Loop guard enabled on etherchannel

Answered Question
Jun 30th, 2012
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Hi Everyone,


I read that when loopguard is enabled on say 4 links in ether channel bundle connecting two switches together

and say 1 link in the bundle becomes  unidirectional then the entire bundle is put in down state.


Need to confirm if entire bundle is in down down state or all 4 ports in bundle went to inconsistentent  port types?


Be default it is said that loopguard puts port in inconsistent state.


Thanks


MAhesh

Correct Answer by soroushm about 5 years 1 month ago

Hello Mahesh,


Note that, BPDU's like other traffic types gets transmitted on one link in the bundle with regards to its src-dst Mac Address and It keeps that link as its path. in other words; bpdu is just sent over ONE link of the bundle.

So with that in mind and knowing that Loop Guard is also dependent on bpdu's to detect potential loops, i guess the situation depends on which member of  the bundle becomes unidirectional. If that member is the one which BPDU is traveling on it, it'll put the whole bundle/Port-Channel in blocking state (loop inconsistent) for that particular VLAN (it is a per-VLAN process), because STP considers the ether-channel one logical interface.


but I dont know what happenes if something happenes on an interface other than the one responsible for BDPUs.


Hope it Helps,

Soroush.

Correct Answer by jimmysands73_2 about 5 years 1 month ago

From :

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml


Configuration Considerations

The loop guard feature is enabled on a per-port basis. However, as long as it blocks the port on the STP level, loop guard blocks inconsistent ports on a per-VLAN basis (because of per-VLAN STP). That is, if BPDUs are not received on the trunk port for only one particular VLAN, only that VLAN is blocked (moved to loop-inconsistent STP state). For the same reason, if enabled on an EtherChannel interface, the entire channel is blocked for a particular VLAN, not just one link (because EtherChannel is regarded as one logical port from the STP point of view).

      


I would have to lab this up to test, but looks like its not down down, but blocked from an STP point of view.

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Correct Answer
jimmysands73_2 Sat, 06/30/2012 - 11:10
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From :

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094640.shtml


Configuration Considerations

The loop guard feature is enabled on a per-port basis. However, as long as it blocks the port on the STP level, loop guard blocks inconsistent ports on a per-VLAN basis (because of per-VLAN STP). That is, if BPDUs are not received on the trunk port for only one particular VLAN, only that VLAN is blocked (moved to loop-inconsistent STP state). For the same reason, if enabled on an EtherChannel interface, the entire channel is blocked for a particular VLAN, not just one link (because EtherChannel is regarded as one logical port from the STP point of view).

      


I would have to lab this up to test, but looks like its not down down, but blocked from an STP point of view.

mahesh18 Sat, 06/30/2012 - 21:30
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hi Jimmysands73,


Many thanks again for prompt reply.


Mahesh

Correct Answer
soroushm Sat, 06/30/2012 - 11:53
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Hello Mahesh,


Note that, BPDU's like other traffic types gets transmitted on one link in the bundle with regards to its src-dst Mac Address and It keeps that link as its path. in other words; bpdu is just sent over ONE link of the bundle.

So with that in mind and knowing that Loop Guard is also dependent on bpdu's to detect potential loops, i guess the situation depends on which member of  the bundle becomes unidirectional. If that member is the one which BPDU is traveling on it, it'll put the whole bundle/Port-Channel in blocking state (loop inconsistent) for that particular VLAN (it is a per-VLAN process), because STP considers the ether-channel one logical interface.


but I dont know what happenes if something happenes on an interface other than the one responsible for BDPUs.


Hope it Helps,

Soroush.

mahesh18 Sat, 06/30/2012 - 21:33
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Hi  Soroushm,


Thanks a lot again.

I understand the behaviour of links in port channel while loopguard is enabled.


mahesh

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