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Community Member

Spanning-Tree Diameter

Hi,

I am having some difficulties to understand the spanning-tree protocol. When this protocol mentions that the maximum diameter is 7, does that means that a maximum of 7 or 8 bridges allowed in a spanning-tree domain.

If we consider that the Message_age_overestimate is
= (dia – 1) x overestimate_per_bridge (1 as per Cisco standard)
= dia – 1
= 6

If we consider the max_age

max_age
= End-to-end_BPDU_propa_delay + essage_age_overestimate
= 14 + 6
= 20 sec

whrere End_to_end_BPDU_Propa_delay is

End-to-end_BPDU_propa_delay
= ((lost_msg + 1) x hello) + ((BPDU_Delay x (dia – 1))
= ((3 + 1) x hello) + ((1 x (dia – 1))
= 4 x hello + dia – 1
= 4 x 2 + 6
= 14 sec

My question is how can used the value 6 as Message_age_overestimate with 7 bridge when the age between the root bridge and the first bridge is 0, the max age when reaching the last switch should be set to 5 not 6.

I am also trying to understand why the 6 seconds for the BPDU propagation delays since the worst case scenario is when we loose the first root switch and only BPDU must only go through 5 switches to reach the last switch, BPDU propagation delay should correspond to 5 switches. I have attached a pdf drawing just to clarify my question.

Thanks for your help

Stephane

1 REPLY
Community Member

Re: Spanning-Tree Diameter

http://slaptijack.com/networking/max-spanning-tree-stp-diameter/

This limitation is not really relevant for RSTP and MST

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