Loopguard blocking single feed port on Cisco 3750

[randyclark]

Loopguard on a 3750 is blocking the single feed port several times a day. My question is why? Could a hub be connected to the switch with a loop on the hub? I would then think the port with the hub would be blocked not the feed port to the rest of the network.

[mahmoodmkl]

Hi

Can u please elaborate more,we r not able to understand u topology.

Thanks

Mahmood

[Giuseppe Larosa · Accepted solution]

Hello Randy,

loop guard is triggered by the missing of superior BPDUs received on the link.

Instead of promoting the port to Designated Port on the segment the port is placed in STP inconsistent state.

This behaviour is Vlan specific.

see example of messages I got on one of our devices

sh log | inc Jan 28

Jan 28 00:33:19: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/28 on VLAN0001.

Jan 28 00:33:19: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/28 on VLAN0001.

Jan 28 08:12:27: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/28 on VLAN0001.

Jan 28 08:12:28: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/28 on VLAN0001.

Jan 28 09:40:03: %SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port GigabitEthernet0/28 on VLAN0001.

Jan 28 09:40:03: %SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port GigabitEthernet0/28 on VLAN0001.

to be noted g0/28 is the root port for vlan1

sh spanning-tree vlan 1

VLAN0001

Spanning tree enabled protocol rstp

Root ID Priority 1

Address 0017.0fad.1900

Cost 4

Port 28 (GigabitEthernet0/28)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 0023.0419.2680

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Interface Role Sts Cost Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Gi0/27 Altn BLK 4 128.27 P2p

Gi0/28 Root FWD 4 128.28 P2p

Po1 Desg FWD 3 128.56 P2p

sh spanning-tree interface g0/28 det

Port 28 (GigabitEthernet0/28) of VLAN0001 is root forwarding

Port path cost 4, Port priority 128, Port Identifier 128.28.

Designated root has priority 1, address 0017.0fad.1900

Designated bridge has priority 1, address 0017.0fad.1900

Designated port id is 128.8, designated path cost 0

Timers: message age 15, forward delay 0, hold 0

Number of transitions to forwarding state: 27

Link type is point-to-point by default

Loop guard is enabled on the port

BPDU: sent 78, received 587997

the port receives BPDUs from the root bridge on g0/28 if it misses to receive them (three in a row) it triggers the STP loop guard and instead of trying to put in forwarding the port it moves to inconsistent state.

In our scenario there are two uplinks and core switches are root bridge for different vlans.

However, if this is the only uplink of the switch you could consider to disable loop guard and to have bpu guard configured on all the access ports. This should be enough.

Hope to help

Giuseppe