From: steven ellis (steven.j.ellis@gmail.com)
Date: Wed Feb 21 2007 - 06:44:12 ART
Whoa - I am totally confused on what was asked and what has been
answered.....
At the risk of being flamed for stating the bleeding obvious....
My understanding of bridge priority is a very simplistic one - a value
signifying the switch priority for each instance of spanning tree - which
out of the box, a switch runs per VLAN spanning tree - so each VLAN (read
spanning tree instance) has a priority number. By default, this number is
32768- always. As has been pointed out, if priority numbers are the same for
two switches, MAC addresses determines the root.
This value can be changed to alter the priority therefore manipulating which
switch becomes root for spanning tree for each instance (also - you can have
different root switches for each VLAN(spanning tree instance)). The values
are in the range 0 to 61440 but can ONLY be altered in multiples of 4096;
any other values are rejected. The global command is:
spanning-tree vlan <vlan-id/s> priority <value in multiples of 4096>
So, if I wanted to configure one switch to be the spanning-tree root for
VLAN's 1 - 1005:
spanning-tree vlan 1-1005 priority 16384
Providing all other switches are either default or a value higher than this
value, it will become spanning-tree root for those VLAN's.
If you want to see this in black and white:
http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12225see/cr/cli3.htm#wp1947142
Cheers,
Steve
On 2/21/07, Gobind <gobind@inbox.com> wrote:
>
> OK,I got it but as said that priority would be multiples of 4096,so there
> is certain limit,I mean multiples of 4096 upto certain value? Also Scoot,the
> p;d way for Bridge ID (16 bit proirity +MAC) isnt used now a days(or some
> use it now a days also?)?
>
> So it would be like this:-
>
> Multiples of 4096+Vlan Id (like 10 for Vlan 10)
>
> Right? Or I am misunderstanding the concept of Vlan ID/System Ext Id,as
> far I am thinking and got to know from gus(correct me if I am wrong,but bit
> confused):-
>
> Sys Ext=Vlan Id=Vlan Number
>
> Gobind
>
> > -----Original Message-----
> > From: swm@emanon.com
> > Sent: Tue, 20 Feb 2007 19:39:37 -0500
> > To: louisccie_r_s@yahoo.com, gobind@inbox.com, ccielab@groupstudy.com
> > Subject: RE: Bridge Priority
> >
> > I think you'd have to sit there with a lot of math to verify that, but
> > it's
> > not accurate anyway, so save yourself some time. :)
> >
> > The old way for Bridge ID was 16-bit priority + MAC address = 64 bits.
> >
> > Now, the bridge ID is still 64 bits, but instead of having 16 bits for
> > priority (who has 65000 switches anyway?), there are 4 bits used for
> > priority and then 12 bits used for VLAN ID. So that's why you can get
> > some
> > strange values.
> >
> > So the priority field is still 16 bits:
> >
> > ppppvvvvvvvvvvvv
> >
> > Where p = priority and v = vlan ID.
> >
> > HTH,
> >
> >
> > Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> JNCIE
> > #153, CISSP, et al.
> > CCSI/JNCI-M/JNCI-J
> > IPexpert VP - Curriculum Development
> > IPexpert Sr. Technical Instructor
> > smorris@ipexpert.com
> > http://www.ipexpert.com
> >
> >
> >
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> > Lou
> > Ioanni
> > Sent: Tuesday, February 20, 2007 5:28 PM
> > To: Gobind; ccielab@groupstudy.com
> > Subject: Re: Bridge Priority
> >
> > Right. You do not deduct vlan number from default to get priority. What
> > confused you is the way they map vlans to priorities ex. 2-32768,
> 3-4094,
> > etc
> >
> > I never heard of such formula. I will be very surprised if there is
> > any.
> >
> > Loizos Y.
