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Module 4 – Switching Concepts. CCNA 3 Cabrillo College. Overview – Review of CCNA 1. The first part of this presentation should be mostly a review from CCNA 1: Describe the history and function of shared, half-duplex Ethernet Define collision as it relates to Ethernet networks

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module 4 switching concepts

Module 4 – Switching Concepts

CCNA 3

Cabrillo College

overview review of ccna 1
Overview – Review of CCNA 1

The first part of this presentation should be mostly a review from CCNA 1:

  • Describe the history and function of shared, half-duplex Ethernet
  • Define collision as it relates to Ethernet networks
  • Define micro-segmentation
  • Define CSMA/CD
  • Describe some of the key elements affecting network performance
  • Describe the function of repeaters
  • Define network latency
  • Define transmission time
  • Describe the basic function of Fast Ethernet
overview new concepts
Overview – New Concepts
  • Define network segmentation using routers, switches, and bridges
  • Describe the basic operations of a switch
  • Define Ethernet switch latency
  • Explain the differences between Layer 2 and Layer 3 switching
  • Define symmetric and asymmetric switching
  • Define memory buffering
  • Compare and contrast store-and-forward and cut-through switching
  • Understand the differences between hubs, bridges, and switches
  • Describe the main functions of switches
  • List the major switch frame transmission modes
  • Describe the process by which switches learn addresses
  • Identify and define forwarding modes
  • Define LAN segmentation
  • Define microsegmentation using switching
  • Describe the frame-filtering process
  • Compare and contrast collision and broadcast domains
  • Identify the cables needed to connect switches to workstations
  • Identify the cables needed to connect switches to switches
overview
Overview
  • Ethernet networks used to be built using repeaters.
  • When the performance of these networks began to suffer because too many devices shared the same segment, network engineers added bridges to create multiple collision domains.
  • As networks grew in size and complexity, the bridge evolved into the modern switch, allowing micro-segmentation of the network.
  • Today’s networks typically are built using switches and routers, often with the routing and switching function in the same device.

