Exploration LAN Switching Chapter5-Updated

8/3/2019 Exploration LAN Switching Chapter5-Updated

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Chapter 5

CCNA 3

STP Spanning Tree Protocol


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More detail than you need to know

2

In this presentation we will discussmuch of the detail of STP.

Much of the detail is not needed forCCNA, however we will discuss it toget a better understanding of how

STP operates.

I am not concerned that youcompletely understand orremember the detail, but rather getan appreciation for what STP is

doing.

Even with the added detail, muchmore detail has been intentionallyleft out and will be discussed inCCNP 3


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Configuring STP

3

By default, STP is enabled for every port on theswitch.

If for some reason STP has been disabled, youcan reenable it.

To re-enable STP, use the

Switch(config)#spanning-tree vlanvlan-id

To disable STP, on a per-VLAN basis:

Switch(config)#no spanning-tree vlanvlan-id


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Spanning Tree Protocol (STP)

4

STP is a loop-preventionprotocol

Uses the Spanning TreeAlogithm

STP allows L2 devices tocommunicate with each otherto discover physical loops inthe network.

STP specifies an algorithmthat L2 devices can use tocreate a loop-free logicaltopology.

STP creates a tree structure ofloop-free leaves and branchesthat spans the entire Layer 2

network.


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Redundancy Creates Loops

5


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Spanning Tree Only for Loops

Rick Graziani [email protected]

6

Loops may occur in yournetwork as part of a adesign strategy forredundancy.

STP is not needed ifthere are no loops inyour network.

However, DO NOT

disable STP! Loops can occur

accidentally fromnetwork staff or even

users!

Two users interconnecting theswitches in their cubicles.


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L2 Loops


7

Broadcasts and Layer 2loops can be a dangerouscombination.

Ethernet frames have noTTL field

After an Ethernet framestarts to loop, it will probablycontinue until someoneshuts off one of the switchesor breaks a link.

IP Packet


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L2 Loops - Flooded unicast frames


8

Bridge loops can occur any

time there is a redundantpath or loop in the bridgenetwork.

The switches will flip flop thebridging table entry for

Station A (creating extremelyhigh CPU utilization).

Bridge Loops can cause:

Broadcast storms

Multiple copies of Ethernetframes

MAC address tableinstability in switches


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STP Prevents Loops

17

The purpose of STP is to avoid and eliminate loops in the network by

negotiating a loop-free path through a root bridge. STP determines where the are loops and blocks links that are redundant.

Ensures that there will be only one active path to every destination.

X


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Spanning Tree Algorithm

18

STP executes an algorithmcalled Spanning TreeAlgorithm.

STA chooses a referencepoint, called a root bridge,and then determines the

available paths to thatreference point.

If more than two pathsexists, STA picks the bestpath and blocks the rest

X


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Two-key STP Concepts

19

STP calculations make extensive use of two key concepts

in creating a loop-free topology: Bridge ID

Path Cost

Link SpeedCost (Revised IEEESpec)

Cost (Previous IEEESpec)

10 Gbps 2 11 Gbps 4 1

100 Mbps 19 10

10 Mbps 100 100


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Bridge ID (BID)

20

Bridge ID (BID) is used to identify each bridge/switch.

The BID is used in determining the center of the network, in respect toSTP, known as the root bridge.

Consists of two components:

A 2-byte Bridge Priority: Cisco switch defaults to 32,768 or0x8000.

A 6-byte MAC address Bridge Priority is usually expressed in decimalformat and the MAC

address in the BID is usually expressed in hexadecimalformat.


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Priority = Priority (Default 32,768) +VLAN

21

Access2#show spanning-tree

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 24577

Address 000f.2490.1380

Cost 23

Port 1 (FastEthernet0/1)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)

Address 0009.7c0b.e7c0


Aging Time 300

VLAN0010



Address 000b.fd13.9080

Cost 19






Aging Time 300


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Bridge ID (BID)


22

Used to elect a root bridge (coming) Lowest Bridge ID is the root.

