Cisco Systems WSC4500X16SFP Manual De Usuario
15-8
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 15 Understanding and Configuring Multiple Spanning Trees
MST Configuration Restrictions and Guidelines
MST-to-PVST+ Interoperability
Keep these guidelines in mind when you configure MST switches (in the same region) to interact with
PVST+ switches:
PVST+ switches:
•
Configure the root for all VLANs inside the MST region as shown in this example:
Switch# show spanning-tree mst interface gigabitethernet 1/1
GigabitEthernet1/1 of MST00 is root forwarding
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 switch at the boundary simulate PVST+ and send PVST+ BPDUs
for all the VLANs.
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 renewable loop guard on that PVST+ switch.
when the MST switches change their configuration. To correct the loop-inconsistent state, you must
disable and renewable 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.
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.
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 such a case, the topology changes are propagated
only in the instance to which the VLAN is mapped. The topology change stays local to the first MST
region, and the Cisco Access Manager (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.
switch does not pass beyond the first MST region. In such a case, the topology changes are propagated
only in the instance to which the VLAN is mapped. The topology change stays local to the first MST
region, and the Cisco Access Manager (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.
MST Configuration Restrictions and Guidelines
Follow these restrictions and guidelines to avoid configuration problems:
•
Do not disable spanning tree on any VLAN in any of the PVST bridges.
•
Do no use PVST bridges as the root of CST.
•
Do not connect switches with access links, because access links may partition a VLAN.
•
Ensure that all PVST root bridges have lower (numerically higher) priority than the CST root bridge.
•
Ensure that trunks carry all of the VLANs mapped to an instance or do not carry any VLANs at all
for this instance.
for this instance.
•
Complete any MST configuration that incorporates a large number of either existing or new logical
VLAN ports during a maintenance window because the complete MST database gets reinitialized
for any incremental change (such as adding new VLANs to instances or moving VLANs across
instances).
VLAN ports during a maintenance window because the complete MST database gets reinitialized
for any incremental change (such as adding new VLANs to instances or moving VLANs across
instances).