caexs3c5stp-091211035914-phpapp02

7/29/2019 caexs3c5stp-091211035914-phpapp02

1/58

CCNA Semester 3

Chapter 5 -

Spanning Tree Protocol

CCNA Exploration 4.0


2/58

2

Objectives

Explain the role of redundancy in a converged

network

Summarize how STP works to eliminate Layer 2

loops in a converged network Explain how the STP algorithm uses three steps to

converge on a loop-free topology

Implement rapid per VLAN spanning tree (rapid

PVST+) in a LAN to prevent loops between redundantswitches.


3/58

3

Role of redundancy


4/58

4

Redundancy in a hierarchical network

The hierarchical design model addresses issues found in theflat model network topologies. One of the issues is

redundancy.

Having multiple paths for data to traverse the network allows

for a single path to be disrupted without impacting theconnectivity of devices on the network.


5/58

5

Issues with Redundancy : Layer 2 Loop

When multiple paths exist between two devices on thenetwork, a Layer 2 loop can occur.

Ethernet frames do not have a time to live (TTL) like IP

packets traversing routers. So, if they are not terminated

properly on a switched network, they continue to bouncefrom switch to switch endlessly.


6/58

6

Issues with Redundancy : Broadcast Storm

A broadcast storm occurs when there are so many broadcastframes caught in a Layer 2 loop that all available bandwidth

is consumed.

As a result, no bandwidth is available bandwidth for

legitimate traffic, and the network becomes unavailable fordata communication.


7/587

Issues with Redundancy : Duplicate Unicast Frames

Unicast frames sent onto a looped network can result induplicate frames arriving at the destination device.


8/588

Real-world redundancy issues

Network loops that are a result of accidental duplicateconnections in the wiring closets are a common occurrence.

The example displays a loop that occurs if a switch is

connected to two different switches on a network that are

both also interconnected. The impact of this type of loop ismuch greater because it affects more switches directly.


9/589

The Spanning Tree Algorithm


10/5810

Spanning Tree Protocol

STP ensures that there is only one logical path between alldestinations on the network by intentionally blocking

redundant paths that could cause a loop.

STP prevents loops from occurring by configuring a loop-free

path through the network using strategically placed blockingstate ports

A port is considered

blocked when

network traffic isprevented from

entering or leaving

that port


11/5811

Spanning Tree Algorithm (STA)

The STA designates a single switch as the root bridge anduses it as the reference point for all path calculations.

After the root bridge has been determined, the STA

calculates the shortest path to the root bridge. Each switch

uses the STA to determine which ports to block.


12/5812

Spanning Tree Algorithm (STA) (cont)

When the STA has determined which paths are to be leftavailable, it configures the switch ports into distinct port

roles.

Root ports : Switch ports closest to the root bridge.

Designated ports : All non-root ports that are stillpermitted to forward traffic on the network.

Non-designated ports :

All ports configured to be

in a blocking state toprevent loops


13/5813

Root Bridge & Election Process

The root bridge serves as a reference point for all spanning-treecalculations to determine which redundant paths to block.

An election process determines which switch becomes the root

bridge.

1. After a switch boots, it sends out BPDU frames (more detail

later) containing the switch BID and the root ID every 2seconds.

2. Initially, each switch identifies itself as the root bridge after

bootup.

3. If the root ID from the BPDU received is lower than the root ID on

the receiving switch, the receiving switch updates its root ID

identifying the adjacent switch as the root bridge

4. The switch then forwards new BPDU frames with the lower root

ID to the other adjacent switches.

5. Eventually, the switch with the lowest BID ends up beingidentified as the root bridge for the spanning-tree instance.


14/5814

Root Bridge & Election Process


15/5815

Root Bridge & Election Process (cont)

BID Structure

Root Bridge


16/58

16

Best Path to the Route Bridge

The path information is determined by summing up theindividual port costs along the path from the destination to

the root bridge.

The default port costs are defined by the speed at which the

port operates.

Although switch ports have a default port cost associatedwith them, the port cost is configurable


17/58

17

Best Path to the Route Bridge (cont)

Path cost is the sum of all the port costs along the path tothe root bridge.

The paths with the lowest path cost become the preferred

path, and all other redundant paths are blocked.


18/58

18

Port Roles

Root Port :

The root port exists on non-root bridges and is the switch port withthe best path to the root bridge. Root ports forward traffic toward theroot bridge.

Designated Port :

For root bridges, all switch ports are designated ports.

For non-root bridges, a designated port is the switch port thatreceives and forwards frames toward the root bridge as needed

Only one designated port is allowed per segment

Non-designated Port ;

The non-designated port is a switch port that is blocked, so it is not

forwarding data frames and not populating the MAC address tablewith source addresses

Disabled Port :

The disabled port is a switch port that is administratively shut down. Adisabled port does not function in the spanning-tree process


19/58

19

Port Roles (cont)

When determining the root port on a switch, the switchcompares the path costs on all switch ports participating in

the spanning tree.

The switch port with the lowest overall path cost to the root is

automatically assigned the root port role because it is closest

to the root bridge.

When there are two switch ports that have the same lowest

path cost to the root bridge, the switch uses the

customizable port priority value, or the lowest port ID if both

port priority values are the same.

The port ID is the interface ID of the switch port.


20/58

20

Port Roles (cont) example


21/58

21



22/58

22


After a switch determines which of its ports is the root port,the remaining ports must be configured as either a

designated port (DP) or a non-designated port (non-DP)

When two switches exchange their BPDU frames, they

examine the sending BID of the received BPDU frame to see

if it is lower than its own.

