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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter14 Configuring STP Understanding Spanning-Tree Features
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN
basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that
VLAN to be subject to accelerated aging. Dynamic add re sses on ot h er V LAN s ca n be un affecte d an d
remain subject to the aging interval entered for the switch.
Spanning-Tree Modes and Protocols
The switch supports these spanning-tree modes and protocols:
PVST+—This spanning-tree mode is based on the IEEE 802.1D sta ndar d and Ci sco pr opriet ary
extensions. It is the default spanning-tree mode used on all Ethernet, Fast Ethernet, and Gigabit
Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum
supported, ensuring that each has a loop-free path through the n etwork.
The PVST+ provides Layer 2 load balancing for the VLAN on which it runs. You can create different
logical topologies by using the VLANs on your network to ensure that all of your links are used but
that no one link is oversubscribed. Each instance of PVST+ on a VLAN has a single root switch.
This root switch propagates the spanning-tree information associated with that VLAN to all other
switches in the network. Because each switch has the same information about the network, this
process ensures that the network topology is maintained.
•Rapid PVST+—This spanning-tree mode is the same as PVST+ except that is uses a rapid
convergence based on the IEEE 802.1w standard. To provide rapid convergence, the rapid PVST+
immediately deletes dynamically learned MAC address entries on a per-port basis upon receiving a
topology change. By contrast, PVST+ uses a short aging time for dynam ically learned MAC address
entries.
The rapid PVST+ uses the same configuration as PVST+ (except where noted) , and the sw itch needs
only minimal extra configuration. The benefit of rapid PVST+ is that you can migrate a large PVST+
install base to rapid PVST+ without having to learn the complexities of the MSTP configuration and
without having to reprovision your network. In rapid-PVST+ mode, each V LAN run s i ts own
spanning-tree instance up to the maximum supported.
•MSTP—This spanning-tree mode is based on the IEEE 802.1s standard. You can map multiple
VLANs to the same spanning-tree instance, which reduces the number of spanning-tree instances
required to support a large number of VLANs. The MSTP runs on top of the RSTP (based on IEEE
802.1w, which provides for rapid convergence of the spanning tree by eliminating the forward delay
and by quickly transitioning root ports and designated ports to the forwarding state. You cannot run
MSTP without RSTP.
The most common initial deployment of MSTP is in the backbone and distribution layers of a
Layer 2 switched network. For more information, see Chapter 15, “Configuring MSTP.” For
information about the number of supported spanning-tree instan ce s, see the next s ec tio n.
Supported Spanning-Tree Instances
In PVST+ or rapid-PVST+ mode, the switch supports up to 64 spanning-tree instances.
In MSTP mode, the switch supports up to 16 MST instances. The number of VLANs that can be mapped
to a particular MST instance is unlimited.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP ), see the
“Spanning-Tree Configuration Guidelines” section on page14-12.