Re: root bridge election from garry baker on 2010-10-19 (Ccielab archives 10/2010)

From: garry baker <baker.garry_at_gmail.com>
Date: Tue, 19 Oct 2010 13:53:14 -0500

maybe this will help out...

run a test with the command 'spanning-tree vlan 1 root primary' and then
change the value on a second bridge to 1 lower and watch it change then run
the command 'spanning-tree vlan 1 root primary' again and watch it lower the
value based of the new information from the BPDU of the lower value of say
switch 2...

also try the same thing with the priority command which is a static value,
same result?

example (i did this in GNS3 with switch modules but same thing for this demo
purpose):
R1(config)#do sh spann bri

VLAN1
  Spanning tree enabled protocol ieee
  Root ID Priority 8191
             Address c200.11e0.0000
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 8191
             Address c200.11e0.0000
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

Interface Designated
Name Port ID Prio Cost Sts Cost Bridge ID Port
ID
-------------------- ------- ---- ----- --- ----- --------------------
-------
FastEthernet1/0 128.41 128 19 FWD 0 8191 c200.11e0.0000
128.41

R2(config)#do sh spann bri

VLAN1
  Spanning tree enabled protocol ieee
  Root ID Priority 8191
             Address c200.11e0.0000
             Cost 19
             Port 41 (FastEthernet1/0)
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 8191
             Address c202.11e0.0000
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

RUN COMMAND to change ROOT priority on R2:
R2(config)#spanning-tree vlan 1 priority 8000
R2(config)#do sh spann bri

VLAN1
  Spanning tree enabled protocol ieee
  Root ID Priority 8000
             Address c202.11e0.0000
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 8000
             Address c202.11e0.0000
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

THEN CHANGE R1 to ROOT PRIMARY and see it lower value (or in this case equal
and the MAC address breaks the tie) to be lower than the R2 bridge to become
ROOT:
R1(config)#spanning-tree vlan 1 root primary
VLAN 1 bridge priority set to 8000
VLAN 1 bridge max aging time unchanged at 20
VLAN 1 bridge hello time unchanged at 2
VLAN 1 bridge forward delay unchanged at 15
R1(config)#do sh spann bri

VLAN1
  Spanning tree enabled protocol ieee
  Root ID Priority 8000
             Address c200.11e0.0000
             This bridge is the root
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 8000
             Address c200.11e0.0000
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

R2(config)#do sh spann bri

VLAN1
  Spanning tree enabled protocol ieee
  Root ID Priority 8000
             Address c200.11e0.0000
             Cost 19
             Port 41 (FastEthernet1/0)
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

  Bridge ID Priority 8000
             Address c202.11e0.0000
             Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
             Aging Time 300

--
Garry L. Baker
"There is no 'patch' for stupidity." - www.sqlsecurity.com
On Tue, Oct 19, 2010 at 1:37 PM, smith <wilsmith0_at_gmail.com> wrote:
> Thanks for the reply,
>
> But If I am asked to configure a switch as root without explicitly saying
> anything about which one to use,
> how to decide which command to use ?
>
>
>
> On Wed, Oct 20, 2010 at 12:01 AM, Sarad <tosara_at_gmail.com> wrote:
>
> > Hi smith,
> >
> > Root primary command is a macro, It actually changes the bridge priority
> in
> > the background,
> >
> > Rgds
> >
> > Sara
> >
> > On Tue, Oct 19, 2010 at 11:53 PM, smith <wilsmith0_at_gmail.com> wrote:
> >
> >> Hi folks,
> >>
> >> how to decide whether to use Priority or the Root  primary / secondary
> >> command while electing a root bridge ?
> >> Is there any specific case to select one over the other ?
> >>
> >> thanks
> >>
> >> --
> >> smith
> >>
> >>
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> >
>
>
> --
> smith
>
>
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Received on Tue Oct 19 2010 - 13:53:14 ART

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