couple of spanning tree questions

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Sep 7th, 2010
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When the bridges decide which port to block, is it the one that provides the best bpdu on the segment that will forward and the other switch will block?


also i have been reading about the max age, how in the bpdu it gets decremented each time its relayed from the root, does that mean on switch 7, the port would age out aftersay 14 seconds? or would it still use the 20 second max age for the port? and is this the same for RSTP, apart from the lower default max age?


cheers


Carl

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Giuseppe Larosa Tue, 09/07/2010 - 13:47
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Hello Carl,


>>When the bridges decide which port to block, is it the one that provides the best bpdu on the segment that will forward and the other switch will block?

your understanding is correct


RSTP generally speaking uses max-age as a sort of max-hops, so max-age should be increased in ring topologies where a fault can create a long chain.


It is the BDPU itself that is considered aged out.


Hope to help

Giuseppe

Peter Paluch Tue, 09/07/2010 - 13:49
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Hello Carl,


When the bridges decide which port to block, is it the one that
provides the best bpdu on the segment that will forward and the other
switch will block?


If you are asking about the Designated port for a segment then yes, you are correct. The port sending the best BPDUs will be the Designated port for this segment, all other ports will be Blocking.


also i have been reading about the max age, how in the bpdu it gets
decremented each time its relayed from the root, does that mean on
switch 7, the port would age out aftersay 14 seconds?


Yes, this is correct. The BPDU in 802.1D STP is originated on the root switch with the Message Age of 0 and incremented each time the BPDU is relayed further by non-root switches. The first switch expires this BPDU after 20 seconds, the second switch after 19 seconds (20-1), the third switch after 18 seconds (20-2) and so on. This article describes this concept quite nicely:


http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml#stp_timers


and is this the same for RSTP, apart from the lower default max age?


Very good question! I don't know for certain but I have had a quick look into the 802.1D-2004 where the RSTP is described. It seems that the Message Age field is still maintained and incremented, however, it is not subtracted from the max_age field, rather it simply serves as an anti-looping - if the Message Age is equal or higher to the max_age field, the BPDU will be immediately discarded. I haven't seen any other use of the Message Age in the RSTP standard but I may be wrong on this.


Best regards,

Peter

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