Tài liệu giảng dạy CCNA - module 04 chapter 15-Spanning Tree Protocol

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Module 04 LAN Switching

Chapter 15

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Objectives

Upon completion of this chapter, you will be able to perform the following tasks:

Describe redundancy in switched networkDescribe how STP works

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Redundant Topology

Redundant topology eliminates single points of failure

Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems

Segment 1

Segment 2

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Multiple Frame Copies

•Host X sends an unicast frame to Router Y

•Router Y MAC Address has not been learned by either Switch yet

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MAC Database Instability

•Host X sends an unicast frame to Router Y

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MAC Database Instability

•Host X sends an unicast frame to Router Y

•Router Y MAC Address has not been learned by either Switch yet

•Switch A and B learn Host X MAC address on port 0

•Frame to Router Y is flooded

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Solution: Spanning-Tree Protocol

Provides a loop free redundant network topology by placing certain ports in the blocking state

Block

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Spanning-Tree Operations

Spanning-Tree Operations

•One root bridge per network

•One root port per nonroot bridge

•One designated port per segment

x

Designated port (F) Root port (F)

Designated port (F)Nondesignated port (B)

Root bridgeSW XSW YNonroot bridge100baseT

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Switch YDefault priority 32768 (8000 hex)MAC 0c0022222222Switch XDefault priority 32768 (8000 hex) MAC 0c0011111111

Spanning-Tree Protocol Root Bridge Selection

Spanning-Tree Protocol Root Bridge Selection

BPDU

BPDU = Bridge protocol data unit

(default = sent every 2 seconds)

Root bridge = Bridge with the lowest bridge IDBridge ID = Bridge priority + bridge MAC address

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Spanning-Tree Protocol Path Cost

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Switch YMAC 0c0022222222Default priority 32768Switch XMAC 0c0011111111Default priority 32768 Port 0Port 1Port 0Port 1Switch ZMac 0c0011110000Default priority 32768Port 0

Can you figure out:

• What is the root bridge?

• What are the designated, nondesignated, and root ports?100baseT

100baseT

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Switch YMAC 0c0022222222Default priority 32768Switch XMAC 0c0011111111Default priority 32768 Port 0Port 1Port 0Port 1Switch ZMac 0c0011110000Default priority 32768Port 0

Can you figure out:

• What is the root bridge?

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Blocking (20 sec)Listening (15 sec) Learning (15 sec)Forwarding

Spanning-Tree Port States

Spanning-Tree PortStates

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Reacting to Network Change

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Key Issue: Time to Convergence

•Convergence occurs when all the switch and bridge ports have transitioned to

either the forwarding or blocking state

•When network topology changes,

switches and bridges must recompute the Spanning-Tree Protocol, which

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Optional to SPT Features

Etherchanel

• Etherchanel provides a way to prevent STP

convergence from being needed when only one port or cable failed

•Etherchanel combines from 2 to 8 parallel

ethernet trunks between the same pair of switches-> etherchanel

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Optional to SPT Features

Portfast

• Portfast allows a switch to place a port in forwading state immediately when the port becomes physically active.

• The port should not connect to bridges, switches or other STP speaking device•The Cisco BPDU Guard feature, if

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Rapid Spanning Tree

Pt-pt link

Share link

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Summary

Upon completion of this chapter, you will be able to perform the following tasks:

Describe redundancy in switched networkDescribe how STP works