spanning tree

Unanswered Question
Jul 28th, 2009
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Hi all

I am running rstp on my switch, however it says the below

Spanning tree enabled protocol rstp

Root ID Priority 32836

Address 0026.5182.6980

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

I thought the max age should be 6 secs and the forward day under a sec ?


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Overall Rating: 4 (1 ratings)
Giuseppe Larosa Tue, 07/28/2009 - 00:20
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Hello Carl,

max age is 20 seconds but it is used only on ports where a legacy PVST neighbor is detected or on an half duplex port.

on point-to-point links where a RSTP neighbor is present a device waits up to 3 times the hello timer (6 seconds)

Hope to help


Peter Paluch Tue, 07/28/2009 - 01:49
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I've just tested it: the max_age of 20 seconds is not used on half duplex - or better said, shared links for RSTP neighbors. For them, the normal 6 second timeout applies.

I did a quick test: I have interconnected two switches by a crossover cable, forced the ports to half duplex, run the RSTP on both switches and made one to be the root bridge by lowering the priority to 0. On the second switch, I have turned on STP debugging and back on the root bridge, I have activated the BPDU Filter on its port towards the second switch. After 6 seconds, the second switch reported a loss of previous root and elected itself as a new root bridge.

So the max_age parameter seems to apply only to legacy 802.1D STP neighbors.

Best regards,



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