ccna3 3.0-07 spanning tree protocol

8/11/2019 Ccna3 3.0-07 Spanning Tree Protocol

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1Version 3.0

Module 7Spanning Tree

Protocol


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2Version 3.0

Redundancy

Redundancy in a network is needed in case there

is loss of connectivity in one segment.

But redundancy in itself presents problems

loops. The Spanning-Tree Protocol is used in switched

networks to create a loop free logical topology

from a physical topology that has loops.

Links, ports, and switches that are not part of theactive loop free topology do not participate in the

forwarding of data frames.


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3Version 3.0

Redundancy

Companies want 100% uptime, but 99.999%(5 nines) is the goal.

Remember the goal is reliability without

faults. Fault tolerance is achieved byredundancy.

Example of having 1 car versus 2 cars1is always availableredundancy

So companies should:

elim inate sing le po ints o f fai lure and

design alternate rou tes to a dest inat ion


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4Version 3.0

Reliability and 24x7 network

demands have compelled LAN

designers to construct multiple

paths between user and resource


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5Version 3.0

Redundant Switched Topologies

Again, if one path fails, the other path or

device can take over.

This is good, but there is a downside that has

to be accounted for:

Broadcast storms

Multiple (or duplicate) frame copies

MAC address table instabilities


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6Version 3.0

Redundant Paths and No

Spanning Tree. . .


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00-A1 BB-44

AA-11 CC-23

SAT

Port 1 Port 2

00-A1

SAT

Port 1 Port 2

00-A1

Port 1 Port 1

Port 2 Port 2

LAN Switch 1 LAN Switch 2

(1) 00-A1 sends frame to CC-23

Switch 1 and Switch 2 learn about 00-A1


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00-A1 BB-44

AA-11 CC-23

SAT

Port 1 Port 2

00-A1

SAT

Port 1 Port 2

00-A1

Port 1 Port 1

Port 2 Port 2


(2) LAN Switch 1 Floods packet out Port 2

since CC-23 is not known


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00-A1 BB-44

AA-11 CC-23

SAT

Port 1 Port 2

00-A1

SAT

Port 1 Port 2 00-A1

Port 1 Port 1

Port 2 Port 2


(3) LAN Switch 2 learns (incorrectly) that

(Source MAC ) 00-A1 is on Port 2


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Version 3.0

Or, A Broadcast

Storm. . .


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Version 3.0

Broadcast Storms, like ARP requests

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Host B


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Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Because it is a Layer 2 broadcast frame, both switches,

Switch A and Switch B, flood the frame out all ports,

including their port As.

Host B


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Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch A

Host A

A

1

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Duplicate

frameDuplicate

frame

Both switches receive the same broadcast, but on a

different port. Doing what switches do, both switches flood

the duplicate broadcast frame out their other ports.

Host B


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14Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

100BaseT Ports

A

Switch A

Switch B

A

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

DuplicateFrame

Duplicate

Frame

Here we go again, with the switches flooding the same

broadcast again out its other ports. This results in

duplicate frames, known as a broadcast storm!

Host A

Host B


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15Version 3.0

10BaseT Ports (12)

10BaseT Ports (12)

A

Switch A

Switch B

A

1 2

00-90-27-76-96-93

00-90-27-76-5D-FE

Hub

Layer 2 broadcasts not only take up network bandwidth,

but must be processed by each host. This can severely

impact a network, to the point of making it unusable.

Host A

Host B


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16Version 3.0

Redundant Topology

The traffic that switches flood out all ports can becaught in a loop, because in the Layer 2 headerthere is no TTL.

(Remember that in Layer 3 the TTL is

decremented and the packet is discarded whenthe TTL reaches 0)

You need switching (bridging) for reliability, butnow the problem of loopsa switched network

cannot have loops if it is to do what it issupposed to do.

Solution? Allow physical loops, but create a loop-free topology


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17Version 3.0

Spanning Tree

Protocol


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18Version 3.0

Standby Link

Switches forward broadcast frames

Prevents loops

Loops can cause broadcast storms and duplicate frames

Allows redundant links

Prunes topology to a minimal spanning tree

Resilient to topology changes and device failures

Main function of the Spanning Tree Protocol (STP) is to allow redundant

switched/bridged paths without suffering the effects of loops in the

network

Spanning Tree Protocol


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19Version 3.0

Root Bridge

RootBridge

Server

Server

= Backup Link

= Forwarding Path

The Spanning-Tree Protocol specifies an algorithm (Spanning-Tree Algorithm) that ultimately creates a logicalloop-free

topology

A

B

C

H

J

IE

G

FD


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20Version 3.0

The STA is used to calculate a loop-free logical

topology.