The switch with the lower BID wins the competition and its

port is configured in the designated role. The losing switch

configures its switch port to be non-designated and,

therefore, in the blocking state to prevent the loop from

occurring.


23/58

23



24/58

24

BPDU

STP determines a root bridge for the spanning-tree instanceby exchanging BPDUs.

BPDU Fields


25/58

25

BPDU (cont)

By default, BPDU frames are sent every 2 seconds after aswitch is booted.

When adjacent switches receive a BPDU frame, they

compare the root ID from the BPDU frame with the local root

ID.

If the root ID in the BPDU is lower than the local root ID,

the switch updates the local root ID and the ID in its

BPDU messages

If the local root ID is lower than the root ID received in the

BPDU frame, the BPDU frame is discarded.


26/58

26

BID

The BID field of a BPDU frame contains three separatefields: bridge priority, extended system ID, and MAC

address. Each field is used during the root bridge election.


27/58

27

BID (cont)


28/58

28

Port States and BPDU Timers

To facilitate the learning of the logical spanning tree, each switch

port transitions through five possible port states and three BPDUtimers.


29/58

29

Port States and BPDU Timers (cont)

Blocking - The port is a non-designated port and does notparticipate in frame forwarding.

Listening - In this state, the switch port is not only receiving

BPDU frames, it is also transmitting its own BPDU frames

and informing adjacent switches that the switch port is

preparing to participate in the active topology.

Learning - The port prepares to participate in frame

forwarding and begins to populate the MAC address table.

Forwarding - The port is considered part of the active

topology and forwards frames and also sends and receives

BPDU frames

Disabled - The Layer 2 port does not participate in spanning

tree and does not forward frames.


30/58

30


Cisco PortFast Technology When a switch port configured with PortFast is configured

as an access port, that port transitions from blocking to

forwarding state immediately, bypassing the typical STP

listening and learning states.


31/58

31



32/58

32

STP Topology Change

A switch considers it has detected a topology change eitherwhen a port that was forwarding is going down (blocking for

instance) or when a port transitions to forwarding and the

switch has a designated port.

When a change is detected, the switch notifies the root

bridge of the spanning tree. The root bridge then broadcasts

the information into the whole network.


33/58

33

Cisco and STP Variants


34/58

34

Cisco and STP Variants


35/58

35

PVST +

Cisco developed PVST+ so that a network can run an STPinstance for each VLAN in the network. With PVST+, more

than one trunk can block for a VLAN and load sharing can

be implemented.

In a Cisco PVST+ environment, you can tune the spanning-

tree parameters so that half of the VLANs forward on each

uplink trunk


36/58

36

PVST + Bridge ID

Bridge priority - A 4-bit field carries the bridge priority.Because of the limited bit count, the priority is conveyed in

discrete values in increments of 4096. The default priority is

32,768.

Extended system ID - A 12-bit field carrying the VID for

PVST+.

MAC address - A 6-byte field with the MAC address of a

single switch.


37/58

37

Configure PVST +

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.


38/58

38

Configure PVST + (cont)


39/58


40/58

40

RSTP (cont)


41/58

41

RSTP BPDU

RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTPbridge can communicate 802.1D on any shared link or with

any switch running 802.1D

Protocol information can be immediately aged on a port if

hellos are not received for three consecutive hello times,

6 seconds by default, or if the max age timer expires

Because BPDUs are used as a keepalive mechanism,

three consecutively missed BPDUs indicate lost

connectivity between a bridge and its neighboring root or

designated bridge


42/58


43/58

43

RSTP (cont) : Edge Port

An RSTP edge port is a switch port that is never intended tobe connected to another switch device. It immediately

transitions to the forwarding state when enabled.

Unlike PortFast, an RSTP edge port that receives a BPDU

loses its edge port status immediately and becomes a

normal spanning-tree port.


44/58

44

RSTP (cont) : Link Types

The link type provides a categorization for each portparticipating in RSTP

The link type is automatically determined, but can be

overwritten with an explicit port configuration.

Root ports do not use the link type parameter. Root ports areable to make a rapid transition to the forwarding state as

soon as the port is in sync.

Alternate and backup ports do not use the link type

parameter in most cases.

Designated ports make the most use of the link type

parameter. Rapid transition to the forwarding state for the

designated port occurs only if the link type parameter

indicates a point-to-point link.


45/58

45

RSTP (cont) : Port States

RSTP provides rapid convergence following a failure orduring re-establishment of a switch, switch port, or link.

There are three possible RSTP port states: discarding,

learning, and forwarding.


46/58

46

RSTP (cont) : Port Roles


47/58

47

Configuring rapid PVST +


48/58


49/58

49

Design STP for Trouble Avoidance

Know Where the Root Is

Minimize the Number of Blocked Ports


50/58

50

Design STP for Trouble Avoidance (contd)

VTP Pruning


51/58

51


Use Layer 3 Switching

There is no speed penalty with the routing hop and an

additional segment between C1 and C2.

Core switch C1 and core switch C2 are Layer 3 switches.

VLAN 20 and VLAN 30 are no longer bridged between C1and C2, so there is no possibility for a loop.


52/58

52



53/58

53

Troubleshoot STP Operation


54/58

54

Troubleshoot STP Operation

To troubleshoot a bridging loop, you need to know:

The topology of the bridge network

The location of the root bridge

The location of the blocked ports and the redundant links

Some possible failure Switch or Link Failure

PortFast Configuration Error

Network Diameter Issues


55/58

55

Switch or Link Failure


56/58

56

PortFast Configuration Error


57/58

57

Network Diameter Issues


58/58

Summary

caexs3c5stp-091211035914-phpapp02

Documents