Routers

Switches, Bridges

Hub, Repeaters

ethernet 802 3 lan development
Ethernet/802.3 LAN development
  • Distance limitations
  • Ethernet is fundamentally a shared technology where all users on a given LAN segment compete for the same available bandwidth.
  • This situation is analogous to a number of cars all trying to access a one-lane road at the same time.
  • Because the road has only one lane, only one car can access it at a time.
  • The introduction of hubs into a network resulted in more users competing for the same bandwidth.
  • Collisions are a by-product of Ethernet networks.
bridges8
Bridges
  • A bridge is a Layer 2 device used to divide, or segment, a network.
  • A bridge is capable of collecting and selectively passing data frames between two network segments.
  • Bridges do this by learning the MAC address of all devices on each connected segment. Using this information, the bridge builds a bridging table and forwards or blocks traffic based on that table.
  • This results in smaller collision domains and greater network efficiency.
  • Bridges do NOT restrict broadcast traffic.
switches
Switches
  • Switches create a virtual circuit between two connected devices, establishing a dedicated communication path between two devices.
  • Switches on the network provide micro-segmentation.
  • This allows maximum utilization of the available bandwidth.
  • Broadcast frames to all connected devices on the network.
router
Router
  • A router is a Layer 3 device.
  • Used to “route” traffic between two or more Layer 3 networks.
  • Routers make decisions based on groups of network addresses, or classes, as opposed to individual Layer 2 MAC addresses.
  • Routers use routing tables to record the Layer 3 addresses of the networks that are directly connected to the local interfaces and network paths learned from neighboring routers.
elements of ethernet 802 3 networks
Elements of Ethernet/802.3 networks
  • Broadcast data frame delivery of Ethernet/802.3
  • The carrier sense multiple access/collision detect (CSMA/CD) method allows only one station to transmit at a time.
  • Multimedia applications with higher bandwidth demand such as video and the Internet, coupled with the broadcast nature of Ethernet, can create network congestion.
  • Normal latency as the frames travel across the layers
  • Extending the distances and increasing latency of the Ethernet/802.3 LANs by using Layer 1 repeaters.
half duplex
Half-Duplex
  • Originally Ethernet was a half-duplex technology.
  • Using half-duplex, a host could either transmit or receive at one time, but not both.
  • If the network is already in use, the transmission is delayed.
  • When a collision occurs, the host that first detects the collision will send out a jam signal to the other hosts.
  • Upon receiving the jam signal, each host will stop sending data, then wait for a random period of time before attempting to retransmit.
  • The back-off algorithm generates this random delay.
  • As more hosts are added to the network and begin transmitting, collisions are more likely to occur.
duplex transmissions
Duplex Transmissions
  • Simplex Transmission: One way and one way only.
    • One way street
  • Half-duplex Transmission: Either way, but only one way at a time.
    • Two way street, but only one way at a time (land slide).
  • Full-duplex Transmission: Both ways at the same time.
    • Two way street
latency
Latency
  • Latency, or delay, is the time a frame or a packet takes to travel from the source station to the final destination.
  • It is important to quantify the total latency of the path between the source and the destination for LANs and WANs.
  • Latency has at least three sources:
    • the time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses.
    • the actual propagation delay as the signal takes time to travel along the cable.
    • the latency added according to which networking devices, whether they are Layer 1, Layer 2, or Layer 3, are added to the path between the two communicating computers.
ethernet 10 base t transmission time
Ethernet 10 BASE-T transmission time
  • Transmission time equals the number of bits being sent times the bit time for a given technology.
  • Another way to think about transmission time is the time it takes a frame to be transmitted.
  • Small frames take a shorter amount of time. Large frames take a longer amount of time.
  • Each 10 Mbps Ethernet bit has a 100 ns transmission window.
    • Therefore, 1 byte takes a minimum of 800 ns to transmit.
    • A 64-byte frame, the smallest 10BASE-T frame allowing CSMA/CD to function properly, takes 51,200 ns ( 51.2 microseconds).
    • Transmission of an entire 1000-byte frame from the source station requires 800 microseconds.
the benefits of using repeaters
The benefits of using repeaters
  • The distance that a LAN can cover is limited due to attenuation.
  • Attenuation means that the signal weakens as it travels through the network.
  • The resistance in the cable or medium through which the signal travels causes the loss of signal strength.
  • An Ethernet repeater is a physical layer device on the network that boosts or regenerates the signal on an Ethernet LAN.
slide19
Full-duplex Ethernet allows the transmission of a packet and the reception of a different packet at the same time.
  • To transmit and receive simultaneously, a dedicated switch port is required for each node.
  • The full-duplex Ethernet switch takes advantage of the two pairs of wires in the cable by creating a direct connection between the transmit (TX) at one end of the circuit and the receive (RX) at the other end.
  • Ethernet usually can only use 50%-60% of the available 10 Mbps of bandwidth because of collisions and latency.
  • Full-duplex Ethernet offers 100% of the bandwidth in both directions.
  • This produces a potential 20 Mbps throughput, which results from 10 Mbps TX and 10 Mbps RX. 
duplex transmissions20
Duplex Transmissions
  • Simplex Transmission: One way and one way only.
    • One way street
  • Half-duplex Transmission: Either way, but only one way at a time.
    • Two way street, but only one way at a time (land slide).
  • Full-duplex Transmission: Both ways at the same time.
    • Two way street
sending and receiving ethernet frames on a bus
Sending and receiving Ethernet frames on a bus

Abbreviated MAC Addresses

1111

2222

3333

nnnn

  • When an Ethernet frame is sent out on the “bus” all devices on the bus receive it.
  • What do they do with it?

3333

1111

sending and receiving ethernet frames on a bus22
Sending and receiving Ethernet frames on a bus

Hey, that’s me!

Nope

Nope

Abbreviated MAC Addresses

1111

2222

3333

nnnn

  • Each NIC card compares its own MAC address with the Destination MAC Address.
  • If it matches, it copies in the rest of the frame.
  • If it does NOT match, it ignores the rest of the frame.
    • Unless you are running a Sniffer program

3333

1111

sending and receiving ethernet frames on a bus23
Sending and receiving Ethernet frames on a bus

Abbreviated MAC Addresses

1111

2222

3333

nnnn

  • So, what happens when multiple computers try to transmit at the same time?
sending and receiving ethernet frames on a bus24
Sending and receiving Ethernet frames on a bus

Abbreviated MAC Addresses

1111

2222

3333

nnnn

Collision!

