Computer networks ethernet switching

Computer networks ethernet switching, mạng máy tính, chuyển mạch ethernet

Module 8:

Ethernet Switching

Outline

8.1 Ethernet Switching

Layer 2 bridging Layer 2 switching

Switch operation

Latency

Switch modes Spanning- Tree Protocol

8.2 Collision Domains and Broadcast Domains Shared media environments

Collision domains segmentation Layer 2 broadcasts

Broadcast domains Introduction to data flow

What is a network segment?

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8.1 Ethernet Switching

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Layer 2 bridging

Ethernet is a shared media

Only one node can transmit data at a time

Within Ethernet physical segment

more nodes

more contention more retransmissions Break the large segment into parts and separate it into isolated collision domains

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Layer 2 briaging (cont )

Example : Host A is pinging Host B

The address of Host A is added to its bridge table

The address of Host B has not been

recorded yet as only the source

address of a frame Is recorded

Host B processes the ping request and transmits a ping reply back to Host A

The address of Host B Is added to Its

bridge table

Host A is now going to ping Host C

The address of Host C has not been recorded yet as only

the source address of a frame is recorded

Host C processes the ping request and transmits a ping reply back to Host A

The address of Host C is added to its bridge table

When Host D transmits data, its MAC address will also be recorded in the bridge table

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00000CAAAAAA Source Destination 00000CBBBBBB

Address Address D0000CDDDDDDI|00000CCCCCCCI

MAC Address Port

1

Layer 2 briaging (cont )

Switch Operation k5

Layer 2 switching

Generally, a bridge has only two ports and divides a collision

domain into two parts

All decisions made by a bridge are based on MAC or Layer 2 addressing and do not affect the logical or Layer 3 addressing

A switch dynamically builds and

maintains a Content-Addressable -._ mm ™® -

information for each port

A bridge will divide a collision domain but has no effect on a logical or broadcast domain

switch operation

A switch is essentially a multi-port bridge

When only one host is connected to a switch port, the two nodes

(the switch port & host) share this small segment, or collision domain The small physical segment is called microsegment

Most switches are capable of supporting full duplex

No contention for the full duplex media

The bandwidth is doubled when using full duplex

Content-addressable memory (CAM) is memory that essentially works backwards compared to conventional memory

Entering data into the memory will return the associated address Using CAM allows a switch to directly find the port that is

associated with a MAC address without using search algorithms Application-specific integrated circuit (ASIC) -> soeed up

Latency

Latency is the delay between the time a frame first starts to leave the source device and the time the first part of the frame reaches its destination

A wide variety of conditions can cause delays as a frame travels from source to destination:

Media delays caused by the finite soeed (10/100/1000Mbps) that signals can travel through the physical media

Circuit delays caused by the electronics that process the Signal along the path

software delays caused by the decisions that software must make to implement switching and protocols

Delays caused by the content of the frame

For example, a device cannot route a frame to a destination until the destination MAC address has been read (RARP in routers)

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The switch receives the entire frame before sending it out the destination port

To verify the Frame Check Sum (FCS)

Fail > it is discarded

Fragment-free

The switch reads the first 64 bytes (frame header)

This mode verifies the reliability of the addressing and Logical Link Control (LLC) protocol information to ensure the destination and handling of the data will be

correct

switch modes (cont.)

synchronous switching

Both the source port and destination port must be operating

at the same bit rate

cut-through Asynchronous switching

The bit rates of both sides are not the same, the frame must

be stored at one bit rate before it is sent out at the other bit

Spanning- Tree Protocol

To prevent switch loops and broadcast storms

Usually caused by design errors or accident

redundant paths : to provide for reliability and fault tolerance Each switch in a LAN using STP sends special messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know

of its existence and to elect a root bridge for the network

The switches then use the Spanning-Tree Algorithm (STA) to resolve and shut down the redundant paths

Each port on a switch using Spanning-Tree Protocol exists in one of the following five states:

Listening | Building “active” topology Learning ‘Building bridging table Forwarding Sending and receiving user data

Disabled | Administratively down

Spanning- Tree Protocol(cont.)

8.2 Collision Domains and Broadcast Domains

Shared media environments

Layer 1 media and topologies :

Shared media environment Extended shared media environment Accommodate for multiple access or longer cable distances

Point-to-point network environment

dialup network connections

Collisions only occur in a shared environment

oe

Point-to-Point

Collision domains

Collisions cause the network to be inefficient

All transmission stops for a period of time

The length of this period of time without transmissions varies and is determined by a backoff algorithm for each network device

Collision domains (cont )

Collision domains (cont )

In a small network a single collosion domain can work just fine

as there is little contention for the network media This type of network is fine for an isolated network that does not require much data transmission

But as the network starts to grow, the contention for the line becomes greater and a larger number of collisions start to occur

As the network continues to grow, the contention for the line becomes greater and even starts to effect the performance of the computers on the network

Finally when the collision domain becomes too big and network

transmission demands become too great The number of

collisions practically shuts the network down

Collision domains (cont )

must be within certain limits otherwise all the workstations will not be able to hear all the collisions

on the network

Repeater latency, propagation delay, and NIC latency all contribute to the four repeater rule

A late collision is when a collision happens after the first 64 bytes (512 bits) of the frame are transmitted

The chipsets in NICs are not |

required to retransmit automatically when a late collision occurs

The 5-4-3-2-1 rule :

5 segments of network media

4 repeaters or hubs

3 host segments of the network

2 link sections (no hosts)

1 large collision domain

Round- Trio Delay

Maximum round-trip delay (the 10BASE-T bit time of 0.1 microseconds times the minimum frame size of 512 bits) is 51.2 microseconds

segmentation

Layer 2 devices segment or divide collision domains

Keep tracking of the MAC addresses and which segment they are

Layer 2 broadcasts (cont )

Because the NIC must interrupt the CPU to process each broadcast or multicast group it belongs to (no discard), broadcast radiation atfects the performance of hosts in the network

Workstations broadcast an Address Resolution Protocol (ARP) request every time they need to locate a MAC address that is not

In the ARP table

Broadcast domains

Broadcasts are forwarded by Layer 2 devices

Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts

Layer 3 forwarding is based on the destination IP address and not the MAC address

Use router to segment broadcast domains

2008/7/19 By using a router in place of a bridging device a layer two

broadcast is contained Layer three devices are the only devices

that contain broadcasts

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Introduction to data flow

Layer 1 devices do no filtering, so everything that is received is passed on

to the next segment

Layer 2 devices filter data frames based on the destination MAC address Layer 3 devices filter data packets based on IP destination address

Data flow through a routed IP based network

a Network Network † Network Network 3 Network

2 Data Link Beta Link Data Link Data Link |} [2 Data Link

1 Physical 211 mem Physical Physical 1 Physical

What is a network segment?

Data Stream from Session Layer

Segment1 || Segment2 || Segment3

Data Stream Segmentation

Two Network Segments

Litt

Wire Segment

There are different types of segments in networking The meaning

of the term segments depends on the context of a sentence

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