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13-6
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 13 Understanding and Configuring STP
Default STP Configuration
STP and IEEE 802.1Q Trunks
802.1Q VLAN trunks impose some limitations on the spanning tree strategy for a network. In a network
of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree
for each VLAN allowed on the trunks. However, non-Cisco 802.1Q switches maintain only one instance
of spanning tree for all VLANs allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device (that supports 802.1Q) through an 802.1Q trunk,
the Cisco switch combines the spanning tree instance of the 802.1Q native VLAN of the trunk with the
spanning tree instance of the non-Cisco 802.1Q switch. However, all per-VLAN spanning tree
information is maintained by Cisco switches separated by a network of non-Cisco 802.1Q switches. The
non-Cisco 802.1Q network separating the Cisco switches is treated as a single trunk link between the
switches.
Note For more information on 802.1Q trunks, see Chapter 11, “Configuring Layer 2 Ethernet Interfaces.”
Per-VLAN Rapid Spanning Tree
Per-VLAN Rapid Spanning Tree (PVRST+) is the same as PVST+, although PVRST+ utilizes a rapid
STP based on IEEE 802.1w rather than 802.1D to provide faster convergence. PVRST+ uses roughly the
same configuration as PVST+ and needs only minimal configuration. In PVRST+, dynamic CAM entries
are flushed immediately on a per-port basis when any topology change is made. UplinkFast and
BackboneFast are enabled but not active in this mode, since the functionality is built into the Rapid STP.
PVRST+ provides for rapid recovery of connectivity following the failure of a bridge, bridge port, or
LAN.
For enabling information, see “Enabling Per-VLAN Rapid Spanning Tree” on page 20.
Default STP Configuration
Table 13-4 shows the default spanning tree configuration.
Table 13-4 Spanning Tree Default Configuration Values
Feature Default Value
Enable state Spanning tree enabled for all VLANs
Bridge priority value 32,768
Spanning tree port priority value (configurable on a
per-interface basis—used on interfaces configured as
Layer 2 access ports)
128
Spanning tree port cost (configurable on a per-interface
basis—used on interfaces configured as Layer 2 access
ports)
• 10-Gigabit Ethernet: 2
• Gigabit Ethernet: 4
• Fast Ethernet: 19
Spanning tree VLAN port priority value (configurable on
a per-VLAN basis—used on interfaces configured as
Layer 2 trunk ports)
128