CCNP Routing and Switching SWITCH 300-115 Official Cert Guide (2015)

Tuning Spanning-Tree Convergence

STP uses several timers, a sequence of states that ports must move through, and specific topology change conditions to prevent bridging loops from forming in a complex network. Each of these parameters or requirements is based on certain default values for a typical network size and function. For the majority of cases, the default STP operation is sufficient to keep the network loop free and enable users to communicate.

However, in certain situations, the default STP can cause network access to be delayed while timers expire and while preventing loops on links where loops are not possible. For example, when a single PC is connected to a switch port, a bridging loop is simply not possible. Another situation relates to the size of a Layer 2 switched network: The default STP timers are based on a benchmark network size.

In a network that is smaller, waiting until the default timer values expire might not make sense when they could be safely set to shorter values. In situations like this, you can safely make adjustments to the STP convergence process for more efficiency.

Modifying STP Timers

Recall that STP uses three timers to keep track of various port operation states and communication between bridges. The three STP timers can be adjusted by using the commands documented in the sections that follow. Remember that the timers need to be modified only on the root bridge because the root bridge propagates all three timer values throughout the network as fields in the configuration BPDU.

Manually Configuring STP Timers

Use one or more of the following global configuration commands to modify STP timers:

Switch(config)# spanning-tree [vlan vlan-id] hello-time seconds
Switch(config)# spanning-tree [vlan vlan-id] forward-time seconds
Switch(config)# spanning-tree [vlan vlan-id] max-age seconds

Notice that the timers can be changed for a single instance (VLAN) of STP on the switch by using the vlan vlan-id parameters. If you omit the vlan keyword, the timer values are configured for all instances (all VLANs) of STP on the switch.

The Hello timer triggers periodic “hello” (actually, the configuration BPDU) messages that are sent from the root to other bridges in the network. This timer also sets the interval in which a bridge expects to hear a hello relayed from its neighboring bridges. Configuration BPDUs are sent every 2 seconds, by default. You can modify the Hello timer with the hello-time keyword, along with a value of 1 to 10 seconds, as in the following command:

Switch(config)# spanning-tree hello-time 1

The Forward Delay timer determines the amount of time a port stays in the Listening state before moving into the Learning state, and how long it stays in the Learning state before moving to the Forwarding state. You can modify the Forward Delay timer with the forward-time keyword. The default value is 15 seconds, but this can be set to a value of 4 to 30 seconds. This timer should be modified only under careful consideration because the value depends on the diameter of the network and the propagation of BPDUs across all switches. A value that is too low allows loops to form, possibly crippling a network.

The Max Age timer specifies a stored BPDU’s lifetime that has been received from a neighboring switch with a designated port. Suppose that BPDUs are being received on a nondesignated switch port every 2 seconds, as expected. Then an indirect failure, or one that does not involve a physical link going down, occurs that prevents BPDUs from being sent. The receiving switch waits until the Max Age timer expires to listen for further BPDUs. If none is received, the nondesignated port moves into the Listening state, and the receiving switch generates configuration BPDUs. This port then becomes the designated port to restore connectivity on the segment.

To modify the Max Age timer, use the max-age keyword. The timer value defaults to 20 seconds but can be set from 6 to 40 seconds.

Automatically Configuring STP Timers

Modifying STP timers can be tricky, given the conservative nature of the default values and the calculations needed to derive proper STP operation. Timer values are basically dependent on the Hello Time and the switched network’s diameter, in terms of switch hops. Catalyst switches offer a single command that can change the timer values in a more controlled fashion. As described earlier, the spanning-tree vlan vlan-list root macro command is a better tool to use than setting the timers with the individual commands. This global configuration command has the following syntax:

Switch(config)# spanning-tree vlan vlan-list root {primary | secondary} [diameter
diameter [hello-time hello-time]]

Here, STP timers will be adjusted according to the formulas specified in the 802.1D standard by giving only the network’s diameter (the maximum number of switches that traffic will traverse across a Layer 2 network) and an optional hello-time. If you do not specify a hello time, the default value of 2 seconds is assumed.

This command can be used only on a per-VLAN basis to modify the timers for a particular VLAN’s spanning tree instance. The network diameter can be a value from 1 to 7 switch hops; the default STP timers are based on a diameter of 7. Because this command makes a switch become the root bridge, all the modified timer values resulting from this command will be propagated to other switches through the configuration BPDU.

Suppose, for example, that a small network consists of three switches connected in a triangle fashion. The command output in Example 7-6 shows the current (default) STP timer values that are in use for VLAN 100.

Example 7-6 Displaying the STP Timer Values in Use