Cisco IE 4000, Software Configuration Manual

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Configuring MSTP
Information About Configuring MSTP
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Partitioning the network into a large number of regions is not recommended. However, if this situation is unavoidable,
we recommend that you partition the switched LAN into smaller LANs interconnected by routers or non-Layer 2
devices.
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For configuration information about UplinkFast and BackboneFast, see “Information About Configuring the Optional
Spanning-Tree Features” section on page 1 .
MST Region Configuration and Enabling MSTP
For two or more switches to be in the same MST region, they must have the same VLAN-to-instance mapping, the same
configuration revision number, and the same name.
A region can have one member or multiple members with the same MST configuration; each member must be capable
of processing RSTP BPDUs. There is no limit to the number of MST regions in a network, but each region can only support
up to 65 spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.
Root Switch
The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A switch ID, consisting of the switch
priority and the switch MAC address, is associated with each instance. For a group of VLANs, the switch with the lowest
switch ID becomes the root switch.
To configure a switch to become the root, use the spanning-tree mst instance-id root global configuration command
to modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the
root switch for the specified spanning-tree instance. When you enter this command, the switch checks the switch
priorities of the root switches. Because of the extended system ID support, the switch sets its own priority for the
specified instance to 24576 if this value will cause this switch to become the root for the specified spanning-tree
instance.
If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own priority
to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value
as shown in
Table 1 on page 4 .)
If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch
with the extended system ID support will become the root switch. The extended system ID increases the switch priority
value every time the VLAN number is greater than the priority of the connected switches running older software.
The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an access
switch as the spanning-tree primary root.
Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is,
the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the
network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a
network of that diameter, which can significantly reduce the convergence time. You can use the
hello keyword to override
the automatically calculated hello time.
Secondary Root Switch
When you configure a switch with the extended system ID support as the secondary root, the switch priority is modified
from the default value (32768) to 28672. The switch is then likely to become the root switch for the specified instance if
the primary root switch fails. This is assuming that the other network switches use the default switch priority of 32768
and therefore are unlikely to become the root switch.
You can execute this command on more than one switch to configure multiple backup root switches. Use the same
network diameter and hello-time values that you used when you configured the primary root switch with the
spanning-tree mst instance-id root primary global configuration command.