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Configuring MSTP
Information About Configuring MSTP
If the switch receives an IEEE 802.1D BPDU after the port migration-delay timer has expired, it assumes that it is
connected to an IEEE 802.1D switch and starts using only IEEE 802.1D BPDUs. However, if the RSTP switch is using
IEEE 802.1D BPDUs on a port and receives an RSTP BPDU after the timer has expired, it restarts the timer and starts
using RSTP BPDUs on that port.
Default MSTP Settings
MSTP Configuration Guidelines
These are the configuration guidelines for MSTP:
When you enable MST by using the spanning-tree mode mst global configuration command, RSTP is automatically
enabled.
For two or more switches to be in the same MST region, they must have the same VLAN-to-instance map, the same
configuration revision number, and the same name.
The switch supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance
is unlimited.
PVST+, rapid PVST+, and MSTP are supported, but only one version can be active at any time. (For example, all
VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For more information, see “Spanning-Tree
Interoperability and Backward Compatibility” section on page 10.
VTP propagation of the MST configuration is not supported. However, you can manually configure the MST
configuration (region name, revision number, and VLAN-to-instance mapping) on each switch within the MST region
by using the command-line interface (CLI) or through the SNMP support.
For load balancing across redundant paths in the network to work, all VLAN-to-instance mapping assignments must
match; otherwise, all traffic flows on a single link.
All MST boundary ports must be forwarding for load balancing between a PVST+ and an MST cloud or between a
rapid-PVST+ and an MST cloud. For this to occur, the IST master of the MST cloud should also be the root of the
CST. If the MST cloud consists of multiple MST regions, one of the MST regions must contain the CST root, and all
of the other MST regions must have a better path to the root contained within the MST cloud than a path through the
PVST+ or rapid-PVST+ cloud. You might have to manually configure the switches in the clouds.
Table 44 Default MSTP Settings
Feature Default Setting
Spanning-tree mode PVST+ (Rapid PVST+ and MSTP are disabled)
Switch priority (configurable on a per-CIST port basis) 32768
Spanning-tree port priority (configurable on a per-CIST port basis) 128
Spanning-tree port cost (configurable on a per-CIST port basis) 1000 Mbps: 4
100 Mbps: 19
10 Mbps: 100
Hello time 2 seconds
Forward-delay time 15 seconds
Maximum-aging time 20 seconds
Maximum hop count 20 hops
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