Mst-To-Pvst+ Interoperability; Understanding How Bpdu Skewing Works - Cisco WS-C6506 Software Manual

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:
•
•
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:
•
•
•
The skew causes BPDUs to reflood the network to keep the spanning-tree topology database current.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
7-24
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
Instance
VLANs Mapped:
Designated Root
Designated Root Priority
Designated Root Cost
Designated Root Port
Bridge ID MAC ADDR
Bridge ID Priority
Port
------------------------ ------------- ---- -------- ------------------------
6/1
Boundary(PVST)
6/2
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.
Spanning-tree timers lapse.
Expected BPDUs are not received by the switch.
Spanning tree detects topology changes.
MST
3
31-40
00-10-7b-bb-2f-00
8195 (root priority:8192, sys ID ext:3)
0 Remaining Hops 20
1/0
00-10-7b-bb-2f-00
8195 (bridge priority:8192, sys ID ext:3)
State Role Cost
forwarding BDRY
blocking BDRY
Chapter 7 Configuring Spanning Tree
Prio Type
10000 30 P2P,
20000 32 P2P,
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