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New Member

Spanning-tree bpduguard not work on interface when connect with IPDSLAM

Hello Experts,

I need your help regarding loop prevent when occure.

I have facing the problem in any cisco switch like 3550 , or 2950.

i have configured on interface in cisco 3550 as bellow.


description *** connect with IPDSLAM ***
switchport mode access
switchport access vlan 2
switchport protected
spanning-tree portfast
spanning-tree bpduguard enable


Cisco 3550 port fas0/3 >>>> IPDSLAM (48 port) >>> ADSL Modem (CPE) >>>> normal swith (port1 at CPE) >>> custpmer PC (connect with port 2 in normal switch.

when loop occure in this normal switch that time cisco fas0/3 port not disable itself when connect with IPDSLAM its idicate up


cisco 3550 port fas0/3 >>> normal switch port 1 >>> customer pc ( connect with normal switch port 2 )

but when this same port directly connect with same normal switch and loop occure that time same interface of cisco 3550 disable itself.

so what is the issue when connect with IPDSLAM that protocol spanning-tree bpduguard enable not work but when it's connect with normal switch it;s working fine.

Please help me it's very urgent for me.

Thanks in ADV,

New Member

Re: Spanning-tree bpduguard not work on interface when connect w

Hi Csawest.dc ,

as you mentioned while you connecting pc thru toplogy 1 cisco switch port not enable bpdu gaurd once loop detected on switch port , well to run bpdu on network each device need to understand wiht bpdu , where adsl modem does not understand with bpdugaurd so it's not pass loop information to cisco router.

in topology 2 where u connected your pc thur switch to cisco switch then all devices comes under one collison domain so that time once loop occured that time loop information comes to switch port and then bpdu works.

let me know incase you have any other question regarding this.


Sanjay Khurana