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Bronze

spanning-tree vlan (vlan id) root and priority

hi every body!

Let say we have sw1 using extended system, priority 4096. It is currently a root.

Now we introduce the another switch sw2 with command " spanning-tree vlan 2 root primary.

Will the new switch sw2 will set its priority as 4096(current root)-4096=0?

2)According to my book,the command" spanning-tree vlan 2 root diameter 3" will adjust the 1) hello timer 2)forward delay 3)max-age.

But i find the in 802.1d specification that command enters the two vaiable, hello time and diameter in following equations to determine forward delay and max age;

Max-age= 4*hello= 2*diameter-2

forward-delay=[(4*hello)+(3*diameter)]/2

If hello timer is not specified, default helo time=2 sec is assumed.

The question is according to book the command" spanning-tree vlan 2 root diameter 3" sets hello timer, forward delay, max-age.

But according to rfc for 802.1q, it sets only two timers foward delay and max age if given hello time and diameter.

Which one is correct?

thanks a lot!

3 REPLIES
Bronze

Re: spanning-tree vlan (vlan id) root and priority

Well I found the answer for first one .

1) sw2 will set its priority to zero.

Hall of Fame Super Silver

Re: spanning-tree vlan (vlan id) root and priority

Hello Sarah,

1) 0 when using extended-id because

prioriy in vlan X = base pri + X

2) hello-time can be specified as an optional parameter in spanning-tree root macro.

As observed by Jon in another post the diameter parameter can only be between 2-7:

SW-TO-CRN-P1-C-1(config)#spanning-tree vlan 889 root prim dia ?

>>>> <2-7> Maximum number of bridges between any two end nodes

SW-TO-CRN-P1-C-1(config)#spanning-tree vlan 889 root prim dia 4 ?

hello-time Hello interval for this spanning tree

SW-TO-CRN-P1-C-1(config)#spanning-tree vlan 889 root prim dia 4 hello-time ?

<1-10> Hello interval in seconds

Hope to help

Giuseppe

Bronze

Re: spanning-tree vlan (vlan id) root and priority

Thanks a lot Giuseppe!

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