Cisco Systems IE 2000 Default MSTP Settings, MSTP Configuration Guidelines

21-13
Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 21 Configuring MSTP
Information About Configuring MSTP
• Propagation—When an RSTP switch receives a TC message from another switch through a
designated or root port, it propagates the change to all o f its nonedge, designated ports and to the
root port (excluding the port on which it is received). The switch starts the TC-while timer for all
such ports and flushes the information learned on them.
• Protocol migration—For backward compatibility with IEEE 802.1D switches, RSTP selectively
sends IEEE 802.1D configuration BPDUs and TCN BPDUs on a per-port basis.
When a port is initialized, the migrate-delay timer is started (specifies the minimum time during
which RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch
processes all BPDUs received on that port and ignores the protocol type.
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 start s 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 VLAN s run MSTP.) For more
information, see the “Spanning-Tree Interoperability and Backward Compatibility” section on
page 20-10.
Tab l e 21-4 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