Hi Francois,

Thank You very much for clear answer.

BR,

Tomas

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

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