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Beginner

Quick Spanning-tree Question

Hello there,

I'm  trying to understand how spanning-tree works and i do understand that  the link between switch 22 and switch 17 should have one port blocked by  spanning tree. So far so good, but that link should choose its  designated port based on the switch with the lowest bridge_ID, which in  this case is switch 22.

So,  switch 22 int g1/2 should be a designated port and switch 17 int g1/1  should be block by spanning tree (non-designated). But, according to  cisco packet tracer it is the other way around. Why is that?

Note: all links have a cost of 4

spann.jpg

Thanks,

Abdy,

4 REPLIES 4
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Rising star

I think you posted at CLN as well. Looks like a bug to me. Show output from show spanning-tree

Daniel Dib
CCIE #37149

Daniel Dib
CCIE #37149
CCDE #20160011

Please rate helpful posts.
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Daniel,

Information shown by "show spanning-tree" only improves my point. But, i have noticed when i was analyzing the STP packets send from each switch that STP used the mac address of the gigabit interface ports and not the mac address of the switch, which is displayed as the mac address of vlan 1. Is that a bug? because bridge_ID has the mac address information of the switch and not the indiviual ports.

Kindly see the picture

Abdy,

Highlighted

That's just the encapsulated frame. The BID should be contained in the payload of that frame so that is fine. If you change the priority does the topology converge correctly then?

Daniel Dib
CCIE #37149

Daniel Dib
CCIE #37149
CCDE #20160011

Please rate helpful posts.
Highlighted

You got the right reply on CLN now. Actually PT is behaving correctly here.

Daniel Dib
CCIE #37149

Daniel Dib
CCIE #37149
CCDE #20160011

Please rate helpful posts.
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