04. Improving Performance with Spanning Tree

Interconnection Technologies

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1

 

Determining Equipment and Cabling Needs

Each link provides adequate bandwidth for the total aggregate traffic over that link.

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Advantages of EtherChannel

  • Logical aggregation of similar links between switches
  • Load-shares across links
  • Viewed as one logical port to STP
  • Redundancy
  • Picture3

 

Redundant Topology

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  • Redundant topology eliminates single points of failure.
  • Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems.

 

Broadcast Frames

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  • Station D sends a broadcast frame.
  • Broadcast frames are flooded to all ports except the originating port.

 

Broadcast Storms

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  • Host X sends a broadcast.
  • Switches continue to propagate broadcast traffic over and over.

 

Multiple Frame Copies

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  • Host X sends a unicast frame to router Y.
  • The MAC address of router Y has not been learned by either switch.
  • Router Y will receive two copies of the same frame.

 

MAC Database Instability

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  • Host X sends a unicast frame to router Y.
  • The MAC address of router Y has not been learned by either switch.
  • Switches A and B learn the MAC address of host X on port 1.
  • The frame to router Y is flooded.
  • Switches A and B incorrectly learn the MAC address of host X on port 2.

 

Loop Resolution with STP

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  • Provides a loop-free redundant network topology by placing certain ports in the blocking state
  • Published in the IEEE 802.1D specification
  • Enhanced with the Cisco PVST+ implementation

 

Spanning-Tree Operation

  • One root bridge per broadcast domain.
  • One root port per nonroot bridge.
  • One designated port per segment.
  • Nondesignated ports are unused.
  • Picture10

 

STP Root Bridge Selection

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2

 

Spanning-Tree Port States

Spanning tree transits each port through several different states:

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Describing PortFast

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PortFast is configured on access ports, not trunk ports.

 

Configuring and Verifying PortFast

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Spanning-Tree Operation Example

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Spanning-Tree Path Cost

4

 

Spanning-Tree Recalculation

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Per VLAN Spanning Tree Plus

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PVST+ Extended Bridge ID

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Rapid Spanning Tree Protocol

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Default Spanning-Tree Configuration

  • Cisco Catalyst switches support three types of STPs:

–PVST+

–PVRST+

–MSTP

  • The default STP for Cisco Catalyst switches is PVST+ :

–A separate STP instance for each VLAN

–One root bridge for all VLANs

–No load sharing

PVRST+ Configuration Guidelines

1.Enable PVRST+.

2.Designate and configure a switch to be the root bridge.

3.Designate and configure a switch to be the secondary root bridge.

4.Verify the configuration.

PVRST+ Implementation Commands

6

 

Verifying PVRST+

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Configuring the Root and Secondary Bridges

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Configuring the Root and Secondary Bridges: SwitchA

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Configuring the Root and Secondary Bridges: SwitchB

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Summary

  • A redundant switched topology includes multihomed switches and EtherChannel.
  • A redundant switched topology causes looping issues such as broadcast storms.
  • The 802.1D STP establishes a loop-free network.
  • The original STP has been enhanced by PVST+ and RSTP.

 


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