Spanning-tree frames called bridge protocol data units

(BPDUs) are sent and received by all switches in thenetwork at regular intervals and are used to determine

the spanning tree topology.

These BPDUs are used to determ ine the sho rtest path

to the root br idge, and wh ich po rts wi l l forward framesas part of the spanning tree BPDUs sent out every 2

seconds

A separate instance of STP runs within each

configured VLAN.

Spanning Tree Algorithm


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21Version 3.0

Spanning Tree

For every switchednetwork:

One root bridge

One root port per

non root bridgeOne designated

port per segment

Unused, non-

designated ports


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22Version 3.0

Step 1: Electing a Root Bridge

Bridge Priority

Bridge ID

Root BridgeStep 2: Electing Root Ports

Path Cost or Port Cost

Root Path Cost

Root Port

Step 3: Electing Designated Ports

Path Cost or Port Cost

Root Path Cost

3 Steps to Spanning Tree


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23Version 3.0

Step 1: Electing a Root Bridge The first step is for switches to select a

Root Bridge.

The root bridge is the bridge from which

all other paths are decided.

Only one switch can be the root bridge.

Election of a root bridge is decided by:

1. Lowest Bridge Priority

2. Lowest Bridge ID (tie-breaker)


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24Version 3.0

Bridge Priority

This is a numerical value.

The switch with the with the lowest bridge

priority is the root bridge.

The switches use BPDUs to accomplishthis.

All switches consider themselves as the

root bridge until they find out otherwise.

All Cisco Catalyst switches have the

default Bridge priority of 32768.


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25Version 3.0

A B

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseT

Ports

100BaseT

Ports

100BaseT

Ports

Bridge Priorities


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26Version 3.0

Catalyst 1900 - Spanning Tree Configuration - Option 1

----------------------- Information ------------------------------------

[V] VLANs assigned to option 1-1005----------------------- Settings ---------------------------------------[B] Bridge priority 32768 (8000 hex)[M] Max age when operating as root 20 second(s)[H] Hello time when operating as root 2 second(s)

[F] Forward delay when operating as root 15 second(s)

Switch A: Bridge Priority


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27Version 3.0

In case of a tie, the Bridge ID is used

Bridge ID The Bridge ID is the MAC addressassigned

to the individual switch.

The lower Bridge ID (MAC address) is the

tiebreaker.

Because MAC addresses are unique, this

ensures that only one bridge will have the

lowest value.

NOTE: There are other tie breakers, if these

values are not unique, but we will not cover

those situations.


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28Version 3.0

Catalyst 1900 Management ConsoleCopyright (c) Cisco Systems, Inc. 1993-1998

All rights reserved.Enterprise Edition Software

Ethernet Address: 00-B0-64-26-6D-00

PCA Number: 73-3122-04PCA Serial Number: FAB03503222Model Number: WS-C1912-EN

System Serial Number: FAB0351U08MPower Supply S/N: PHI033301VQPCB Serial Number: FAB03503222,73-3122-04


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29Version 3.0

A B

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseTPorts

100BaseT

Ports

Priority: 32768 ID: 00-B0-64-26-6D-00

Priority: 32768 ID: 00-B0-64-58-CB-80

Priority: 32768 ID: 00-B0-64-58-DC-00

Bridge Priorities and Bridge Ids

Which one is the lowest?


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30Version 3.0

A B

A B

1

1

A

B

C

10BaseT Ports (12)

10BaseT Ports (24)

10BaseT Ports (24)

100BaseT

Ports

Priority: 32768 ID: 00-B0-64-26-6D-00

Priority: 32768 ID: 00-B0-64-58-CB-80

Priority: 32768 ID: 00-B0-64-58-DC-00

Lowest: A becomes the root bridge

A B


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31Version 3.0

States initially set, later modified by STP

Server ports can be configured to

immediately enter STP forward mode

Understanding STP States

Blocking

Listening Learning

Forwarding

Disabled


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32Version 3.0

Blocking- No frames forwarded, BPDUsreceived

Listening- No frames forwarded,

listening for frames

Learning- No frames forwarded, but

learning MAC addresses

ForwardingReceiving BPDUs,

Forwarding data traffic, receiving datatraffic, learns MAC addresses

Disabled- No frames forwarded, no

BPDUs heard

Understanding STP States

50 seconds

from

blocking to

forwarding


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33Version 3.0

Rapid Spanning Tree Protocol

IEEE 802.1w

Will eventually replace 802.1d

Port states and roles will be clarified

A set of link types will be defined that will allow going to a

forwarding stage quicker

All switches will generate their own BPDUs instead of relying on

the root bridge.

Link types would be:

Point to point Edge-type

Shared

Can go to forward stateimmediately


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Module 7Spanning Tree

Protocol

ccna3 3.0-07 spanning tree protocol

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