Forwarding State; Disabled State; Stp Address Management; Stp And Ieee 802.1Q Trunks - Cisco WS-C3550-12G Software Configuration Manual

Understanding Basic STP Features
•
•

Forwarding State

A Layer 2 interface in the forwarding state forwards frames. The Layer 2 interface enters the forwarding
state from the learning state.
A Layer 2 interface in the forwarding state performs as follows:
•
•
•
•

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or STP. A Layer 2
interface in the disabled state is nonoperational.
A disabled Layer 2 interface performs as follows:
•
•
•
•

STP 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 STP state, the switch receives but does not forward packets destined for addresses
between 0x0180c2000000 and 0x1080C200000F.
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 STP interoperability. It 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.
Catalyst 3550 Multilayer Switch Software Configuration Guide
10-8
Learns addresses
Receives BPDUs
Receives and forwards frames received on the port
Forwards frames switched from another port
Learns addresses
Receives BPDUs
Discards frames received on the port
Discards frames switched from another interface for forwarding
Does not learn addresses
Does not receive BPDUs
Chapter 10 Configuring STP
78-11194-03