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Community Member

Loop guard enabled on etherchannel

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

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2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: Loop guard enabled on etherchannel

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.

Gold

Re: Loop guard enabled on etherchannel

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.

Hope it Helps!

Soroush.
4 REPLIES

Re: Loop guard enabled on etherchannel

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.

Community Member

Loop guard enabled on etherchannel

hi Jimmysands73,

Many thanks again for prompt reply.

Mahesh

Gold

Re: Loop guard enabled on etherchannel

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.

Hope it Helps!

Soroush.
Community Member

Loop guard enabled on etherchannel

Hi  Soroushm,

Thanks a lot again.

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

mahesh

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