Cisco 7604 Configuration Manual page 299
Ios software configuration guide
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Also See for 7604:
- Configuration manual (742 pages) ,
- Installation manual (324 pages) ,
- User manual (98 pages)
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Chapter 20
Configuring STP and Prestandard IEEE 802.1s MST
When you enter the spanning-tree vlan vlan_ID root command, the router checks the bridge priority of
the current root bridges for each VLAN. When the extended system ID is disabled, the router sets the
bridge priority for the specified VLANs to 8192 if this value will cause the router to become the root for
the specified VLANs. When the extended system ID is enabled, the router sets the bridge priority for the
specified VLANs to 24576 if this value will cause the router to become the root for the specified VLANs.
If the extended system ID is disabled and if any root bridge for the specified VLANs has a bridge priority
lower than 8192, the router sets the bridge priority for the specified VLANs to 1 less than the lowest
bridge priority.
If the extended system ID is enabled and if any root bridge for the specified VLANs has a bridge priority
lower than 24576, the router sets the bridge priority for the specified VLANs to 4096 less than the lowest
bridge priority. (4096 is the value of the least significant bit of a 4-bit bridge priority value; see
Table 20-2 on page
The spanning-tree vlan vlan_ID root command fails if the value required to be the root bridge is less
Note
than 1.
The spanning-tree vlan vlan_ID root command can cause the following effects:
If the extended system ID is disabled, and if all network devices in VLAN 100 have the default
•
priority of 32768, entering the spanning-tree vlan 100 root primary command on the router sets
the bridge priority for VLAN 100 to 8192, which causes the router to become the root bridge for
VLAN 100.
If the extended system ID is enabled, and if all network devices in, for example, VLAN 20 have the
•
default priority of 32768, entering the spanning-tree vlan 20 root primary command on the router
sets the bridge priority to 24576, which causes the router to become the root bridge for VLAN 20.
The root bridge for each instance of STP should be a backbone or distribution router. Do not configure
Caution
an access router as the STP primary root.
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of
bridge hops between any two end stations in the Layer 2 network). When you specify the network
diameter, the Cisco 7600 series router automatically selects an optimal hello time, forward delay time,
and maximum age time for a network of that diameter, which can significantly reduce the STP
convergence time. You can use the hello keyword to override the automatically calculated hello time.
Note
To preserve a stable STP topology, we recommend that you avoid configuring the hello time, forward
delay time, and maximum age time manually after configuring the Cisco 7600 series router as the root
bridge.
OL-4266-08
20-3.)
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Configuring STP
20-25
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