Configuring The Root Bridge - Cisco 7604 Configuration Manual
Ios software configuration guide
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Chapter 19
Configuring Standard-Compliant IEEE MST
Configuring MST
Configuring the Root Bridge
The router maintains a spanning tree instance for the group of VLANs mapped to it. A switch ID,
consisting of the switch priority and the router MAC address, is associated with each instance. For a
group of VLANs, the router with the lowest switch ID becomes the root bridge.
To configure a router to become the root bridge, 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 router becomes the root bridge for the specified spanning tree instance. When you
enter this command, the router checks the switch priorities of the root bridges. Because of the extended
system ID support, the router sets its own priority for the specified instance to 24576 if this value will
cause this router to become the root bridge for the specified spanning tree instance.
If any root bridge for the specified instance has a switch priority lower than 24576, the router 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 20-2 on page
20-3.)
If your network consists of routers that both do and do not support the extended system ID, it is unlikely
that the router with the extended system ID support will become the root bridge. The extended system
ID increases the switch priority value every time the VLAN number is greater than the priority of the
connected routers running older software.
The root bridge for each spanning tree instance should be a backbone or distribution router. Do not
configure an access router as the spanning tree primary root bridge.
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 Layer 2 hops between any two end stations in the Layer 2
network). When you specify the network diameter, the router 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.
With the router configured as the root bridge, do not manually configure the hello time, forward-delay
Note
time, and maximum-age time with the spanning-tree mst hello-time, spanning-tree mst
forward-time, and spanning-tree mst max-age global configuration commands.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
19-19
OL-4266-08
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