Introduction Multiple Spanning Tree Protocol (MSTP)
Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)
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For Bridges that are running MSTP, the priority component of the Port ID is repeated for the CIST, and each
MSTI defined for the bridge. This allows the user complete independent control over the port config-
urations for each Spanning Tree instance.
4.5.8.5 Port Path Costs
Port path costs are used in root path cost calculations, which are a factor in selecting root ports and designated
bridges. By default, port path costs are related to the bandwidth capacity of the ports; however, the default val-
ues may be changed by the user to reflect other factors (e.g. propagation delay, link quality, desired traffic level,
etc.)
The values for port path costs are listed in 4.5.3.9. For MSTP, the internal port path cost and the external port
path cost are represented by one port path cost parameter described there.
4.5.8.6 Timer Durations
The spanning tree convergence process is dependent on certain timers, explained in 4.5.3.10. As explained
there, RSTP has a parameter, Migrate Time, with a set default of 3 seconds.
4.5.8.7 Port participation
If ports on a switch are members of an extended LAN or VLAN that does not require use of the spanning tree
protocol (i.e., if the VLAN is administered such that no network loops could exist), then spanning tree protocol
operations can be disabled for those ports. However, if a port is a member of multiple VLANs, then the span-
ning tree protocol must be enabled on that port for all those VLANs or none of them; a mixed configuration is
not supported.
If spanning tree protocol operations are disabled for a port, it may still pass bearer traffic to and from other
ports, regardless of whether or not the spanning tree protocol is used for those other ports.
For bridges that run MSTP, port participation in the spanning tree may be disabled on a per MSTI basis. This
means that VLAN traffic associated with the disabled MSTI may flow freely through those ports
Note: Spanning tree instances (MSTIs) themselves may not be disabled individually.
4.5.8.8 Force Version
Refer to 4.5.3.12. The value specifies whether STP or RSTP is to be used on a bridge. For MSTP the value 2
(RSTP) is used.
4.5.8.9 Edge Port
Refer to 4.5.3.13, and is part of RSTP processing in that it identifies a port is directly connected to the host. For
MSTP, this value is also used.
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