Cisco WS-X6416-GE-MT - Interface Module - Expansion Instrukcja Użytkownika Pobierz pdf (Strona 24)

White Paper

Spanning Tree Protocol

Spanning Tree Protocol prevents loops from being formed when switches or bridges are

interconnected using multiple paths. Spanning Tree Protocol implements the 802.1D IEEE

algorithm by exchanging BPDU messages with other switches to detect loops and then removes

the loop by shutting down selected bridge interfaces. This algorithm guarantees that there is one

and only one active path between two network devices.

Common Spanning Tree (CST) assumes one spanning-tree instance for the entire bridged

network, regardless of the number of VLANs. This implementation reduces CPU load since only

one Spanning Tree instance is maintained for the entire network. This implementation can be used

when only one Layer 2 topology is needed in the network.

Multiple Instance Spanning Tree Protocol (MISTP) (802.1s) is an IEEE standard that allows

several VLANs to be mapped to a reduced number of spanning-tree instances. This is possible

since most networks do not need more than a few logical topologies. Each instance handles

multiple VLANs that have the same Layer 2 topology.

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in

the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while

blocking it for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability

to load balance traffic (at Layer 2) by forwarding some VLANs on one trunk and other VLANs on

another trunk without causing a Spanning Tree Protocol loop. PVST+ (additional advantages are

described later) provides the same functionality with 802.1Q trunking technology and is only

supported on Cisco switches.

Rapid Spanning Tree Protocol (RSTP) is an evolution of Spanning Tree Protocol (802.1D

standard) and provides for faster spanning tree convergence after a topology change. The

standard also includes features equivalent to Cisco PortFast, UplinkFast, and BackboneFast for

faster network reconvergence.

This section presents the configuration differences between the Cisco Catalyst OS and Cisco IOS

Software for basic Spanning Tree Protocol configuration, PVST+ (802.1d), IEEE 802.1s (MST),

IEEE 802.1w (RSTP), and Rapid PVST+.

Basic Spanning Tree Protocol Configuration

Cisco Catalyst OS Cisco IOS Software

set spantree root 10 dia 5 hello 2

set spantree root sec 11 dia 5 hello 2

set spantree priority 4096 10,11

IOS(config)# spanning-tree vlan 10 root

primary dia 5 hello 2

IOS(config)# spanning-tree vlan 11 root

sec dia 5 hello 2

IOS(config)# spanning-tree vlan 10 pri 4096

IOS(config)# spanning-tree vlan 11 pri 4096

PVST Enhancements

PVST+ enhances basic spanning tree algorithms by allowing for faster convergence times using

the implementation and integration of Cisco proprietary protocols, including UplinkFast,

BackboneFast, and PortFast, into the PVST+ protocol itself.

Spanning Tree UplinkFast allows for faster convergence in a Layer 2 network after a direct root link

failure. If a link from one bridge to the root bridge goes down, then the bridge will move one

blocking port to forwarding immediately rather than waiting for the normal spanning tree timers to

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