Chapter 5 - Spanning Tree Protocol CCNA pdf

Port RolesPort Role Decisions Học viện mạng Bách Khoa - Website: www.bkacad.com 44... Port RolesPort Role Decisions Học viện mạng Bách Khoa - Website: www.bkacad.com 45... Port RolesPort

Trang 1

Chapter 5 - Spanning Tree Protocol

CCNA Exploration 4.0

Trang 2

network

network.

converge on a loop-free topology.

between redundant switches.

Học viện mạng Bách Khoa - Website: www.bkacad.com 2

Trang 3

Layer 2 redundancy

Trang 4

Redundancy in a hierarchical network

• Layer 2 redundancy improves the availability of the network by

implementing alternate network paths by adding equipment and

cabling

Trang 5

Examine a redundant design

• In a hierarchical design, redundancy is achieved at the distribution and core layers through additional hardware and alternate paths through the additional hardware

the additional hardware

Trang 6

Trang 7

Trang 8

Trang 9

Trang 10

• Known Unicast: Destination addresses are in Switch Tables

• Unknown Unicast: Destination addresses are not in Switch Tables

• Multicast: Traffic sent to a group of addresses

• Broadcast: Traffic forwarded out all interfaces except incoming

interface

Trang 11

Issues with Redundancy

Trang 12

A redundant switched topology (STP disabled) may cause:

Trang 13

Layer 2 Loops

• When multiple paths exist between two devices on the network and

STP has been disabled on those switches, a Layer 2 loop can occur

Trang 15

Duplicate Unicast Frames

• Unicast frames sent onto a looped network can result in duplicate

frames arriving at the destination device

Trang 16

Trang 17

Real-world Redundancy Issues

Loops in the Wiring Closet

• If the network cables are not properly labeled when they are terminated

in the patch panel in the wiring closet Network loops that are a result

of accidental duplicate connections in the wiring closets are a common occurrence

Trang 18

Real-world Redundancy Issues

Loops in the Cubicles

Trang 19

Prevent loop, storm bordcast…?

• Block redundant link and auto unblock redundant link when primary link down.

Trang 20

Spanning Tree Protocol p g

Trang 21

The Spanning Tree Algorthm

• STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop This

disabled to prevent the loops from occurring.

• If the path is ever needed to compensate for a network cable or switch failure, STP recalculates the paths and unblocks the necessary ports to allow the

redundant path to become active

redundant path to become active.

Trang 22

Spanning-Tree Protocol

• Layer 2 protocol

• STP executes an algorithm called Spanning Tree Algorithm (STA)

• STA chooses a reference point, called a root bridge, and then

determines the available paths to that reference point

– If more than two paths exists, STA picks the best path and blocks o e t a t o pat s e sts, S p c s t e best pat a d b oc s

Trang 23

STP Algorithm

• STP uses the Spanning Tree Algorithm (STA) to determine which

switch ports on a network need to be configured for blocking to prevent loops from occurring

• Root Bridge – The lowest BID

• Root Ports - Switch ports closest to the root bridge.

• Designated ports - All non-root ports that are still permitted to forward traffic

Trang 24

The Root Bridge

• Every spanning-tree instance (switched LAN or broadcast domain) has

a switch designated as the root bridge The root bridge serves as a

reference point for all spanning tree calculations to determine which

reference point for all spanning-tree calculations to determine which redundant paths to block

Trang 25

Bridge ID (BID)

• For each Network, the switch with the highest switch priority (the

lowest numerical priority value) is elected as the root switch

• Th d t il f h BID fi ld di d l t b t it i f l t

• The details of each BID field are discussed later, but it is useful to

know now that the BID is made up of a priority value, an extended

system ID, and the MAC address of the switch

Trang 26

Bridge ID (BID)

Priority-based decision

Trang 27

Bridge ID (BID)

Priority-based decision

Trang 28

Bridge ID (BID)

Configure and Verify the BID

Trang 29

Best Paths to the Root Bridge

• When the root bridge has been designated for the spanning-tree

instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain

the root bridge from all destinations in the broadcast domain

Trang 30

Best Paths to the Root Bridge

Example

Trang 31

STP BPDU

• The BPDU Fields

Trang 32

STP BPDU

The BPDU Process

Trang 33

STP BPDU

Trang 34

STP BPDU

Trang 35

STP BPDU

Trang 36

STP BPDU

Trang 37

STP BPDU

Trang 38

STP BPDU

Trang 39

STP BPDU

Trang 40

STP BPDU

Trang 41

Port Roles

• There are four distinct port roles that switch ports are automatically configured for during the spanning-tree process.