> > CCIE#10702 R & S
> >
> > Gobind <gobind@inbox.com> wrote:
> > So in no case this is true,right?:-
> >
> > Bridge priority = Default bridge priority - Vlan Number
> >
> > I heard it in CBT was got confused!
> >
> >> -----Original Message-----
> >> From: louisccie_r_s@yahoo.com
> >> Sent: Tue, 20 Feb 2007 11:21:11 -0800 (PST)
> >> To: gobind@inbox.com, ccielab@groupstudy.com
> >> Subject: Re: Bridge Priority
> >>
> >> Yes each vlan has its own root depending on the priority. If
> >> priorities are the same then it uses the mac to select the root. Each
> >> vlan and priorities are combined to create a unique ID. Check the
> >> floowing link for a siminlar question and answers:
> >>
> >> http://www.velocityreviews.com/forums/t37177-bridge-priority-and-sysid
> >> ext-number.html
> >>
> >> Gobind wrote: If the sys-id-ext usually correlates to Vlan Number,it
> >> means like for Vlan Number 1 the sys-id-ext would be 1,right? it is
> >> taken automatically or manually? I am confused about the part that the
> >> If in a switch every Vlan has its own priority then when the Root
> >> Election occurs,does it occurs to every Vlan,lets say there are
> >> 3 Vlans and what if it have different Priority then is it also
> >> possible that on Switch would be Root Bridge for 1 Vlan and not other
> >> two(due to higher Priority)?
> >>
> >>
> >> -----Original Message-----
> >> From: louisccie_r_s@yahoo.com
> >> Sent: Tue, 20 Feb 2007 10:59:29 -0800 (PST)
> >> To: gobind@inbox.com, ccielab@groupstudy.com
> >> Subject: Re: Bridge Priority
> >>
> >>
> >> I am not sure if you refering to a specific lab exercise, but in real
> >> environments you can select which switch you want to be the Root
> >> Bridge for specific vlans by lowering the priority from the default
> >> 32768. The lower the priority the more prefered to be the root
> >> (example 32000). If you want to FORCE a specific switch to be the root
> >> then you set priority 0. For example:
> >>
> >> spanning-tree vlan 10-14 priority 0 (This will set this switch to be
> >> the root for vlans 10,11,12,13,14)
> >>
> >> You can set a different root bridge for each group of vlans for load
> >> balancing. You can direct certain vlans traffic through one
> >> distribution switch and other vlans through the other distribution
> >> reduntant switch for load balancing L2 traffic.
> >>
> >> Usually is good to set the L3 distribution switches as the Root
> >> bridges for the vlans attached to the access layer switches attached to
> > it.
> >>
> >> Thanks,
> >>
> >> Loizos Y.
> >> CCIE#10702 R & S
> >>
> >>
> >>
> >> Gobind wrote: Hi Guys,
> >>
> >> I have some confusion wiht the Bridge Priority.In Some CBTs I am
> >> getting to hear that:-
> >>
> >> Bridge Priority is Default Priority(32768) - Vlan Number
> >>
> >> I asked my friend,he said Default Priority is Default Priority + Vlan
> >> Number
> >>
> >> In one Cisco Dumentation it is written:-
> >>
> >> "Each VLAN on each network device has a unique 64-bit bridge ID
> >> consisting of a bridge priority value, an extended system ID, and an
> >> STP MAC address allocation"
> >>
> >> Source:- http://www.cisco.com/en/U
> >> S/products/hw/switches/ps708/products_configuration_guide_chapter09186
> >> a008016
> >> 0a5b.html#wp1040218
> >>
> >> I am really confused(maybe I am getting it wrong) what Priority(in STP
> >> is),also I am not getting if the Priority is Default priority + Vlan
> >> Number or - Vlan Number then what Vlan Number is it refering to? I
> >> mean the Trunk belongs to every Vlan then which Vlan Number it takes
> >> while making the election.(when it sends BPDU i.e.)
> >>
> >> Gobind
> >>
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