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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08
Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST
Understanding How Prestandard IEEE 802.1s MST Works
Edge port: no (trunk) port guard : none (default)
Link type: point-to-point (auto) bpdu filter: disable (default)
Boundary : boundary (PVST) bpdu guard : disable (default)
Bpdus sent 10, received 310
Instance Role Sts Cost Prio.Nbr Vlans mapped
-------- ---- --- --------- -------- -------------------------------
0 Root FWD 20000 128.1 1-2,4-2999,4000-4094
3 Boun FWD 20000 128.1 3,3000-3999
The ports that belong to the MST router at the boundary simulate PVST+ and send PVST+ BPDUs
for all the VLANs.
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+ router.
•
Do not locate the root for some or all of the VLANs inside the PVST+ side of the MST router
because when the MST router 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 router running MST.
When you connect a PVST+ router to two different MST regions, the topology change from the PVST+
router 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+ router to the two regions
through access links.
Common Spanning Tree
CST (802.1Q) is a single spanning tree for all the VLANs. In a Cisco 7600 series router running PVST+,
the VLAN 1 spanning tree corresponds to CST. In a Cisco 7600 series router running MST, IST (instance
0) corresponds to CST.
MST Instances
This release supports up to 16 instances; each spanning tree instance is identified by an instance ID that
ranges from 0 to 15. Instance 0 is mandatory and is always present. Instances 1 through 15 are optional.
MST Configuration Parameters
MST configuration includes these three parts:
•
Name—A 32-character string (null padded) identifying the MST region.
•
Revision number—An unsigned 16-bit number that identifies the revision of the current MST
configuration.
Note
You must set the revision number when required as part of the MST configuration. The
revision number is not incremented automatically each time you commit the MST
configuration.