X

csma cd

CSMA/CD

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

  • Common contention method used with Ethernet and IEEE 802.3
  • “Let everyone have access whenever they want and we will work it out somehow.”
csma cd and collisions

CSMA/CD and Collisions

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

  • Listens to the network’s shared media to see if any other users on “on the line” by trying to sense a neutral electrical signal or carrier.
  • If no transmission is sensed, then multiple accessallows anyone onto the media without any further permission required.
  • If two PCs detect a neutral signal and access the shared media at the exact same time, a collision occurs and is detected.
  • The PCs sense the collision by being unable to deliver the entire frame (coming soon) onto the network. (This is why there are minimum frame lengths along with cable distance and speed limitations. This includes the 5-4-3 rule.)
  • When a collision occurs, a jamming signal is sent out by the first PC to detect the collision.
  • Using either a priority or random backoff scheme, the PCs wait certain amount of time before retransmitting.
  • If collisions continue to occur, the PCs random interval is doubled, lessening the chances of a collision.
csma cd and collisions27

CSMA/CD and Collisions

Hey, that’s me!

Nope

Nope

Abbreviated MAC Addresses

1111

2222

3333

nnnn

And as we said,

  • When information (frame) is transmitted, every PC/NIC on the shared media copies part of the transmitted frame to see if the destination address matches the address of the NIC.
  • If there is a match, the rest of the frame is copied
  • If there is NOT a match the rest of the frame is ignored.

Notice the location of the DA!

3333

1111

sending and receiving ethernet frames via a hub

Sending and receiving Ethernet frames via a hub

3333

1111

  • So, what does a hub do when it receives information?
  • Remember, a hub is nothing more than a multi-port repeater.

1111

?

2222

5555

3333

4444

slide29

Hubs

Hub or

slide30

Hubs

3333

1111

  • The hub will flood it out all ports except for the incoming port.
  • Hub is a layer 1 device.
  • A hub does NOT look at layer 2 addresses, so it is fast in transmitting data.
  • Disadvantage with hubs: A hub or series of hubs is a single collision domain.
  • A collision will occur if any two or more devices transmit at the same time within the collision domain.
  • More on this later.

1111

2222

Nope

5555

Nope

3333

For me!

4444

Nope

slide31

Hubs

2222

1111

  • Another disadvantage with hubs is that is take up unnecessary bandwidth on other links.

1111

2222

For me!

5555

Wasted bandwidth

Nope

3333

Nope

4444

Nope

switches33

Switches

Source Address Table

PortSource MAC Add.PortSource MAC Add.

3333

1111

switch

  • Switches are also known as learning bridges or learning switches.
  • A switch has a source address table in cache (RAM) where it stores source MAC addresses after it learns about them.
  • A switch receives an Ethernet frame and searches the source address table for the Destination MAC address.
  • If it finds a match, it filters the frame by only sending it out that port.
  • If there is not a match if floods it out all ports.

1111

3333

Abbreviated MAC addresses

2222

4444

no destination address in table flood

No Destination Address in table, Flood

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111

3333

1111

switch

  • How does it learn source MAC addresses?
  • First, the switch will see if the SA (1111) is in it’s table.
  • If it is, it resets the timer (more in a moment).
  • If it is NOT in the table it adds it, with the port number.
  • Next, in our scenario, the switch will flood the frame out all other ports, because the DA is not in the source address table.

1111

3333

Abbreviated MAC addresses

2222

4444

destination address in table filter

Destination Address in table, Filter

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

1111

3333

switch

  • Most communications involve some sort of client-server relationship or exchange of information. (You will understand this more as you learn about TCP/IP.)
  • Now 3333 sends data back to 1111.
  • The switch sees if it has the SA stored.
  • It does NOT so it adds it. (This will help next time 1111 sends to 3333.)
  • Next, it checks the DA and in our case it can filter the frame, by sending it only out port 1.

1111

3333

Abbreviated MAC addresses

2222

4444

destination address in table filter36

Destination Address in table, Filter

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

3333

1111

switch

  • Now, because both MAC addresses are in the switch’s table, any information exchanged between 1111 and 3333 can be sent (filtered) out the appropriate port.
  • What happens when two devices send to same destination?
  • What if this was a hub?
  • Where is (are) the collision domain(s) in this example?

1111

3333

1111

3333

Abbreviated MAC addresses

2222

4444

no collisions in switch buffering

No Collisions in Switch, Buffering

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

3333

1111

switch

  • Unlike a hub, a collision does NOT occur, which would cause the two PCs to have to retransmit the frames.
  • Instead the switch buffers the frames and sends them out port #6 one at a time.
  • The sending PCs have no idea that there was another PC that wanted to send to the same destination.