If all devices have the same priority, the bridge with the lowest MACaddress becomes the root bridge. (Yikes)


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Five-Step STP Decision Sequence

23

When creating a loop-free topology, STP always uses the samefive-step decision sequence:

Five-Step decision Sequence

Step 1 - Lowest BID

Step 2 - Lowest Path Cost to Root Bridge

Step 3 - Lowest Sender BID

Step 4 Lowest Port Priority

Step 5 - Lowest Port ID

Bridges use Configuration BPDUs during this four-step process.

We will assume all BPDUs are configuration BPDUs untilotherwise noted.


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Five-Step STP Decision Sequence

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BPDU key concepts:

Bridges save a copy of only the best BPDU seen on every port.

When making this evaluation, it considers all of the BPDUsreceived on the port, as well as the BPDU that would be sent onthat port.

As every BPDU arrives, it is checked against this five-stepsequence to see if it is more attractive (lower in value) than theexisting BPDU saved for that port.

Only the lowest value BPDU is saved.

Bridges send configuration BPDUs until a more attractive BPDU is

received. Okay, lets see how this is used...


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Elect one Root Bridge

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The STP algorithm uses three simple steps to converge on a loop-free topology:

STP ConvergenceStep 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports

When the network first starts, all bridges are announcing achaotic mix of BPDUs.

All bridges immediately begin applying the five-step sequencedecision process.

Switches need to elect a single Root Bridge. Switch with the lowest BID wins!

Note: Many texts refer to the term highest priority which is thelowest BID value.

This is known as the Root War.


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Elect one Root BridgeLowest BID wins!

26

32768-000f.2490.1380

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0


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27

32768-000f.2490.1380

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

El t R t B id


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Its all done with BPDUs!

Sent every 2 seconds!

28


Determines shortest path to Root Bridge

Determines which ports will forward frames.


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Root Bridge Selection Criteria

29

At the beginning, all bridges assume they are the center of theuniverse and declare themselves as the Root Bridge, by placingits own BID in the Root BID field of the BPDU.


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30

Once all of the switches see that Access2 has the lowest BID, they areall in agreement that Access2 is the Root Bridge.

32768-000f.2490.1380

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge


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Rigging the Root Bridge Election

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The switch with the lowest BID becomes the root. The root switch can be determined by lowering the priority on that

switch, below the default of 32768.

There are two ways to lower the priority on Switch-2 to make it theRoot Bridge

Switch-2(config)#spanning-tree vlan 1 root primary

or

Switch-2(config)#spanning-tree vlan 1 priority 4096

The spanning-tree vlan 1 priority 4096 command lowers thepriority from 32768 to 4096, thus making it the root switch.

The spanning-tree vlan 1 root primary command lowers thepriority to 24576 (on a 2950 switch), thus making it the root switch.


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Elect Root Ports

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STP ConvergenceStep 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports

Now that the Root War has been won, switches move on toselecting Root Ports.

A bridges Root Port is the port closest to the Root Bridge.

Bridges use the cost to determine closeness.

Every non-Root Bridge will select one Root Port!

Specifically, bridges track the Root Path Cost, the cumulativecost of all links to the Root Bridge.

Difference b/t Path Cost and Root Path Cost Root Path Cost


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34

Difference b/t Path Cost and Root Path Cost

Path Cost:

The value assigned to each port.

Added to BPDUs received on that port tocalculate Root Path Cost.

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root BridgeBPDU

Cost=0

BPDU

Cost=0+19=19

BPDU

Cost=0+19=19

BPDU

Cost=0+19=19

0

0

0

19

19

19

Root Path Cost Cumulative cost to the Root Bridge. This is the value transmitted in the BPDU. Calculated by adding the receiving ports

Path Cost to the valued contained in theBPDU.

