Spanning Tree and physical loop on Ethernet Interface

Answered Question
Apr 21st, 2008
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Hello,


According to STP theory and conversations in this forum STP can detect physical loop between two Ethernet Interfaces on the same switch.


Just additional question.


Does Spanning Tree Protocol detect, if physical loop condition exists on single switch Ethernet Interface (Tx pair wires connected with Rx pair wires)?


Will the STP protocol put this port into Blocked State ?


Best Regards,


Tomas

Correct Answer by Francois Tallet about 9 years 1 month ago

Yes it does. This has been laborious on the standard side, but Cisco switches have always (or at least for a long time) been able to do this. It was particularly useful because of cabling adapted to token ring, that used to loop back the interface should the connector be unplugged. Some switch display a special state of "looped back" for this blocking condition.

Regards,

Francois

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Correct Answer
Francois Tallet Mon, 04/21/2008 - 11:01
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Yes it does. This has been laborious on the standard side, but Cisco switches have always (or at least for a long time) been able to do this. It was particularly useful because of cabling adapted to token ring, that used to loop back the interface should the connector be unplugged. Some switch display a special state of "looped back" for this blocking condition.

Regards,

Francois

1pipantom2 Mon, 04/21/2008 - 12:08
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Hi Francois,


Thank You very much for clear answer.


BR,


Tomas

Maximiliano Gus... Sun, 04/27/2008 - 16:34
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Hi Francois, I'm a little confused about this issue. What I undestand is that STP/RSTP/MST standards do not detect a self loopback port, but Cisco do this with the "keepalive" frames that a switch port sends every ten seconds by default. If a switch port receives the same keepalive frame (ETH protocol 0x9000) that it had transmited, then the port goes to "errdisable" blocking the traffic. But this is not true for every Cisco switch, for example the Cisco 7600 switch with IOS 12.2.SXF7 does not support this error disable cause. Am I correct or I am loosing something ?

Thanks.

Regards,


Max

Francois Tallet Mon, 04/28/2008 - 10:53
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Hi Max,

I was talking about STP only. Cisco's STP has always been able to block a looped-back port (by simply receiving its own BPDUs). The standard has attempted to do it by different ways. I can't remember exactly the details, but there was a time when the standard messed up this mechanism. The standard should be ok right now btw.

The "loopback" frame you are referring to is a different mechanism that was introduced in some low end platforms (2K/3K) and that was supposed to detect looped back ports even when STP was disabled. This mechanism is enabled by default (which I think is controversial), and error disable the port if it receives back its own frames.

Regards,

Francois

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