3333

4444

1111

3333

Abbreviated MAC addresses

2222

4444

collision domains

Collision Domains

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

3333

1111

Collision Domains

switch

  • When there is only one device on a switch port, the collision domain is only between the PC and the switch. (Cisco curriculum is inaccurate on this point.)
  • With a full-duplex PC and switch port, there will be no collision, since the devices and the medium can send and receive at the same time.

3333

4444

1111

3333

Abbreviated MAC addresses

2222

4444

other information

Other Information

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

switch

  • How long are addresses kept in the Source Address Table?
    • 5 minutes is common on most vendor switches.
  • How do computers know the Destination MAC address?
      • ARP Caches and ARP Requests
  • How many addresses can be kept in the table?
    • Depends on the size of the cache, but 1,024 addresses is common.
  • What about Layer 2 broadcasts?
    • Layer 2 broadcasts (DA = all 1’s) is flooded out all ports.

1111

3333

Abbreviated MAC addresses

2222

4444

what happens here

What happens here?

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

1 2222 1 3333

1111

3333

  • Notice the Source Address Table has multiple entries for port #1.

3333

1111

2222

5555

what happens here41

What happens here?

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

1 2222 1 5555

1111

3333

  • The switch filters the frame out port #1.
  • But the hub is only a layer 1 device, so it floods it out all ports.
  • Where is the collision domain?

3333

1111

2222

5555

what happens here42

What happens here?

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

1 2222 1 5555

1111

3333

Collision Domain

3333

1111

2222

5555

lan segmentation with routers

LAN segmentation with routers
  • Routers provide segmentation of networks, adding a latency factor of 20% to 30% over a switched network.
  • This increased latency is because a router operates at the network layer and uses the IP address to determine the best path to the destination node.
  • Bridges and switches provide segmentation within a single network or subnetwork.
  • Routers provide connectivity between networks and subnetworks.
  • Routers also do not forward broadcasts while switches and bridges must forward broadcast frames.
layer 2 and layer 3 switching

Layer 2 and layer 3 switching

(routing)

  • A layer 3 switch is typically a layer 2 switch that includes a routing process, I.e. does routing. Layer 3 switching has many meanings and in many cases is just a marketing term.
  • Layer 3 switching is a function of the network layer.
  • The Layer 3 header information is examined and the packet is forwarded based on the IP address.
symmetric and asymmetric

Symmetric and Asymmetric

Note: Most switches are now 10/100, which allow you to use them symmetrically or asymmetrically.

ethernet switch latency
Ethernet switch latency
  • Latency is the period of time from when the beginning of a frame enters to when the end of the frame exits the switch.
  • Latency is directly related to the configured switching process and volume of traffic.
memory buffering

Memory buffering

switch

1111

3333

Abbreviated MAC addresses

2222

4444

memory buffering48
Memory Buffering
  • An Ethernet switch may use a buffering technique to store and forward frames.
  • Buffering may also be used when the destination port is busy.
  • The area of memory where the switch stores the data is called the memory buffer.
  • This memory buffer can use two methods for forwarding frame:
    • port-based memory buffering
    • shared memory buffering
  • In port-based memory buffering frames are stored in queues that are linked to specific incoming ports.
  • Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share.
two switching methods

Two switching methods
  • Store-and-forward – The entire frame is received before any forwarding takes place.
    • The destination and source addresses are read and filters are applied before the frame is forwarded.
    • CRC Check done
  • Cut-through – The frame is forwarded through the switch before the entire frame is received.
    • This mode decreases the latency of the transmission, but also reduces error detection.
  • Depends on the model of the switch.
cut through

Cut-through

Cut-through

  • Fast-forward – Offers the lowest level of latency.
    • Fast-forward switching immediately forwards a packet after reading the destination address.
    • There may be times when packets are relayed with errors.
    • Although this occurs infrequently and the destination network adapter will discard the faulty packet upon receipt.
cut through51

Cut-through

Cut-through

  • Fragment-free – Fragment-free switching filters out collision fragments before forwarding begins.
    • In a properly functioning network, collision fragments must be smaller than 64 bytes.
    • Anything greater than 64 bytes is a valid packet and is usually received without error.
    • Fragment-free switching waits until the packet is determined not to be a collision fragment before forwarding.
two switching methods52

