On 11/27/2012 03:40 AM, Kell, Jeff wrote:
We're doing an eval on some PowerConnect 7048P switches, and have run
into spanning tree issues. They don't like PVST, but will spit out
STP that "in theory" will revert a Cisco to STP (is this process
contagious? or limited to the upstream?).
The STP process on vlan1 emits both Cisco PVST and IEEE PDUs IIRC, and
when it detects a PDU from the other end on a given port, switches to
just using that version of the protocol until the STP state machine is
reset (i.e. link down). I think there's also a CLI command to blip it.
So it's a per-port (well, per virtual-port, really) thing, similar to
the PVST/MST compat. mode that Ciscos do when running MST.
That unfortunately requires an untagged vlan 1 on all trunk ports
(breaks our standards and would require a mass reconfiguration).
Yeah, I always thought it was a bit sucky that the PVST fallback didn't
take place on the untagged/native VLAN per-port, as opposed to being
hardcoded to vlan1. How hard can it be to implement that?
However: IIRC the IEEE-fallback is of limited use in some topologies. If
the downstream (IEEE-only) switch blocks the port, then it blocks it for
all vlans. But, those vlan/port combos on the upstream Ciscos are in FWD
because they can't "hear" each other via that path. This can lead to
surprising behaviour.
Worse, if the blocked port is on the upstream Cisco, it's only blocked
for vlan 1 using this method. The other vlans are blocked by virtue of
the Ciscos hearing each others PVST via that path, and this comes with
the attendant 6/30 second delay on failover/failback.
You can also try MST if you want to convert (again, breaks our
standards and would require a mass reconfiguration).
Otherwise they seem quite tolerable and have some attractive
features... but I'm not quite sold if it requires a campus-wide mass
reconfiguration to get along with existing gear.
Personally I wouldn't go for it; but what the question? ;o)
Normally I'm not a big fan of "proprietary" protocols, but MST is so
awesomely sucky for Campus environments ("map all your VLANs to
instances before you start, and never change it" - yeah, right!) that we
mandate Cisco compatible PVST in all our edge.
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