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8/3/2019 Ccna Press Stp

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SwitchingSwitching


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2002, Cisco Systems, Inc. All rights reserved.


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ObjectivesObjectives Upon completing thismodule, you willbe able

to:

Explain how bridging andswitching operates

Explain thepurpose andoperationsof the Spanning-

Tree Protocol

Verify thedefault configuration of thedevice, given afunctioning accesslayerswitch

Build a functional accessswitch configuration to

support thespecified networkoperationalparameters,given a networkdesign

Execute an add,move,or changeon an accesslayerswitch, given a new networkrequirement


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2002, Cisco Systems, Inc. All rights reserved. 4

Basic Layer 2 Switching and

Bridging Functions


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ObjectivesObjectives

Upon completing thislesson, you willbeable to:

Describe Layer 2 switching andbridging

operations andmodes Describe how LAN switchesuse andpopulate

the MAC address table


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Addresslearning

Forward/filterdecision

Loop avoidance

Ethernet Switches andBridgesEthernet Switches andBridges


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Cut-Through

Switch checks destination

address and immediately

begins forwarding frame.

Fragment-Free

Switch checks the first 64 bytes,

then immediatelybegins forwarding frame.

Store and Forward

Complete frame is received

and checked before

forwarding.

Transmitting Frames


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MAC Address TableMAC Address Table

Initial MAC address table is empty.


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Learning AddressesLearning Addresses

Station A sends a frame to station C.

Switch caches the MAC address of station A to port E0 bylearning the source address of data frames.

The frame from station A to station C is flooded out to all

ports except port E0 (unknown unicasts are flooded).


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Learning Addresses (Cont.)Learning Addresses (Cont.)

Station D sends a frame to station C.

Switch caches the MAC address of station D to port E3 bylearning the source address of data frames.

The frame from station D to station C is flooded out to all ports

except port E3 (unknown unicasts are flooded).


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Filtering FramesFiltering Frames

Station A sends a frame to station C.

Destination is known; frame is not flooded.


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Filtering Frames (Cont.)Filtering Frames (Cont.)

Station A sends a frame to station B.

The switch has the address for station B in the MACaddress table.


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Redundant topology eliminatessinglepointsoffailure.

Redundant topology causesbroadcast storms,multipleframe copies, and MAC address table instability problems.

Redundant TopologyRedundant Topology


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Host X sends a broadcast.

Switches continue to propagate broadcast traffic

over and over.

Broadcast StormsA 5.1.2.2


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Host X sends a unicast frame to router Y.

MAC address of router Y has not been learned by

either switch yet.

Router Y will receive two copies of the same frame.

Multiple Frame Copies orDuplicate Unicast Frames

A 5.1.2.3


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Host X sends a unicast frame to router Y.

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 0.

The frame to router Y is flooded.

Switches A and B incorrectly learn the MAC address of host X on port 1.

MAC Database Instability


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SpanningSpanning--Tree FunctionTree Function

STP logically block all alternativepathsforsolve theredundancy problem


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SpanningSpanning--Tree OperationTree Operation

1. One root bridge per network

2. One root port per nonroot bridge

3. One designated port per segment4. Nondesignated ports are unused

A 1.5.2.1STP Topology


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11--Root bridgeRoot bridge

1. Bpdu = Bridge Protocol Data Unit

(default = sent every two seconds)

2. Root bridge = Bridge with the lowest bridge ID

3. Bridge ID =

4. All port at the root bridge is designated forward

6 byte2 byte 6 byte2 byte 6 byte2 byte


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

S4 S3

S1 S2


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f)

S4 S3

S1 S2


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22-- One root port per nonrootOne root port per nonrootbridgebridge

Thelowest cost toRoot bridge

Ifequal thelowest brideID

Ifequal thelowest port no.


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Spanning-Tree Path Cost


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

S4 S3

S1 S2

Cost=19

Cost=4

Cost=19

Switch 2

Cost=19

Cost=4


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

S4 S3

S1 S2

Cost=4

Cost=19

Cost=19

Cost=19

Cost=19

Switch 3


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

R(F)

S4 S3

S1 S2

Cost=19

Cost=19

Cost=19

Cost=4

Cost=4

Switch 4


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

R(F)

S4 S3

S1 S2

D(f)

D(f)


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

R(F)

S4 S3

S1 S2

D(f)

D(f)


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44-- Non designatedports areNon designatedports areunusedunused


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100Mb/s

100Mb/s

100Mb/s

100Mb/s

D(f)

D(f) R(F)

R(F)

S4 S3

S1 S2

D(f)

D(f)ND(B)

ND(B)


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ExampleExample 22


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)

R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)

R(f)D(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)

R(f)D(f)

D(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)

R(f)D(f)

D(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

S4 S3

S1 S2

D(f)

D(f)

R(f)

R(f)

R(f)D(f)

D(f)

ND(B)


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What happen when link failedWhat happen when link failed??

Block port Steps to arrive at forward port


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1) Block : Wait 20 seconds without receive

BPDU.

2) Listening: theport can receive andsend

Bpdu and wait 15 second beforeenter in

thirdstep .

3) Learn:in thissteplearn mac address

and wait 15 second also .

4) Forward:

thefinalstep and can send


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55..22..55 STP Port States andSTP Port States andBPDU TimersBPDU Timers

55 22 33 B id IDB id ID


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55..22..33 Bridge IDBridge ID


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1. Toensure that theswitch has thelowest bridgepriority value,usethespanning-tree vlan vlan-idroot primary command in globalconfiguration mode. Thepriority for theswitch isset to thepredefinedvalueof 24576 or to the next 4096 increment valuebelow thelowest bridgepriority detectedon the network.

1. If an alternateroot bridge isdesired,use the spanning-treevlan vlan-id root secondary global configuration modecommand. This commandsets thepriority for theswitch to thepredefinedvalueof 28672. Thisensures that thisswitch becomestheroot bridge if theprimary root bridgefails and a new rootbridgeelection occurs and assuming that therest of theswitches

in the network have thedefault 32768 priority valuedefined.

2. In theexample,switch S1 hasbeen assigned as theprimary rootbridgeusing thespanning-tree vlan 1 root primary globalconfiguration mode command, andswitch S2 hasbeen configuredas thesecondary root bridgeusing thespanning-tree vlan 1 rootsecondary global configuration mode command.

3. Method 2 - Anothermethodfor configuring thebridgepriorityvalue isusing thespanning-tree vlan vlan-idpriority value globalconfiguration mode command. This command gives youmoregranular controlover thebridgepriority value. The priorityvalue is configured in increments of 4096 between 0 and65536.


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55..22..33 Bridge IDBridge ID


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The range value can be between 1 and 200,000,000.

Packet Tracer Don,t support this command


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The endThe end


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Cisco PortFast Technology

PortFast is a Cisco technology. When a switch port configured with

PortFast is configured as an access port, that port transitions from

blocking to forwarding state immediately,


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2

D(f)

D(f) R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2

D(f)

D(f) R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2

D(f)

D(f) R(f)

R(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2

D(f)

D(f) R(f)

R(f)

D(f)

D(f)


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1Gb/s

100Mb/s

1Gb/s

1Gb/s

SW4 SW3

SW1 SW2

D(f)

D(f) R(f)

R(f)

D(f)

D(f)

ND(B)


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SW1

SW2

SW3

100Mb/s

100Mb/s


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SW1

SW2

SW3

100Mb/s

100Mb/s

D(f)

D(f)


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SW1

SW2

SW3

100Mb/s

100Mb/s

D(f)

D(f)

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