19

19

19

32768-000f.2490.1380


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show spanning-tree

35

Distribution1#show spanning-tree

VLAN0001




Cost 19




Address 000b.fd13.9080


Aging Time 300

Interface Port ID Designated Port ID

Name Prio.Nbr Cost Sts Cost Bridge ID Prio.Nbr

---------------- -------- --------- --- --------- -------------------- --------

Fa0/1 128.1 19 BLK 19 32769 000b.befa.eec0 128.1

Fa0/2 128.2 19 BLK 19 32769 000b.befa.eec0 128.2

Fa0/3 128.3 19 FWD 0 32769 0009.7c0b.e7c0 128.1

Fa0/4 128.4 19 BLK 0 32769 0009.7c0b.e7c0 128.2

Fa0/5 128.5 19 FWD 19 32769 000b.fd13.9080 128.5Gi0/1 128.25 4 FWD 19 32769 000b.fd13.9080 128.25



---------------- -------- --------- --- --------- -------------------- --------

Gi0/2 128.26 4 BLK 19 32769 000b.befa.eec0 128.26

Switches now send BPDUs with their Root Path Cost out other interfaces.


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Switches now send BPDUs with their Root Path Cost out other interfaces.

Note: STP costs are incremented as BPDUs are received on a port, not as they are sentout a port.

Access 1 uses this value of 19 internally and sends BPDUs with a Root Path Cost of 19out all other ports.

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

BPDU

Cost=4+19=23

BPDU

Cost=4+19=23

19

19

0

0

019

19

19

BPDU

Cost=19

BPDU

Cost=19

32768-000f.2490.1380

Final Results


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39

Ports show ReceivedRoot Path Cost = BPDU Root Path Cost + Path Cost of Interface,after the best BPDU is received on that port from the neighboring switch.

This is the cost of reaching the Root Bridge from this interface towards the neighboringswitch.

Now lets see how this is used!

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

0

0

0

32768-000f.2490.1380

19+4=23

23+4=27

19+19=38

19+4=23

19+4=23

19+4=23

19+4=23

19+4=2319

23+4=27

19+19=38

Next: Elect Root Ports


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40

Next:

Elect Root Ports

Elect Designated Ports

Non-Designated Ports: All other ports

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

23

0

0

023

32768-000f.2490.1380

23

23

23

27

23

27

3838

Every non-Root bridge must select one Root Port. A bridges Root Port is the port closest to the Root

Bridge.


Elect Root Ports: (Review)


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41

( )

Ports show ReceivedRoot Path Cost = BPDU Root Path Cost + Path Cost of Interface,after the best BPDU is received on that port from the neighboring switch.

This is the cost of reaching the Root Bridge from this interface towards the neighboringswitch.

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

23

0

0

023

32768-000f.2490.1380

23

23

23

27

23

27

3838If I go

through D2it costs 38.

If I gothrough

Core it costs27.

If I gothrough A1 it

costs 23.

If I go throughA2 it costs 19.

This is the bestpath to the

Root!

Distribution 1 thought process

Elect Root Ports


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42

Elect Root Ports

Every non-Root bridge must select one Root Port.

A bridges Root Port is the port closest to the Root Bridge.


32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

23

0

0

023

32768-000f.2490.1380

23

23

23

27

23

27

3838

Root PortRoot Port

Root Port

? ?

Elect Root Ports Five-Step decision Sequence


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Core switch has two equal Root Path Costs tothe Root Bridge.

In this case we need to look at the five-stepdecision process.

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

23

0

0

023

32768-000f.2490.1380

23

23

23

27

23

27

3838

Root PortRoot Port

Root Port

? ?

p qStep 1 - Lowest BIDStep 2 - Lowest Path Cost to Root BridgeStep 3 - Lowest Sender BIDStep 4 - Lowest Port PriorityStep 5 - Lowest Port ID

Elect Root Ports Five-Step decision Sequence


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44

Distribution 1 switch has a lower Sender BIDthan Distribution 2.

Core chooses the Root Port of G 0/1.

