Bridge parameters

These parameters are set in STP, RSTP, MSTP, PVST+, and R-PVST+.

Bridge priority

Use this parameter to specify the priority of a device and to determine the root bridge.

Each device has a unique bridge identifier called the bridge ID. The bridge ID is an 8 byte value that is composed of two fields: a 2 B bridge priority field and the 6 B MAC address field. The value for the bridge priority ranges from 0 to 61440 in increments of 4096. The default value for the bridge priority is 32768. You use the bridge-priority command to set the appropriate values to designate a device as the root bridge or root device. A default bridge ID may appear as 32768.768e.f805.5800. If the bridge priorities are equal, the device with the lowest MAC address is elected the root.

After you decide what device to designate as the root, you set the appropriate device bridge priorities. The device with the lowest bridge priority becomes the root device. When a device has a bridge priority that is lower than that of all the other devices, it is automatically selected as the root.

The root device should be centrally located and not in a "disruptive" location. Backbone devices typically serve as the root because they usually do not connect to end stations. All other decisions in the network, such as which port to block and which port to put in forwarding mode, are made from the perspective of the root device.

You may also specify the bridge priority for a specific VLAN. If the VLAN parameter is not provided, the priority value is applied globally for all per-VLAN instances. However, for the VLANs that have been configured explicitly, the per-VLAN configuration takes precedence over the global configuration.

Bridge Protocol data units (BPDUs) carry information between devices. All the devices in the Layer 2 network, participating in any variety of STP, gather information on other devices in the network through an exchange of BPDUs. As the result of exchange of the BPDUs, the device with the lowest bridge ID is elected as the root bridge

When setting the bridge forward delay, bridge maximum aging time, and the hello time parameters keep in mind that the following relationship should be kept:

(2 × (forward-delay — 1)) ≥ max-age ≥ (2 ×(hello-time + 1))

Bridge forward delay

The bridge forward delay parameter specifies how long an interface remains in the listening and learning states before the interface begins forwarding all spanning tree instances. The valid range is from 4 through 30 seconds. The default is 15 seconds.

Additionally, you may specify the forward delay for a specific VLAN. If the VLAN parameter is not provided, the bridge forward delay value is applied globally for all per-VLAN instances. However, for the VLANs that have been configured explicitly, the per-VLAN configuration takes precedence over the global configuration.

Bridge maximum aging time

You can use this setting to configure the maximum length of time that passes before an interface saves its BPDU configuration information.

Keeping with the inequality shown above, when configuring the maximum aging time, you must set the value greater than the hello time. The range of values is 6 through 40 seconds while he default is 20 seconds.

You may specify the maximum aging for a specific VLAN. If the VLAN parameter is not provided, the priority value is applied globally for all per-VLAN instances. However, for the VLANs that have been configured explicitly, the per-VLAN configuration takes precedence over the global configuration.

Bridge hello time

You can use this parameter to set how often the device interface broadcasts hello BPDUs to other devices.

Use the hello-time command to configure the bridge hello time. The range is from 1 through 10 seconds. The default is 2 seconds.

You may also specify the hello time for a specific VLAN. If the VLAN parameter is not provided, the priority value is applied globally for all per-VLAN instances. However, for the VLANs that have been configured explicitly, the per-VLAN configuration takes precedence over the global configuration.