Two switching methods
  • Adaptive cut-through
    • In this mode, the switch uses cut-through until it detects a given number of errors.
    • Once the error threshold is reached, the switch changes to store-and-forward mode.
functions of a switch
Functions of a switch
  • The main features of Ethernet switches are:
    • Isolate traffic among segments
    • Achieve greater amount of bandwidth per user by creating smaller collision domains
learning addresses
Learning Addresses

“Learning bridges” or Learning switches”

  • Bridges and switches learn in the following ways:
    • Reading the source MAC address of each received frame or datagram
    • Recording the port on which the MAC address was received.
  • The bridge or switch learns which addresses belong to the devices connected to each port.
  • The learned addresses and associated port or interface are stored in the addressing table.
  • The bridge examines the destination address of all received frames.
  • The bridge then scans the address table searching for the destination address.
filter or flood
Filter or Flood
  • If a switch has the frame’s destination address in its CAM table (or Source Address Table) it will only send the frame out the appropriate port.
  • If a switch does not have the frame’s destination MAC address in its CAM table, it floods (sends) it out all ports except for the incoming port (the port that the frame came in on) known as an Unknown Unicast, or if the destination MAC address is a broadcast.
  • Note: A CAM table may contain multiple entries per port, if a hub or a switch is attached to that port.
  • Most Ethernet bridges can filter broadcast and multicast frames.
filter or flood56
Filter or Flood
  • Switches flood frames that are:
    • Unknown unicasts
    • Layer 2 broadcasts
    • Multicasts (unless running multicast snooping or IGMP)
      • Multicast are special layer 2 and layer 3 addresses that are sent to devices that belong to that “group”.
why segment lans layer 2 segments
Why segment LANs? (Layer 2 segments)

Hub

Switch

  • to isolate traffic between segments.
  • to achieve more bandwidth per user by creating smaller collision domains.
why segment lans layer 2 segments58

Why segment LANs? (Layer 2 segments)

switch

Collision Domains

  • A switch employs “microsegmentation” to reduce the collision domain on a LAN.
  • The switch does this by creating dedicated network segments, or point-to-point connections.

1111

3333

Abbreviated MAC addresses

2222

4444

broadcast domains

Broadcast domains
  • Even though the LAN switch reduces the size of collision domains, all hosts connected to the switch are still in the same broadcast domain.
  • Therefore, a broadcast from one node will still be seen by all the other nodes connected through the LAN switch.
  • ARP Request
broadcast domains60
Broadcast domains
  • When a device wants to send out a Layer 2 broadcast, the destination MAC address in the frame is set to all ones.
  • A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal.
  • By setting the destination to this value, all the devices will accept and process the broadcasted frame.
using switches

Using Switches
  • Layer 2 devices
  • Layer 2 filtering based on Destination MAC addresses and Source Address Table
  • One collision domain per port
  • One broadcast domain across all switches
other switching features

Other Switching Features

Review

  • Asymmetric ports: 10 Mbps and 100 Mbps
  • Full-duplex ports
  • Cut-through versus Store-and-Forward switching
slide64

Other Switching Features

  • Ports between switches and server ports are good candidates for higher bandwidth ports (100 Mbps) and full-duplex ports.
  • Most switch ports today are full-duplex.
introducing multiple subnets networks without routers

Introducing Multiple Subnets/Networks without Routers
  • Switches are Layer 2 devices
  • Router are Layer 3 devices
  • Data between subnets/networks must pass through a router.
slide66

Switched Network with Multiple Subnets

ARP Request

  • What are the issues?
  • Can data travel within the subnet? Yes
  • Can data travel between subnets? No, need a router!
  • What is the impact of a layer 2 broadcast, like an ARP Request?
slide67

Switched Network with Multiple Subnets

ARP Request

  • All devices see the ARP Request, even those on the other subnets that do not need to see it.
  • One broadcast domain means the switches flood all broadcast out all ports, except the incoming port.
  • Switches have no idea of the layer 3 information contained in the ARP Request.This consumes bandwidth on the network and processing cycles on the hosts.
slide68

One Solution: Physically separate the subnets

  • But still no data can travel between the subnets.
  • How can we get the data to travel between the two subnets?
slide69

Another Solution: Use a Router

  • Two separate broadcast domains, because the router will not forward the layer 2 broadcasts such as ARP Requests.
switches with multiple subnets

Switches with multiple subnets
  • So far this should have been a review.
  • Lets see what happens when we have two subnets on a single switch and we want to route between the two subnets.
slide71

Router-on-a-stick or One-Arm-Router (OAR)

interface e 0

ip address 172.30.1.1 255.255.255.0

ip address 172.30.2.1 255.255.255.0 secondary

ARP Request

Secondary addresses can be used when the router does not support sub-interfaces which will be discussed later.

