Re: PVST - MSTP interop : Error

OK, some thoughts:

#1
You are trying to advertise bridge ID of MST switch to PVST switch using
command "spanning-tree mst 1 priority 8192". In this topology it does
nothing. When MST switch sees PVST switch it will ALWAYS send priority
32768. In binary its 1000 000000000000. By default, any PVST switch will
have higher priority, at least 32769 (1000 000000000001) for vlan 1 and
therefore will not try to become a root. It looks like Cisco took some
precaution so if priority is not forced manually on PVST switch, MST will
become a root using normal election: lower priority wins.

# 2
Now, what if you force PVST to have lower priority than MST, like you do
with command "spanning-tree vlan 1-4094 priority 4096"? I can only suppose
that Cisco really does not want PVST to become a root. MST switch sees this
as an error and puts both links to PVST in blocking state.

----------------------------------------
PS: PZDR to the initiated :)

On Sun, Mar 27, 2011 at 2:40 AM, GAURAV MADAN <gauravmadan1177_at_gmail.com>wrote:

> 2nd try .. any other expert opinion on this ?
> Cant we have PVST running switch as root in case which I mentioned .
>
> Any thoughts are welcomed :)
>
> Gaurav Madan
>
>
> On Sat, Mar 26, 2011 at 1:34 PM, Jakub Drwal <jakub.drwal_at_gmail.com>
> wrote:
>
> > Hi,
> >
> > As far as I remember only option in cisco's pvst-mst interoperability
> > implementation is having root bridge for all vlans/instances at mst
> region.
> >
> > regards,
> >
> > On Sat, Mar 26, 2011 at 8:45 AM, GAURAV MADAN <gauravmadan1177_at_gmail.com
> >wrote:
> >
> >> Hi All
> >>
> >> As far as I know (from readings etc) :
> >>
> >> 1) In case of MSTP - PVSTP inter-op scenarios ; we should either have
> >> [A] root for all vlans (all instances) in MST region
> >> [B] root for all vlans on PVST switch
> >>
> >> 2) Having root for half vlans in MST region and half on PVST switch is
> not
> >> a
> >> valid scenario and we will see PVST_Inc error in this .
> >>
> >> 3) Best practice suggest having root in MSTP region
> >>
> >> 4) However there is no stopping having root on PVST switch as well .
> >>
> >> Now My problem is following (My intent is to get SW2 PVST switch as
> root):
> >>
> >> SW-1 (MSTP) g1/1---------------------g1/1- SW-2(PVST)
> >> g1/2---------------------g1/2
> >>
> >> SW1 config
> >> ===========
> >> spanning-tree mode mst
> >> spanning-tree extend system-id
> >> !
> >> spanning-tree mst configuration
> >> name aa
> >> revision 10
> >> instance 1 vlan 1-5
> >> instance 2 vlan 6-10
> >> !
> >> spanning-tree mst 1 priority 8192
> >> spanning-tree mst 2 config 32768
> >>
> >> SW2 config
> >> ========
> >> !
> >> spanning-tree mode pvst
> >> spanning-tree extend system-id
> >> spanning-tree vlan 1-4094 priority 4096
> >> !
> >>
> >> At SW1 ; I am getting following error :
> >> C4924(config)#
> >> *Mar 26 06:39:35.735: %SPANTREE-2-PVSTSIM_FAIL: Blocking root port
> Gi1/1:
> >> Inconsitent inferior PVST BPDU received on VLAN 2, claiming root
> >> 2:0024.1430.2f00
> >>
> >> C4924#sh span mst 1
> >>
> >> ##### MST1 vlans mapped: 1-5
> >> Bridge address 001d.450d.0700 priority 8193 (8192 sysid 1)
> >> Root this switch for MST1
> >>
> >> Interface Role Sts Cost Prio.Nbr Type
> >> ---------------- ---- --- --------- --------
> >> --------------------------------
> >> Gi1/1 Mstr BKN*20000 144.1 P2p Bound(PVST) *PVST_Inc
> >> Gi1/2 Altn BLK 20000 128.2 P2p Bound(PVST)
> >>
> >> C4924#sh span mst 2
> >>
> >> ##### MST2 vlans mapped: 6-10
> >> Bridge address 001d.450d.0700 priority 32770 (32768 sysid
> 2)
> >> Root this switch for MST2
> >>
> >> Interface Role Sts Cost Prio.Nbr Type
> >> ---------------- ---- --- --------- --------
> >> --------------------------------
> >> Gi1/1 Mstr BKN*20000 128.1 P2p Bound(PVST) *PVST_Inc
> >> Gi1/2 Altn BLK 20000 128.2 P2p Bound(PVST)
> >>
> >> Please let me know where am i wrong ?
> >>
> >> Thanks
> >> Gaurav Madan
> >>
> >>
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> >>
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> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >
> >
> > --
> > Jakub Drwal
>
>
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Received on Mon Mar 28 2011 - 14:01:50 ART