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

23

0

0

023

32768-000f.2490.1380

23

23

23

27

23

27

3838

Root PortRoot Port

Root Port

Step 1 - Lowest BIDStep 2 - Lowest Path Cost to Root BridgeStep 3 - Lowest Sender BIDStep 4 - Lowest Port PriorityStep 5 - Lowest Port ID

Lower BID Root Port


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Elect Designated Ports

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STP Convergence

Step 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports

The loop prevention part of STP becomes evident during this step, electingdesignated ports.

A Designated Port functions as the single bridge port that both sends andreceives traffic to and from that segment and the Root Bridge.

Each segment in a bridged network has one Designated Port, chosenbased on cumulative Root Path Cost to the Root Bridge.

The switch containing the Designated Port is referred to as the Designated

Bridge for that segment. To locate Designated Ports, lets take a look at each segment. Segments perspective: From a device on this segment, Which switch should

I go through to reach the Root Bridge?

Root Path Cost, the cumulative cost of all links to the Root Bridge.

Obviously, the segment has not ability to make this decision, so the

perspective and the decision is that of the switches on that segment.

A Designated Portis elected for every segment.


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32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

g y g The Designated Port is the only port that sends and receives traffic to/from that segment to

the Root Bridge, the best port towards the root bridge. Note: The Root Path Cost shows the SentRoot Path Cost. This is the advertised cost in the BPDU, by this switch out that interface, i.e. this is the cost of

reaching the Root Bridge through me!

Segments perspective:


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47

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

Access 2 has a Root Path Cost = 0 (after all it is the Root Bridge) and Access 1 has a RootPath Cost = 19.

Because Access 2 has the lower Root Path Cost it becomes the Designated Portfor thatsegment.

? DP

What is my best pathto the Root Bridge, 19via Access 1 or 0 via

Access 2?



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32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

The same occurs between Access 2 and Distribution 1 and Distribution 2 switches. Because Access 2 has the lower Root Path Cost it becomes the Designated Portfor those

segments.

?

DP

?DP

DP

Segments perspective: Five-Step decision SequenceSt 1 L t BID


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32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

Segment between Distribution 1 and Access1 has two equal Root Path Costs of 19.

Using the Lowest Sender ID (first two stepsare equal), Access 1 becomes the best pathand the Designated Port.

?

DP

DP

DP

Step 1 - Lowest BIDStep 2 - Lowest Path Cost to Root BridgeStep 3 - Lowest Sender BIDStep 4 - Lowest Port PriorityStep 5 - Lowest Port ID

DP

What is my best pathto the Root Bridge, 19via Distribution 1 or 19via Access 1? Theyare the same! Who

has the lowest BID?



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32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

All other ports, those ports that are not Root Ports or Designated Ports, become Non-Designated Ports.

Non-Designated Ports are put in blocking mode. (Coming)

This is the loop prevention part of STP.

DP

DP

DPDP

DP

DP

DP

NDP

NDP

NDP

X

X

XX

DP

NDP

Port Cost/Port ID


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Port Cost/Port ID

57

If the path cost and bridge IDs are equal (as in the case of parallel links),

the switch goes to the port priority as a tiebreaker. Lowest port priority wins (all ports set to 32).

You can set the priority from 0 63.

If all ports have the same priority, the port with the lowest port numberforwards frames.

Port 0/2 would forward because its the lowest.


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Port Cost/Port ID

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Distribution1#show spanning-tree

VLAN0001




Cost 19



Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)Address 000b.fd13.9080


Aging Time 300



---------------- -------- --------- --- --------- -------------------- --------

Fa0/1 128.1 19 BLK 19 32769 000b.befa.eec0 128.1

Fa0/2 128.2 19 BLK 19 32769 000b.befa.eec0 128.2

Fa0/3 128.3 19 FWD 0 32769 0009.7c0b.e7c0 128.1

Fa0/4 128.4 19 BLK 0 32769 0009.7c0b.e7c0 128.2

Fa0/5 128.5 19 FWD 19 32769 000b.fd13.9080 128.5

Gi0/1 128.25 4 FWD 19 32769 000b.fd13.9080 128.25


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Spanning-Tree Port States

59

STP Timers


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STP Timers

60


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Spanning Tree Port States

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Spanning tree transitions each port through severaldifferent states.

