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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP
Unders tanding Spanni ng-Tree Fea tures

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 fr ame s sw it ched fr om a noth er int erfac e fo r for wardi ng
•Does not learn addresses
•Does not receive BPDUs

Span ning- Tr ee Add re ss Ma na geme nt

IEEE 802.1D specifi es 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to
be used by di fferen t br idge p roto cols. The se addr ess es are st atic a ddre sses tha t c annot be re moved.
Regardless of the spanning-tree state, the switch receives but does not forward packets destined for
addresse s betw een 0x 018 0C20000 00 and 0x1 080 C200 000F.
If STP is ena bled, the swi tch C P U rec eives packets destine d for 0 x01 80C2 000000 a nd
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 requir es onl y one spanning-tree inst ance for all VLANs allowed on the trunks.
However, in a network of Cisco switche s connecte d through 802 .1Q trunks, the switches mai ntain 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. It combines the
spannin g-tr ee inst anc e of the 802.1Q VL AN of t he t runk w ith t he spa nning -tre e inst anc e of the
non-Cisc o 802.1Q switc h.
However, all PVST+ 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 li nk between the switc hes.
The extern al spanning -tree behavior on access ports and trunk por ts is not affecte d by PVST+.
For more inf orm ati on on 8 02. 1Q tr unks , s ee Chapter 14, “Configuring VLAN s.”

Spanning Tree and Redundant Connectivity

You can create a redundant backbone with spanning tree by connecting two switch interfaces to another
device or to two different devices. Spanning tree automatically disables one interface but enables it if
the other one fails, as shown in Figure 11-3. If o ne li nk is hi g h-sp eed an d the othe r is low-spee d, th e
low-speed link is always disabled. If the speeds are the same, the port priority and port ID are added
together, and spanning tree disables the link with the lowest value.