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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 7 Configuring Spanning Tree
Understanding How BPDU Skewing Works
MST-to-PVST+ Interoperability
These guidelines apply in a topology where you configure MST switches (all in the same region) to
interact with PVST+ switches that have VLANs 1–100 set up to span throughout the network:
•
Configure the root for all VLANs inside the MST region. The ports that belong to the MST switch
at the boundary simulate PVST+ and send PVST+ BPDUs for all the VLANs. This example shows
the ports simulating PVST:
Console> (enable)
show spantree mst 3
Spanning tree mode MST
Instance 3
VLANs Mapped: 31-40
Designated Root 00-10-7b-bb-2f-00
Designated Root Priority 8195 (root priority:8192, sys ID ext:3)
Designated Root Cost 0 Remaining Hops 20
Designated Root Port 1/0
Bridge ID MAC ADDR 00-10-7b-bb-2f-00
Bridge ID Priority 8195 (bridge priority:8192, sys ID ext:3)
Port State Role Cost Prio Type
------------------------ ------------- ---- -------- ------------------------
6/1 forwarding BDRY 10000 30 P2P,
Boundary(PVST)
6/2 blocking BDRY 20000 32 P2P,
Boundary(PVST)
If you enable loop guard on the PVST+ switches, the ports might change to a loop-inconsistent state
when the MST switches change their configuration. To correct the loop-inconsistent state, you must
disable and reenable loop guard on that PVST+ switch.
•
Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST switch
because when the MST switch at the boundary receives PVST+ BPDUs for all or some of the
VLANs on its designated ports, root guard sets the port to the blocking state. Do not designate
switches with a slower CPU running PVST+ as a switch running MST.
When you connect a PVST+ switch to two different MST regions, the topology change from the PVST+
switch does not pass beyond the first MST region. In this case, the topology changes are only propagated
in the instance to which the VLAN is mapped. The topology change stays local to the first MST region
and the CAM entries in the other region are not flushed To make the topology change visible throughout
other MST regions, you can map that VLAN to IST or connect the PVST+ switch to the two regions
through access links.
Understanding How BPDU Skewing Works
BPDU skewing is the difference in time between when the BPDUs are expected to be received by the
switch and when the BPDUs are actually received by that switch. Skewing occurs as follows:
•
Spanning-tree timers lapse.
•
Expected BPDUs are not received by the switch.
•
Spanning tree detects topology changes.
The skew causes BPDUs to reflood the network to keep the spanning-tree topology database current.