From Blocking to Forwarding:

20 sec + 15 sec + 15 sec = 50 seconds



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62

Blocked: All ports start in blocked mode

in order to prevent the bridgefrom creating a bridging loop.

Port are listening (receiving)

BPDUs. No user data is being passed.

The port stays in a blocked stateif Spanning Tree determinesthat there is a better path to

the root bridge.

May take a port up to 20seconds to transition out of thisstate (max age). - coming soon.

BPDUs sent and received


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Learn: The learn state is very similar

to the listen state, except thatthe port can add informationit has learned to its address

table. Adds addresses to MAC

Address Table

Still not allowed to send orreceive user data

Learns for a period of timecalled the forward delay(default 15 seconds)


S i T P S


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65

Forward: The port can send and

receive user data.

A port is placed in theforwarding state if:

There are no redundantlinks

or

It is determined that it hasthe best path to the root



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66

Disabled: The port isshutdown.



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67

Non-Designated Ports

Designated Ports & Root Ports



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68

32768-000b.fd13.9080 32768-000b.fd13.cd80

32768-000b.befa.eec0 32768-0009.7c0b.e7c0

Root Bridge

19

19

19

19

0

0

019

32768-000f.2490.1380

19

19

23

19

23

19

1919

RP

RP

RP

RP

DP

DP

DPDP

DP

DP DP

NDP

NDP

NDPX

X

XX

Active links

DP

NDP


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Topology Change

69

Much of the detail has been omitted. If there is a change in the topology, a link is

added or removed:

User traffic will be disrupted until the switch

recalculates paths using the Spanning TreeAlgorithm.

A delay of up to 50 seconds may occur beforeswitches start forwarding frames.


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PVST, RSTP & Rapid PVST+(More flavours of STP)


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71


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PVST+

72

In the figure, switch S3 is the root bridge for VLAN 20, and switch S1 is the rootbridge for VLAN 10. Creating different STP root switches per VLAN creates a

more redundant network.

C fi i PVST


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Configuring PVST+

73

Step 1. Select the switches you want for the primary and secondary root bridges for

each VLAN.

Step 2. Configure the switch to be a primary bridge for one VLAN, for example

switch S3 is a primary bridge for VLAN 20.

Step 3. Configure the switch to be a secondary bridge for the other VLAN, for

example, switch S3 is a secondary bridge for VLAN 10.


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Rapid Spanning Tree Protocol

74

The immediate hindrance of STP is convergence. Depending on the type of failure, it takes

anywhere from 30 to 50 seconds, to convergethe network.

RSTP helps with convergence issues thatplague legacy STP.


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RSTP vs STP

76

RSTP is based on IEEE 802.1w standard. Numerous differences exist between RSTP and STP.

RSTPrequires full-duplex point-to-point connectionbetween adjacent switchesto achieve fast convergence.

Half duplex, denotes a shared medium, multiple devices.

As a result, RSTP cannot achieve fast convergence in half-duplex mode.

STP and RSTP also have port designation differences.

RSTP has alternateport and backupport designations.

Ports not participating in spanning treeare known as edgeports.

The edge port becomes a nonedgeport immediately if aBPDU is heard on the port.


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RSTP vs STP

77

RSTP is proactive and therefore negates the need forthe 802.1D delay timers.

RSTP (802.1w) supersedes 802.1D, while stillremaining backward compatible.

RSTP BPDU format is the same as the IEEE 802.1DBPDU format, except that the Version field is set to 2to indicate RSTP.

The RSTP spanning tree algorithm (STA) elects a rootbridge in exactly the same way as 802.1D elects a

root. Critical differences that make RSTP the preferred

protocol for preventing Layer 2 loops in a switchednetwork environment.

