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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 1 Product Overview
Layer 2 Software Features
MST allows you to build multiple spanning trees over trunks. You can group and associate VLANs to
spanning tree instances. Each instance can have a topology independent of other spanning tree instances.
This new architecture provides multiple forwarding paths for data traffic and enables load balancing.
Network fault tolerance is improved because a failure in one instance (forwarding path) does not affect
other instances (forwarding paths).
For information on configuring MST, see Chapter 15, “Understanding and Configuring Multiple
Spanning Trees.”
PVRST+
Per-VLAN Rapid Spanning Tree (PVRST+) is the implementation of 802.1w on a per-VLAN basis. It is
the same as PVST+ with respect to STP mode and runs RSTP protocol based on 802.1w.
For information on configuring PVRST+, see Chapter 13, “Understanding and Configuring STP.”
QoS
The quality of service (QoS) feature prevents congestion by selecting network traffic and prioritizing it
according to its relative importance. Implementing QoS in your network makes network performance
more predictable and bandwidth use more effective.
The Catalyst 4500 series switch supports the following QoS features:
• Classification and marking
• Ingress and egress policing, including per-Port per-VLAN policing
• Sharing and shaping
Catalyst 4500 series switch supports trusted boundary, which uses the Cisco Discovery Protocol (CDP)
to detect the presence of a Cisco IP phone (such as the Cisco IP Phone 7910, 7935, 7940, and 7960) on
a switch port. If the telephone is not detected, the trusted boundary feature disables the trusted setting
on the switch port and prevents misuse of a high-priority queue.
The Catalyst 4500 series switch also supports QoS Automation (Auto QoS), which simplifies the
deployment of existing QoS features through automatic configuration.
For information on QoS and Auto QoS, see Chapter 27, “Configuring Quality of Service.”
Spanning Tree Protocol
The Spanning Tree Protocol (STP) allows you to create fault-tolerant internetworks that ensure an active,
loop-free data path between all nodes in the network. STP uses an algorithm to calculate the best
loop-free path throughout a switched network.
For information on configuring STP, see Chapter 13, “Understanding and Configuring STP.”
The Catalyst 4500 series switch supports the following STP enhancements:
• Spanning tree PortFast—PortFast allows a port with a directly attached host to transition to the
forwarding state directly, bypassing the listening and learning states.
• Spanning tree UplinkFast—UplinkFast provides fast convergence after a spanning-tree topology
change and achieves load balancing between redundant links using uplink groups. Uplink groups
provide an alternate path in case the currently forwarding link fails. UplinkFast is designed to
decrease spanning-tree convergence time for switches that experience a direct link failure.