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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 20 Configuring STP Understanding Spanning-Tree Features
Regardless of the spanning-tree state, each switch in the stack receives but does not forward packets
destined for addresses between 0x0180C2000000 and 0x0180C200000F.
If spanning tree is enabled, the CPU on the switch or on each switch in the stack receives packets
destined for 0x0180C2000000 and 0x0180C2000010. If spanning tree is disabled, the switch or each
switch in the stack forwards those packets as unknown multicast addresses.
Accelerated Aging to Retain Connectivity
The default for aging dynamic addresses is 5 minutes, the default setting of the mac address-table
aging-time global configuration command. However, a spanning-tree reconfiguration can cause many
station locations to change. Because these stations could be unreachable for 5 minutes or more during a
reconfiguration, the address-aging time is accelerated so that station addresses can be dropped from the
address table and then relearned. The accelerated aging is the same as the forward-delay parameter value
(spanning-tree vlan vlan-id forward-time seconds global configuration command) when the spanning
tree reconfigures.
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 addresses on other VLANs can be unaffected and
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 standard and Cisco proprietary
extensions. It is the default spanning-tree mode used on all 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 network.
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 dynamically learned MAC address
entries.
The rapid PVST+ uses the same configuration as PVST+ (except where noted), and the switch 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 VLAN runs its 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