Specifications

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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 12 Configuring STP
Understanding Spanning-Tree Features
Forwarding State
A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state
from the learning state.
An interface in the forwarding state performs as follows:
• Receives and forwards frames received on the port
• Forwards frames switched from another port
• Learns addresses
• Receives BPDUs
Disabled State
A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree.
An interface in the disabled state is nonoperational.
A disabled interface performs as follows:
• Discards frames received on the port
• Discards frames switched from another interface for forwarding
• Does not learn addresses
• Does not receive BPDUs
Spanning-Tree Address Management
IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to
be used by different bridge protocols. These addresses are static addresses that cannot be removed.
Regardless of the spanning-tree state, the switch receives but does not forward packets destined for
addresses between 0x0180C2000000 and 0x0180C200000F.
If STP is enabled, the switch CPU receives packets destined for 0x0180C2000000 and
0x0180C2000010. If STP is disabled, the switch forwards those packets as unknown multicast addresses.
STP and IEEE 802.1Q Trunks
The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for
a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks.
However, in a network of Cisco switches connected through 802.1Q trunks, the switches maintain one
spanning-tree instance for each VLAN allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch uses
per-VLAN spanning tree (PVST) to provide spanning-tree interoperability. If per-VLAN rapid spanning
tree (PVRST ) is enabled, the switch uses PVRST instead of PVST to provide spanning-tree
interoperability. The switch combines the spanning-tree instance of the 802.1Q VLAN of the trunk with
the spanning-tree instance of the non-Cisco 802.1Q switch.
However, all PVST or PVRST information is maintained by Cisco switches separated by a cloud of
non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a
single trunk link between the switches.