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Beginner

Loop Inconsistent

I am getting Loop Inconsistent for a port in a vlan. I have two links for that vlan for a client. But all the time it is required to shutdown and again no shutdown of switchport. How can I get rid of it permanently. The switch log is given below:

SW#show spanning-tree inconsistentports

Name                     Interface                   Inconsistency
---------------         -------------------              ---------------------
VLAN0206    FastEthernet0/24 Loop      Inconsistent

Number of inconsistent ports (segments) in the system : 1

2 ACCEPTED SOLUTIONS

Accepted Solutions

Hello,

Hello,

I belive loop guard is configured and which is doing its role as expected.

STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. 

When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes designated and moves to a forwarding state. This situation creates a loop. 

The loop guard feature makes additional checks. If BPDUs are not received on a non-designated port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state, instead of crossing all states.

Without the loop guard feature, the port assumes the designated port role. The port moves to the STP forwarding state and creates a loop.

Hope it Helps..

-GI

Rate if it Helps..

Beginner

If Loop guard is enabled, the

If Loop guard is enabled, the port that was placed in a loop-inconsistent STP state will automatically transition to another STp state whenever BPDUs are received on this port.

When you mentioned you have 2 links for a client in VLAN206, can you be more specific.  Does the client have 2 hosts connected or are they connecting via a switch?

I do not believe you can configure errdisable automactic recovery for loop-inconsitent issues.  The best way to avoid this is to determine why it is happening and resolve that.

8 REPLIES 8

Hello,

Hello,

I belive loop guard is configured and which is doing its role as expected.

STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops). An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the forwarding state. This usually happens because one of the ports of a physically redundant topology (not necessarily the STP blocking port) no longer receives STP BPDUs. 

When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes designated and moves to a forwarding state. This situation creates a loop. 

The loop guard feature makes additional checks. If BPDUs are not received on a non-designated port, and loop guard is enabled, that port is moved into the STP loop-inconsistent blocking state, instead of crossing all states.

Without the loop guard feature, the port assumes the designated port role. The port moves to the STP forwarding state and creates a loop.

Hope it Helps..

-GI

Rate if it Helps..

Beginner

Is there any way to avoid

Is there any way to avoid this manual shut/no shut. 

Beginner

If Loop guard is enabled, the

If Loop guard is enabled, the port that was placed in a loop-inconsistent STP state will automatically transition to another STp state whenever BPDUs are received on this port.

When you mentioned you have 2 links for a client in VLAN206, can you be more specific.  Does the client have 2 hosts connected or are they connecting via a switch?

I do not believe you can configure errdisable automactic recovery for loop-inconsitent issues.  The best way to avoid this is to determine why it is happening and resolve that.

Beginner

Client is connected via

Client is connected via switch at theor end. That means it is switch to switch coonection.
Collaborator

Hello,

Hello,

Are those switches connected by two links or just one link? Do you have more switches in your network with the topology which may cause loop? If no, remove loop guard on the link.

no spanning-tree guard loop [ under the interface]

If you have more switches in the netwok, there reason might be the inconsistency of spanning tree configuration.

Masoud

Highlighted
VIP Advisor

Hello

Hello

there any Is way to avoid this manual shut/no shut

NO there isnt - Errdisable recovery doesnt cover this feature -

Shoud loopguard be appied to tis interface - Does it need to a desinated port

Due to loss of bpdu towards that given interface I would want to know why that specifc port is failing to receive bpdus and then constantley transnissioning into loop inconsitant state.

res
Paul





.



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future
Cisco Employee

Re: Hello

Beginner

Re: Hello

Loop guard recovery is automatic in the sense that once BPDUs are received on the port, it will no longer be in an inconsistent state.   However recovery in the sense of err-disable recovery, that it does not participate, since loop guard does not put a port in err-disable state.

 

Loop guard will put a port in inconsistent state if it fails to receive BPDUs.   The other item mentioned in documentation is when the port is in an STP blocking state, but I'm not sure that's a requirement, and not just an intention.

 

Although you have a downlink switch to connect two clients (PCs, servers, etc) to this port, it sounds like the downlink switch is a "dumb", unmanaged switch.   Such switches do not participate in STP, so they do not generate BPDUs, though they may pass though BPDUs received from a managed switch.

Effectively then, you have this port as an edge port so you should remove loop guard from it.

With an unmanaged switch, you do have the danger of creating a loop if another port of the unmanaged switch is connected to one of the uplink switches.   However the BPDU passthrough may have a connected ports on one of the uplink switches go into a blocking state as if the unmanaged switch were an Ethernet cable.

 

If the downlink switch is a managed switch, then it may have STP disabled.  You may want to enable STP on it but with a very high priority number so that it never becomes the root bridge.

 

Whether the switch with the edge devices is managed with STP turned off or an unmanged switch, if you trust that nobody will ever connect two or more of its ports to your uplink switch environment, then you can treat it as an end device and configure the port on the upstream switch as if connected to a single edge device.

 

One common example of emulating this small switch and causing havoc is with VOIP phones that let you piggyback a PC behind it.  Some silly users get the idea of connecting both of the phone's Ethernet ports to live Ethernet wallports.   This creates a loop, particularly when the switchports have voice vlan configured which for Cisco, automatically adds Portfast to the port configuration.

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