Tài liệu giảng dạy CCNA - module 04 chapter 13-Cisco LAN Switching Basics

How Switches And Bridges Learn Addresses  Reading the source MAC address of each received frame  Recording the port on which the MAC address was received  If the address is not fou

Module 04 LAN Switching

Chapter 13 Cisco LAN Switching Basics

1 The Case for Bridging and

Switching

Collisions domain: Share access

Collisions domain: Repeater

Collisions domain: HUB

Collisions domain

 Collision Domains are the area where collisions occur.

 All of layer 1 interconnections are part of the collision domain.

 Extending a network with a repeater or a hub, results in a

Segmenting Collision Domain

LAN Segmentation With Bridges

How Switches And Bridges Learn

Addresses

 Reading the source MAC address of each received frame

 Recording the port on which the MAC address was received

 If the address is not found, the bridge forwards the frame out all

ports except the port on which it was received

 If the address is found in an address table and the address is

associated with the port on which it was received, the frame is

discarded

 If the address is found in an address table and the address is not

associated with the port on which it was received, the bridge

forwards the frame to the port associated with the address

How Switches And Bridges Filter

Transparent Bridging

 Transparent bridging is called “transparent” because the

endpoint devices do not need to know that the bridge(s)

exist(s)

 In other words, the computers attached to the LAN do not

behave any differently in the presence or absence of

transparent bridges

2 LAN Switching

 Dynamically builds and maintains a Content-Addressable

Memory (CAM) table

Half-duplex Networks

Full-duplex Transmitting

 Full-duplex Ethernet allows the transmission of a packet

and the reception of a different packet at the same time

 This connection is considered point-to-point and is collision

Microsegmentation Implementation

Switch modes

Switch modes

 Store-and-forward:

 The entire frame is received before forward.

 The latency is greater with larger frames

 Error detection is high.

 Must be used for asynchronous switching.

3 LAN Segmentation

LAN Segmentation

LAN Segmentation With Bridges

LAN Segmentation With Routers

LAN Segmentation With Switches

• Switches eliminate the impacts of collisions

Switches And Collision Domains

 The network area where frames originate and collide is

called the collision domain

 A switch builds a switching table by learning the MAC

addresses of the hosts that are connected to each switch

port

 When two connected hosts want to communicate with each

other, the switch looks up the switching table and

establishes a virtual connection between the ports

 The virtual circuit is maintained until the session is

terminated

Switches And Broadcast Domains

 Broadcasting is when one transmitter tries to reach all the

receivers in the network

 When a device wants to send out a Layer 2 broadcast, the

destination MAC address in the frame is set to all ones

 The broadcast domain at Layer 2 in referred to as the MAC

broadcast domain

 The MAC broadcast domain consists of all devices on the

LAN that receive frame broadcasts by a host to all other

machines on the LAN

4 The Need for Spanning Tree

Redundant topology and spanning

tree

 Redundant networking topologies are designed to ensure that

networks continue to function in the presence of single points

of failure

 Switches flood traffic out all ports when the traffic is broadcast

or multicast or sent to a destination that is not yet known

 In the Layer 2 header there is no Time To Live (TTL) If a

frame is sent into a Layer 2 looped topology of switches, it can loop forever

The solution is to allow physical loops, but create a loop free

logical topology

Spanning Tree Protocol

 The ST Algorithm, implemented by the STP, prevents

loops by calculating a stable spanning-tree network

topology

 Spanning-tree frames, called bridge protocol data units

(BPDUs), are sent and received by all switches in the

network at regular intervals and are used to determine

the spanning-tree topology

 All bridge interfaces eventually stabilize at either a

forwarding or a blocking

 One of the bridges is elected as root

 All root bridge interfaces are in forwarding

 Each bridge receives BPDU from the root, either directly or

forwarded by some other bridge

 The port in which the least-cost BPDU is received is

called the root port of a bridge is placed in forwarding

Algorithm (cont.)

 For each LAN segment:

 Designated bridge is one bridge sends the forwarded

BPDU with the lowest cost

 The designated bridge’s interface is placed in

forwarding state

 All interfaces of other bridges are placed in blocking

state

Algorithm (cont.)

 Hello Time (2s):

 The root sends BPDU every Hello time

 All bridges use the same value

 MaxAge time (20s):

 If a bridge does not receive a BPDU for MaxAge time It

begins the process of causing the Spanning Tree to change

 Forward Delay time (15s):

Listening time between blocking and forwarding After

 LAN segmentation with bridges, switches, routers

 Spanning Tree Protocol