Mac Address Allocation; Stp Address Management; Stp And Ieee 802.1Q Trunks; Stp And Redundant Connectivity - Cisco Catalyst 2950 Software Manual

Understanding Basic STP Features

MAC Address Allocation

The switch has a pool of MAC addresses, one for each instance of STP, that is used as the bridge IDs for
the VLAN spanning-tree instances. MAC addresses are allocated sequentially.

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.
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 link between the switches.
PVST+ is automatically enabled on 802.1Q trunks, and no user configuration is required. The external
spanning-tree behavior on access and port trunks is not affected by PVST+.
For more information on 802.1Q trunks, see

STP and Redundant Connectivity

You can create a redundant backbone with STP by connecting two switch interfaces to another device or
to two different devices. STP automatically disables one interface but enables it if the other one fails, as
shown in
disabled. If the speeds of the two links are the same, the port priority and port ID are added together, and
STP disables the link with the lowest value.
Catalyst 2950 Desktop Switch Software Configuration Guide
9-8
Figure 9-3. If one link is high-speed and the other is low-speed, the low-speed link is always
Chapter 8, "Configuring VLANs."
Chapter 9 Configuring STP
78-11380-03