Rapid PerVLAN Spanning Tree Protocol

Chapter 8, "Traditional Spanning Tree Protocol," described PVST+ as the default STP mode on Catalyst switches. In PVST+, one spanning tree instance is created and used for each active VLAN that is defined on the switch. Each STP instance behaves according to the traditional 802.1D STP rules.

You can improve the efficiency of each STP instance by configuring a switch to begin using RSTP instead. This means that each VLAN will have its own independent instance of RSTP running on the switch. This mode is known as Rapid PVST+ (RPVST+).

You need only one configuration step to change the STP mode and begin using RPVST+. You can use the following global configuration command to accomplish this:

Switch(config)# spanning-tree mode rapid-pvst Be careful when you use this command on a production network because any STP process that is currently running must be restarted. This can cause functioning links to move through the traditional STP states, preventing data from flowing for a short time.

TIP To revert back to the default PVST+ mode, using traditional 802.1D STP, you can use the following command: Switch(config)# spanning-tree mode pvst

After you enable the RPVST+ mode, the switch must support both RSTP and 802.1D STP neighbors. The switch can detect the neighbor's STP type by the BPDU version that is received. You can see the neighbor type in the output of the show spanning-tree vlan vlan-id command, as demonstrated in Example 11-1.

Example 11-1 Detecting a Neighboring Switch's STP Type

Switch# show spanning-tree vlan

171

VLAN0171

Spanning

tree enabled protocol

rstp

Root ID

Priority

4267

Address

00d0.0457.38aa

Cost

3

Port

833 (Port-channel1)

Hello Time 2 sec

Max Age 20 sec Forward Delay

15 sec

Bridge ID

Priority

32939

(priority 32768 sys-id-ext 171

Address

0007.0d55.a800

Hello Time 2 sec

Max Age 20 sec Forward Delay

15 sec

Aging Time 300

Interface

Role

Sts Cost

Prio.Nbr Type

Gi7/8

Desg

FWD 4

128.392 P2p

Gi9/6

Altn

BLK 4

128.518 P2p Peer(STP)

Po1

Root

FWD 3

128.833 P2p

Po2

Desg

FWD 3

128.834 P2p

Po3

Desg

FWD 3

128.835 P2p

Switch#

The output in Example 11-1 shows information about the RSTP instance for VLAN 171. The first shaded line confirms that the local switch indeed is running RSTP. (The only other way to confirm the STP mode is to locate the spanning-tree mode command in the running configuration.)

In addition, this output displays all the active ports participating in the VLAN 171 instance of RSTP, along with their port types. The string P2p denotes a point-to-point RSTP port type in which a full-duplex link connects two neighboring switches that both are running RSTP. If you see P2p Peer(STP), the port is a point-to-point type but the neighboring device is running traditional 802.1D STP.

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