Root Port

• The root port exists on non-root bridges and is the switch port with the best

path to the root bridge.

Designated Port

• The designated port exists on root and non-root bridges.

• For root bridges , all switch ports are designated ports.

• For non-root bridges For non root bridges , a designated port is the switch port that receives and , a designated port is the switch port that receives and

forwards frames toward the root bridge as needed.

• Only one designated port is allowed per segment

Non-designated Port g

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

data frames and not populating the MAC address table with source addresses.

• A non-designated port is not a root port or a designated port

Trang 42

Port Roles

Disabled Port

• The disabled port is a switch port that is administratively shut down A

disabled port does not function in the spanning-tree process

Trang 43

Port Roles

Configure Port Priority

• The port priority values range from 0 - 240, in increments of 16 The

d f lt t i it l i 128

default port priority value is 128

Trang 44

Port Roles

Port Role Decisions

Trang 45

Port Roles

Trang 46

Port Roles

Trang 47

Port Roles

Trang 48

Port Roles

Trang 49

Port Roles

Trang 50

Port Roles

Trang 51

Port Roles - Summary

• One Root bridge per network

• One Root port on nonroot bridge p g

• One Designated port on segment

• Nondesignated ports are blocked

Trang 52

STP Port States and BPDU Timers

Port States

• Blocking - The port is a non-designated port and does not participate in frame forwarding The port receives BPDU frames to determine the location and root

forwarding The port receives BPDU frames to determine the location and root

ID of the root bridge switch and what port roles each switch port should

assume in the final active STP topology.

• Listening g - STP has determined that the port can participate in frame p p p

forwarding according to the BPDU frames that the switch has received thus far

At this point, 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

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 Forwarding - The port is considered part of the active topology and forwards - 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 The disabled state is set when the switch port is

p administratively disabled.

Trang 53

Trang 54

BPDU Timers

Trang 55

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 configured as an access port, that port transitions from blocking to forwarding state immediately, bypassing the typical STP listening and learning states.

Trang 56

Step 1 Elect one Root Bridge

Step 2 Elect Root Ports p

Step 3 Elect Designated Ports

• All STP decisions are based on a the following predetermined

sequence:

Four

Four Step decision Sequence Step decision Sequence

Step 1 - Lowest BID

Step 2 - Lowest Path Cost to Root Bridge

Step 3 - Lowest Sender BID

Step 4 - Lowest Port ID

Trang 57

STP Convergence

Step 2 Elect Root Ports

Trang 58

STP Convergence

Trang 59

Trang 60

Trang 61

STP Convergence

Trang 62

Trang 63

Trang 64

Trang 65

Trang 66

Verify the Root Port

Trang 67

STP Convergence

Trang 68

Step 3 Electing Designated Ports and

Non-Designated Ports

Trang 69

Trang 70

Trang 71

Trang 72

Trang 73

Trang 74

Trang 75

Verify DP and Non-DP

Trang 76

STP Topology Change

STP Topology Change Notification Process

Trang 77

STP (802.1D) Enhancements

• UplinkFast is an access-layer STP solution that provides fast failover when the root port or root switch fails.

• BackboneFast is a distribution and access-layer STP

solution that provides fast convergence in the network for indirect link failures

indirect link failures.

• PortFast is an access-layer STP solution that causes a

port to enter the spanning tree forwarding state

immediately, bypassing the listening and learning states.

Trang 78

Cisco and STP Variants

Trang 79

• Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network And Creating different STP root switches

per VLAN creates a more redundant network

• With PVST+, more than one trunk can block for a VLAN and load

sharing can be implemented

Trang 80

• PVST+ Bridge ID

• The following provides more details on the PVST+ fields:

– Bridge priority - A 4-bit field carries the bridge priority

– 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 y g

switch

Trang 81

Trang 82

Trang 83

Trang 84

What is RSTP?

• RSTP (IEEE 802.1w) is an evolution of the 802.1D standard RSTP

can achieve much faster convergence in a properly configured

network, sometimes in as little as a few hundred milliseconds

Trang 85

New Features

• New Port States and Port Roles

• Rapid Transition to Forwarding State

• Proposal/Agreement Handshake Sequence

Trang 86

RSTP Port States and Port Roles

RSTP Port States

• RSTP provides rapid convergence following a failure or during

re-establishment of a switch switch port or link

establishment of a switch, switch port, or link

• An RSTP topology change causes a transition in the appropriate switch ports to the forwarding state through either explicit handshakes or a

proposal and agreement process and synchronization

Định dạng
Số trang	117
Dung lượng	6,84 MB