From: steven ellis (steven.j.ellis@gmail.com)
Date: Thu Feb 22 2007 - 11:24:01 ART
Hi Gobind,
The bridge priority is shown in the bracket - 32768 for VLAN 23. The Bridge
ID becomes the bridge priorty plus the VLAN-ID because "each VLAN is
considered a different logical bridge with PVST+ - the switch must have
different bridge ID's for each configured VLAN" - that from a config guide
for 3560.
Hope that helps.
Cheers,
Steve
On 2/22/07, Gobind <gobind@inbox.com> wrote:
>
> Thanks Steve for reactifying the thing,but I have only one doubt left,as
> said by Scott and as per I read that if sys-ext is enabled then the bridge
> priority is :-
>
> Default Priority(or assigned) + sys-ext number(which is actually Vlan
> number0
>
> But when I checked with ::show spanning-tree bridge:: command,it showed up
> for Vlan number 23:-
>
> VLAN0023 32791 (32768, 23) 000b.467b.3800 2 20 15 ieee
>
> Which means that it is adding Vlan Number/Sys-ext Number in default
> priority but I havent enabled the sys-ext,is it on my default? Or I am
> getting the wrong concept about it?
>
> Thanks again
>
> Gobind
>
>
> -----Original Message-----
> *From:* steven.j.ellis@gmail.com
> *Sent:* Wed, 21 Feb 2007 09:44:12 +0000
> *To:* gobind@inbox.com
> *Subject:* Re: Bridge Priority
>
> 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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