  • When a single interface is used to route between subnets or networks, this is know as a router-on-a-stick.
  • To assign multiple ip addresses to the same interface, secondary addresses or subinterfaces are used.
slide72

Router-on-a-stick or One-Arm-Router (OAR)

interface e 0

ip address 172.30.1.1 255.255.255.0

ip address 172.30.2.1 255.255.255.0 secondary

Advantages

  • Useful when there are limited Ethernet interfaces on the router.

Disadvantage

  • Because a single link is used to connect multiple subnets, one link is having to carry the traffic for multiple subnets.
  • Be sure this is link can handle the traffic.
slide73

Router-on-a-stick or One-Arm-Router (OAR)

interface e 0

ip address 172.30.1.1 255.255.255.0

ip address 172.30.2.1 255.255.255.0 secondary

ARP Request

  • Still the same problem of the switch forwarding broadcast traffic to all devices on all subnets.
slide74

Router-on-a-stick or One-Arm-Router (OAR)

interface e 0

ip address 172.30.1.1 255.255.255.0

ip address 172.30.2.1 255.255.255.0 secondary

Remember to have the proper default gateway set for each host.

  • 172.30.1.0 hosts - default gateway is 172.30.1.1
  • 172.30.2.0 hosts - default gateway is 172.30.2.1
slide75

Interface for each subnet

E0

E1

  • An Ethernet router interface per subnet may be used instead of one.
  • However this may be difficult if you do not have enough Ethernet ports on your router.
slide76

Still one broadcast domain

ARP Request

  • Still the same problem of the switch forwarding broadcast traffic to all devices on all subnets.
introducing vlans

Introducing VLANs
  • VLAN = Subnet
  • VLANs create separate broadcast domains within the switch.
  • Routers are needed to pass information between different VLANs
  • This is only an introduction, as we will discuss VLANs in later chapters.
slide78

Layer 2 Broadcast Segmentation

Switch Port: VLAN ID

ARP Request

  • An ARP Request from 172.30.1.21 for 172.30.1.23 will only be seen by hosts on that VLAN.
  • The switch will flood broadcast traffic out only those ports belonging to that particular VLAN, in this case VLAN 1.
slide79

Layer 2 Broadcast Segmentation

Port-centric VLAN Switches

  • As the Network Administrator, it is your job to assign switch ports to the proper VLAN.
  • This assignment is only done at the switch and not at the host.
  • Note: The following diagrams show the VLAN below the host, but it is actually assigned on the switch.
slide80

Without VLANs – No Broadcast Control

ARP Request

  • Without VLANs, the ARP Request would be seen by all hosts.
  • Again, consuming unnecessary network bandwidth and host processing cycles.
slide81

With VLANs – Broadcast Control

Switch Port: VLAN ID

ARP Request

slide82

Inter-VLAN Traffic

Switch Port: VLAN ID

1. Remember that VLAN IDs (numbers) are assigned to the switch port and not to the host. (Port-centric VLAN switches)

2. Be sure to have all of the hosts on the same subnet belong to the same VLAN, or you will have problems.

  • Hosts on subnet 172.30.1.0/24 - VLAN 1
  • Hosts on subnet 172.30.2.0/24 - VLAN 2
  • etc.
slide83

Inter-VLAN Traffic

Switch Port: VLAN ID

To 172.30.2.12

  • A switch cannot route data between different VLANs.
  • Note: The host will not even send the Packet unless it has a default gateway to forward it to.
slide84

Inter-VLAN Routing needs a Router

  • A router is need to route traffic between VLANs (VLAN = Subnet).
  • There are various methods of doing this including Router-on-a-stick with trunking (more than one VLAN on the link).
  • This will be discussed later when we get to the chapter on VLANs and Inter-VLAN Routing.
module 4 switching concepts85

Module 4 – Switching Concepts

CCNA 3

Cabrillo College