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

 Upon completion of this chapter, you will be able to perform the following tasks:  Describe redundancy in switched network  Describe how STP works  Configure Spanning tree protocol

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Module 04 LAN Switching

Chapter 15 Spanning Tree Protocol

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 Upon completion of this chapter, you will be able to

perform the following tasks:

 Describe redundancy in switched network

 Describe how STP works

 Configure Spanning tree protocol

 Optional STP features

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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

• Router Y will receive two copies of the same frame

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MAC Database Instability

• Router Y MAC Address has not been learned by either

Port 0 Port 1

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MAC Database Instability

• 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

• Switch A and B incorrectly learn Host X MAC address on port 1

Port 0 Port 1

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 Complex topology can cause multiple loops to occur

 Layer 2 has no mechanism to stop the loop

Server/host

Workstations

Loop

Loop Loop Multiple Loop Problems

Broadcast

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Solution: Spanning-Tree Protocol

Provides a loop free redundant network topology by

placing certain ports in the blocking state

Block

x

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Spanning-Tree Operations

• One root bridge per network

• One root port per nonroot bridge

• One designated port per segment

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Switch Y Default priority 32768 (8000 hex)

Spanning-Tree Protocol Root Bridge Selection

BPDU

BPDU = Bridge protocol data unit

(default = sent every 2 seconds) Root bridge = Bridge with the lowest bridge ID Bridge ID = Bridge priority + bridge MAC address

In the example, which switch has the lowest bridge ID?

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Switch Y Default priority 32768 MAC 0c0022222222

10baseT

Nondesignated port (B) Designated port (F)

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Spanning-Tree Protocol Path Cost

Link Speed Cost (reratify IEEE spec) Cost (previous IEEE spec) -

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Switch Y MAC 0c0022222222 Default priority 32768

Can you figure out:

• What is the root bridge?

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

100baseT

Spanning-Tree:

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Can you figure out:

• What is the root bridge?

• What are the designated, nondesignated, and root parts?

• Which are the forwarding and blocking ports?

Root port (F)

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Blocking (20 sec) Listening

(15 sec) Learning

(15 sec) Forwarding

Spanning-Tree Port States

Spanning-tree transitions each port

through several different state:

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Reacting to Network Change

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x

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

disrupts user traffic

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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

•STP treats an etherchanel as a single link

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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

enable, tell the switch to disable Protfast

port of BPDU is received on those ports.

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Rapid Spanning Tree

Pt-pt link

Share link

Edge link

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