Many of the differences stem from the Ciscoproprietary enhancements. (CCNP 3)


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RSTP Port States

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RSTP Port States

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Port State Description

Discarding This state isseen in both a stable active topology and duringtopology synchronization and changes.The discarding state prevents the forwarding of data frames, thusbreaking the continuity of a Layer 2 loop.

Learning This state isseen in both a stable active topology and duringtopology synchronization and changes.The learning state accepts data frames to populate the MAC table inan effort to limit flooding of unknown unicast frames.

Forwarding This state isseen only in stable active topologies.The forwarding switch ports determine the topology. Following a topology change, or during synchronization, the forwardingof data frames occurs only after a proposal and agreement process.


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Port States

80

The table describes STP and RSTP port states.

Operational PortState

STP Port State RSTP Port State

Enabled Blocking Discarding

Enabled Listening Discarding

Enabled Learning Learning

Enabled Forwarding Forwarding

Disabled Disabled Discarding


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Port Roles

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Port Role Description

Root port(Same as STP)

The root port is the switch port on every nonroot bridge that is thechosen path to the root bridge. There can be only one root port onevery switch. The root port assumes the forwarding state in astable active topology.

Designated port(Same as STP)

Each segment has at least one switch port as the designated portfor that segment. In a stable, active topology, the switch with thedesignated port receives frames on the segment that are destined

for the root bridge. There can be only one designated port persegment. The designated port assumes the forwarding state. Allswitches connected to a given segment listen to all BPDUs anddetermine the switch that will be the designated switch for aparticular segment.

Alternative port(Non-Designated Port in STP)

The alternative port is a switch port that offers an alternative pathtoward the root bridge. The alternative port assumes a discarding

state in a stable, active topology. An alternative port is present onnondesignated switches and makes a transition to a designatedport if the current designated path fails.

Backup port The backup port is an additional switch port on the designatedswitch with a redundant link to the segment for which the switch isdesignated. A backup port has a higher port ID than the designatedport on the designated switch. The backup port assumes the

discarding state in a stable, active topology.


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RSTP Link Types

82


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83


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Summary


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STP: Summary

85

Recall that switches go through three steps for their initial

convergence:STP Convergence

Step 1 Elect one Root Bridge: Lowest BID

Step 2 Elect Root Ports: Closest port to Root Bridge

Step 3 Elect Designated Ports: Best switch to Root Bridge

Also, all STP decisions are based on a the following predeterminedsequence:

Five-Step decision Sequence

Step 1 - Lowest BID

Step 2 - Lowest Path Cost to Root Bridge

Step 3 - Lowest Sender BID

Step 4 Lowest Port Priority

Step 5 - Lowest Port ID


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STP: Summary


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BID = Priority + MAC Address One Root Bridge is elected per network

Every non-Root Bridge will select one Root Port!

Switches Perspective: Port closest to Root Bridge

Smallest Root Path Cost, the cumulative cost of all links to

the Root Bridge. Each segment in a bridged network has one Designated Port

Segments perspective: From a device on this segment,Which switch should I go through to reach the Root Bridge?

Chosen based on cumulative Root Path Cost to the RootBridge.

The switch containing the Designated Port is referred to as theDesignated Bridge for that segment.

BPDUs are sent every 2 seconds by a switch


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STP: Summary

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50 Seconds from Blocking to Forwarding: Blocking: Max Age 20 seconds

Listening: Forward Delay 15 seconds

Learning: Forward Delay 15 seconds

Forwarding


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RSTP

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Port States Discarding

Learning

Forwarding

Port Roles Root

Designated

Alternate (NDP)

Backup

Link Types Point-to-point (Switch-to-Switch or Host-to-Switch)

Shared (Hub)

Slides Adapted from Rick


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Slides Adapted from Rick

Graziani

Rick Graziani [email protected] Thanks for the help

Documents

Exploration LAN Switching Chapter5-Updated