STP LOOP ISSUE

[santosh.s.vijapur]

Hi All

we had a issue where an IPT phone was plugged into one of the switch1( SW1) port  in VLAN 200/Voice

What we found is the 2nd port on the same phone was also plugged into  switch1( SW1)  and this caused issues at the customer network.

Once we shut down the ports the services came back.

However noticeed on the port config voice VLAN have “spanning-tree bpdufilter enable” which basically says don’t send or receive BDPU’s..

Definition-This command prevents ports that are in a Port Fast-operational state from sending or receiving BPDUs. (Enabling BPDU filtering on an interface is the same as disabling spanning tree on it and can result in spanning-tree loops.)  ???? .

I think having BDPU guard on the ports would have helped us here.

Below is the port configs

interface GigabitEthernet x/y

switchport access vlan 150
switchport mode access
switchport voice vlan 200
qos trust dscp
spanning-tree portfast
spanning-tree bpdufilter enable
service-policy input VOIP-PHONE

Need your help to know exact root cause for loop in the network  when IPT phone connected to the network,

also If such issue needs to avoid in future  what will be the standard port configuration to be defined . please help

[davy.timmermans]

Receiving a BPDU on a BPDU-guard configured port will put the port in err-disabled state. It will stay in err-disabled state until you bring the port down (physic/ admin shut). It's also possible to recover automatically:

errdisable recovery cause (all | bpduguard)

errdisable recovery interval xx

recovery interval is 60 seconds by default. After the recovery interval - the port will try to come up. If the problem (loop in this case) still exists, it will go back to err-disabled state

show errdisable recovery will show you the causes for which it will try to recover automatically.

check which ports are errdisabled:

show interface status err-disabled

Regarding the small chance a loop occurs when connecting two portfast ports. I think the problem will resolve itself because at a certain moment a BPDU packet will be received- STP will break the loop and resolve the problem

[Kishore Chennupati]

santosh,

i believe Peter has aleready answered this.

"Correct. The loop may exist for a very short time and be almost undetectable - but that cannot be guaranteed. Because both ports with PortFast immediately become forwarding, the loop always exists in the moment of connecting those ports together."

HTH

Kishore