871-cdea00 2 Yes No
2 100/1000T | Auto 128 Inconsistent |
3 100/1000T | Auto 128 Disabled |
4 100/1000T | Auto 128 Disabled |
5 100/1000T | Auto 128 Disabled |
6 100/1000T | Auto 128 Disabled |
7 100/1000T | Auto 128 Disabled |
8 100/1000T | Auto 128 Disabled |
Viewing summary spanning tree configuration information
switch(config)# show spanning-tree config
Multiple Spanning Tree (MST) Configuration Information
STP Enabled [No] : Yes
Force Version [MSTP-operation] : MSTP-operation
Default Path Costs [802.1t] : 802.1t
MST Configuration Name : 0024a8d13a40
MST Configuration Revision : 0 Switch Priority : 32768
Forward Delay [15] : 15 Hello Time [2] : 2
Max Age [20] : 20 Max Hops [20] : 20
| Path Prio Admin Auto Admin Hello Root Loop TCN BPDU
Port Type | Cost rity Edge Edge PtP Time Guard Guard Guard Flt
---- --------- + --------- ---- ----- ---- ----- ------ ----- ----- ----- ---
1 100/1000T | Auto 128 No Yes True Global No No No No
2 100/1000T | Auto 128 No Yes True Global No Yes No No
3 100/1000T | Auto 128 No Yes True Global No No No No
4 100/1000T | Auto 128 No Yes True Global No No No No
5 100/1000T | Auto 128 No Yes True Global No No No No
6 100/1000T | Auto 128 No Yes True Global No No No No
.
.
.
Viewing detailed spanning tree configuration information
switch(config)# show spanning-tree detail Status and Counters - CST Port(s) Detailed Information Port : 1 Status : Up . . . Port : 2 Status : Up BPDU Protection : No BPDU Filtering : No PVST Protection : No PVST Filtering : No Errant BPDU Count : 0 Root Guard : No Loop Guard : Yes TCN Guard : No MST Region Boundary : Yes External Path Cost : 20000 External Root Path Cost : 40000 Administrative Hello Time: Global Operational Hello Time : 2 AdminEdgePort : No Auto Edge Port : Yes OperEdgePort : No AdminPointToPointMAC : True OperPointToPointMAC : Yes Aged BPDUs Count : 0 Loop-back BPDUs Count : 0 TC ACK Flag Transmitted : 0 TC ACK Flag Received : 1 MST MST CFG CFG TCN TCN BPDUs Tx BPDUs Rx BPDUs Tx BPDUs Rx BPDUs Tx BPDUs Rx ---------- ---------- ---------- ---------- ---------- ---------- 3 0 24354 1682 0 13
Viewing spanning tree configuration information for a single port
switch(config)# show spanning-tree 2
Multiple Spanning Tree (MST) Information
STP Enabled : Yes
Force Version : MSTP-operation
IST Mapped VLANs : 1-4094
Switch MAC Address : 0024a8-d13a40
Switch Priority : 32768
Max Age : 20
Max Hops : 20
Forward Delay : 15
Topology Change Count : 1
Time Since Last Change : 58 mins
CST Root MAC Address : 001083-847000
CST Root Priority : 0
CST Root Path Cost : 60000
CST Root Port : 1
IST Regional Root MAC Address : 0024a8-d13a40
IST Regional Root Priority : 32768
IST Regional Root Path Cost : 0
IST Remaining Hops : 20
Root Guard Ports :
Loop Guard Ports : 2
TCN Guard Ports :
BPDU Protected Ports :
BPDU Filtered Ports :
PVST Protected Ports :
PVST Filtered Ports :
| Prio | Designated Hello
Port Type | Cost rity State | Bridge Time PtP Edge
------ --------- + --------- ---- ------------ + ------------- ---- --- ----
2 100/1000T | Auto 128 Inconsistent |