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

1
Module 04 LAN Switching
Chapter 15
Spanning Tree Protocol
2
Objectives

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
3
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
Server/host X
Router Y
4
Broadcast Storms
Segment 1

Segment 2
Server/host X
Router Y
Broadcast
Switch A
Switch B
Host X sends a Broadcast
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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
Segment 1
Segment 2
Server/host X
Router Y

Unicast
Switch A
Switch B
Unicast
Unicast
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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
Segment 1
Segment 2
Server/host X
Router Y
Unicast
Unicast
Switch A Switch B
Port 0
Port 1
Port 0
Port 1
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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
•
Switch A and B incorrectly learn Host X MAC address on port 1
Segment 1
Segment 2

Server/host X
Router Y
Unicast
Unicast
Switch A
Switch B
Port 0
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
1

0
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 bridge Nonroot bridge
SW X
SW Y
100baseT
10baseT
1
1
Switch Y
Default priority 32768
(8000 hex)
MAC 0c0022222222
Switch X
Default priority 32768
(8000 hex)
MAC 0c0011111111

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?
1
2
Switch Y
Default priority 32768
MAC 0c0022222222
Switch X
Default priority 32768
MAC 0c0011111111
Spanning-Tree Protocol Port States
Root bridge
x
Port 0
Port 1
Port 0
Port 1
100baseT
10baseT


Designated port (F) Root port (F)
Nondesignated port (B)Designated port (F)
1
3
Spanning-Tree Protocol Path Cost
Link Speed Cost (reratify IEEE spec) Cost (previous IEEE spec)

10 Gbps 2 1
1 Gbps 4 1
100 Mbps 19 10
10 Mbps 100 100
1
4
Switch Y
MAC 0c0022222222
Default priority 32768
Switch X
MAC 0c0011111111
Default priority 32768
Port 0
Port 1
Port 0
Port 1
Switch Z
Mac 0c0011110000
Default priority 32768
Port 0
Can you figure out:
•
What is the root bridge?
•
What are the designated, nondesignated, and root ports?
•
Which are the forwarding and blocking ports?

100baseT
100baseT
Spanning-Tree:
1
5
Switch Y
MAC 0c0022222222
Default priority 32768
Switch X
MAC 0c0011111111
Default priority 32768
Port 0
Port 1
Port 0
Port 1
Switch Z
Mac 0c0011110000
Default priority 32768
Port 0
Can you figure out:
•
What is the root bridge?
•
What are the designated, nondesignated, and root parts?
•
Which are the forwarding and blocking ports?


100baseT
100baseT

Spanning-Tree:
Designated port (F)
Root port (F)
Nondesignated port (BLK)Designated port (F)
Root port (F)
1
6
Blocking
(20 sec)
Listening
(15 sec)
Learning
(15 sec)
Forwarding
Spanning-Tree Port States
Spanning-tree transitions each port
through several different state:
1
7
Reacting to Network Change
Switch Y
MAC 0c0022222222
Default priority 32768
Switch X
MAC 0c0011111111
Default priority 32768
Port 0
Port 1
Port 0
Port 1

10baseT
x
100baseT
Root Bridge
Designated port Root port (F)
Nondesignated port (BLK)Designated port
1
8
Switch Y
MAC 0c0022222222
Default priority 32768
Switch X
MAC 0c0011111111
Default priority 32768
Port 0
Port 1
Port 0
Port 1
10baseT
x
100baseT
Root Bridge
Designated port Root port (F)
Nondesignated port (BLK)Designated port
BPDU
x
MAXAGE
x
Reacting to Network Change
1

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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
2
0
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
2
1
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.
2
2
Rapid Spanning Tree
Pt-pt link
Share link
Edge link
2
3
Summary

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