Re: MSTP backward compatibility ?

Not really, all you need is the following

1) Ensure the CST root is the root for all PVST+ VLANs by lowering bridge
priority for MSTI0. This will ensure proper interop among MSTP and PVST+.
2) On PVST+ side, play with per-VLAN link STP costs to properly elect
blocking ports on per-VLAN basis.

Symmetric load-balancing will be automatically enforced by the way Ethernet
bridges learn MAC addresses. Effectively, the MSTP domain will learn the
MACs on the ports that are NOT blocking for a given VLAN, as configured on
PVST+ side. At the same time every PVST+ STP instance will be part of CST to
ensure loop-less topology for every VLAN.

2011/3/2 Jacek <q.192.168.1.0_at_gmail.com>

> Thanks for correcting me on this. Load balancing will have to be then set
> separately for MST switches using bridge id and for PVST using cost or port
> priority ?
>
> On Wed, Mar 2, 2011 at 1:51 AM, Petr Lapukhov
> <petr_at_internetworkexpert.com>wrote:
>
> > Hi Jacek,
> >
> > Not sure why you concluded that load-balancing is impossible in this
> case.
> > From the viewpoint of PVST+ side, the MSTP CIST Root appears to be a root
> > bridge for every VLAN in the topology. By adjusting STP costs on the
> links
> > connecting PVST+ domain to MSTP domain you can select elect root ports on
> > per-VLAN basis and provide limited form of load-balancing just as usual
> with
> > PVST+ scenarios.
> >
> > --
> > Petr Lapukhov, petr_at_INE.com
> > CCIE #16379 (R&S/Security/SP/Voice)
> > CCDE #20100007
> >
> > Internetwork Expert, Inc.
> > http://www.INE.com <http://www.ine.com/>
> > Toll Free: 877-224-8987
> > Outside US: 775-826-4344
> >
> > 2011/3/1 Jacek <q.192.168.1.0_at_gmail.com>
> >
> > Right, root must be on MST, it works now but not fully...
> >>
> >> When MST switch sends priority to a PVST switch it sends default
> priority
> >> 32768, WITHOUT 12 bits extended system id. This is even the case if MST
> >> switch is configured as root. It will still send 32768 to a PVST switch.
> >> This is good because PVST switch will always see an MST switch with
> lower
> >> priority than itself or other PVST switches. This is assuming that PVST
> >> switches do not have priority configured manually.
> >>
> >> But...
> >> in this topology:
> >>
> >> SW1=======SW2 ---> MSTP
> >> || \\ // ||
> >> || \\// ||
> >> || //\\ ||
> >> || // \\ ||
> >> SW3=======SW4 ---> PVST
> >>
> >> SW1 and SW2 will both send priority 32768 to SW3 and SW4. As a result
> SW3
> >> and SW4 will have to compare MAC addresses to determine the root bridge.
> >> In
> >> my case PVST chooses SW1 is the root for all vlans. Load balancing is
> then
> >> impossible.
> >>
> >> Is my logic OK ?
> >>
> >>
> >>
> >> On Tue, Mar 1, 2011 at 3:45 PM, Rares Donca <rares.donca_at_gmail.com>
> >> wrote:
> >>
> >> > Hello,
> >> >
> >> > The spanning-tree root for all Vlans has to be in MSTP, for that to
> >> work.
> >> >
> >> > I recommend you to read these excellent blog posts who clearly explain
> >> what
> >> >
> >> > is happening there.
> >> >
> >> > http://blog.ine.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/
> >> >
> http://blog.ine.com/2008/09/24/mstp-tutorial-part-ii-outside-a-region/
> >> >
> >> > Regards,
> >> > Rares
> >> >
> >> > On Tue, Mar 1, 2011 at 10:33 PM, Jacek <q.192.168.1.0_at_gmail.com>
> wrote:
> >> >
> >> >> I have a question regarding MSTP backward compatibility with PVST.
> >> >> According
> >> >> to Cisco documentation "MSTP maintain backward compatibility with
> >> >> equipment
> >> >> that is based on PVST+"
> >> >>
> >> >> I am testing this in the lab and and it does not work. In the
> topology
> >> >> below
> >> >> I have MSTP and PVST switches connected. I tried several combination
> of
> >> >> stp
> >> >> priority on different switches and it looks like MST and PVST
> switches
> >> >> will
> >> >> not exchange stp priority at all.
> >> >>
> >> >> Here is the scenario:
> >> >>
> >> >> SW1=======SW2 ---> MSTP
> >> >> || \\ // ||
> >> >> || \\// ||
> >> >> || //\\ ||
> >> >> || // \\ ||
> >> >> SW3=======SW4 ---> PVST
> >> >>
> >> >> For example, if I configure SW4 to be root bridge for vlan 10 I will
> >> end
> >> >> up
> >> >> with two switches claiming to be root for vlan 10: SW1 and sw4 will
> >> claim
> >> >> to
> >> >> be root.
> >> >>
> >> >> It looks to me that MST and PVST switches do not exchange stp
> priority,
> >> >> any
> >> >> ideas about "backward compatibility" ?
> >> >>
> >> >> BTW, I tested a mix of RSTP and PVST+ in the similar topology and it
> >> >> worked
> >> >> OK.
> >> >>
> >> >>
> >> >> Blogs and organic groups at http://www.ccie.net
> >> >>
> >> >>
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Received on Thu Mar 03 2011 - 10:19:03 ART