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Ch. 4 – Switching Concepts. CCNA 3 version 3.0. 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.

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Ch 4 switching concepts l.jpg

Ch. 4 – Switching Concepts

CCNA 3 version 3.0


Overview l.jpg

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 microsegmentation 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 l.jpg

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.


Bridges l.jpg

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 l.jpg

Switches

  • Switches create a virtual circuit between two connected devices, establishing a dedicated communication path between two devices.

  • Switches on the network provide microsegmentation.

  • This allows maximum utilization of the available bandwidth.

  • A switch is also able to facilitate multiple, simultaneous virtual circuit connections.

  • Broadcast frames to all connected devices on the network.


Router l.jpg

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.

  • Routers are not compelled to forward broadcasts.


Factors that impact network performance l.jpg

Factors that impact network performance


Elements of ethernet 802 3 networks l.jpg

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 l.jpg

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 l.jpg

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


Network congestion l.jpg

Network Congestion

  • Today's networks are experiencing an increase in the transmission of many forms of media:

    • Large graphics files

    • Images

    • Full-motion video

    • Multimedia applications


Network latency l.jpg

Network 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:

    • First, there is 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. This is sometimes called NIC delay.

    • Second, there is the actual propagation delay as the signal takes time to travel along the cable.

    • Third, latency is 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 l.jpg

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 l.jpg

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.


Full duplex transmitting l.jpg

Full-duplex transmitting

  • 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 transmissions16 l.jpg

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


Lan segmentation l.jpg

LAN segmentation

  • Not the best diagram, let’s look at some examples…


Sending and receiving ethernet frames on a bus l.jpg

Sending and receiving Ethernet frames on a bus

  • When an Ethernet frame is sent out on the “bus” all devices on the bus receive it.

  • What do they do with it?

Abbreviated MAC Addresses

1111

2222

3333

nnnn

3333

1111


Sending and receiving ethernet frames on a bus19 l.jpg

Sending and receiving Ethernet frames on a bus

  • 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

Hey, that’s me!

Nope

Nope

Abbreviated MAC Addresses

1111

2222

3333

nnnn

3333

1111


Sending and receiving ethernet frames on a bus20 l.jpg

Sending and receiving Ethernet frames on a bus

  • So, what happens when multiple computers try to transmit at the same time?

Abbreviated MAC Addresses

1111

2222

3333

nnnn


Sending and receiving ethernet frames on a bus21 l.jpg

Sending and receiving Ethernet frames on a bus

Collision!

Abbreviated MAC Addresses

1111

2222

3333

nnnn

X


Access methods l.jpg

Access Methods

Two common types of access methods for LANs include

  • Non-Deterministic: Contention methods (Ethernet, IEEE 802.3)

    • Only one signal can be on a network segment at one time.

    • Collisions are a normal occurrence on an Ethernet/802.3 LAN

  • Deterministic: Token Passing (Token Ring)


Csma cd l.jpg

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 l.jpg

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 collisions25 l.jpg

CSMA/CD and Collisions

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.

Hey, that’s me!

Nope

Nope

Abbreviated MAC Addresses

1111

2222

3333

nnnn

Notice the location of the DA!

3333

1111


Sending and receiving ethernet frames via a hub l.jpg

Sending and receiving Ethernet frames via a hub

  • So, what does a hub do when it receives information?

  • Remember, a hub is nothing more than a multiport repeater.

3333

1111

1111

?

2222

5555

3333

4444


Sending and receiving ethernet frames via a hub27 l.jpg

Sending and receiving Ethernet frames via a hub

Hub or


Sending and receiving ethernet frames via a hub28 l.jpg

Sending and receiving Ethernet frames via a hub

  • 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.

3333

1111

1111

2222

Nope

5555

Nope

3333

For me!

4444

Nope


Sending and receiving ethernet frames via a hub29 l.jpg

Sending and receiving Ethernet frames via a hub

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

2222

1111

1111

2222

For me!

5555

Wasted bandwidth

Nope

3333

Nope

4444

Nope


Sending and receiving ethernet frames via a switch l.jpg

Sending and receiving Ethernet frames via a switch


Sending and receiving ethernet frames via a switch31 l.jpg

Sending and receiving Ethernet frames via a switch

Source Address Table

PortSource MAC Add.PortSource MAC Add.

  • 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 address after it learns about them.

  • A switch receives an Ethernet frame it 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.

3333

1111

switch

1111

3333

Abbreviated MAC addresses

2222

4444


No destination address in table flood l.jpg

No Destination Address in table, Flood

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111

  • 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.

3333

1111

switch

1111

3333

Abbreviated MAC addresses

2222

4444


Destination address in table filter l.jpg

Destination Address in table, Filter

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

  • 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

switch

1111

3333

Abbreviated MAC addresses

2222

4444


Destination address in table filter34 l.jpg

Destination Address in table, Filter

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

  • 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?

3333

1111

switch

1111

3333

1111

3333

Abbreviated MAC addresses

2222

4444


No collisions in switch buffering l.jpg

No Collisions in Switch, Buffering

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

  • 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 their was another PC wanting to send to the same destination.

3333

1111

switch

3333

4444

1111

3333

Abbreviated MAC addresses

2222

4444


Collision domains l.jpg

Collision Domains

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

  • 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

1111

Collision Domains

switch

3333

4444

1111

3333

Abbreviated MAC addresses

2222

4444


Other information l.jpg

Other Information

Source Address Table

PortSource MAC Add.PortSource MAC Add.

1 1111 6 3333

9 4444

  • 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.

  • switch

    1111

    3333

    Abbreviated MAC addresses

    2222

    4444


    Side note transparent bridging l.jpg

    Side Note - Transparent Bridging

    • Transparent bridging (normal switching process) is defined in IEEE 802.1d describing the five bridging processes of:

      • learning

      • flooding filtering

      • forwarding

      • aging

    • These will be discussed further in STP (Spanning Tree Protocol)


    Transparent bridge process jeff doyle l.jpg

    Transparent Bridge Process - Jeff Doyle

    Receive Packet

    Learn source address or refresh aging timer

    Is the destination a broadcast, multicast or unknown unicast?

    Yes

    No

    Flood Packet

    Are the source and destination on the same interface?

    No

    Yes

    Filter Packet

    Forward unicast to correct port


    What happens here l.jpg

    What happens here?

    Source Address Table

    PortSource MAC Add.PortSource MAC Add.

    1 1111 6 3333

    1 2222 1 3333

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

    1111

    3333

    3333

    1111

    2222

    5555


    What happens here41 l.jpg

    What happens here?

    Source Address Table

    PortSource MAC Add.PortSource MAC Add.

    1 1111 6 3333

    1 2222 1 5555

    • 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?

    1111

    3333

    3333

    1111

    2222

    5555


    What happens here42 l.jpg

    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 l.jpg

    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 l.jpg

    Layer 2 and layer 3 switching

    • A layer 3 switch is typically a layer 2 switch that includes a routing process, I.e. does routing. (Oh yea, also known as routing. Got to love those people in Marketing.)

    • 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.

    (routing)


    Symmetric and asymmetric switching l.jpg

    Symmetric and asymmetric switching

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


    Ethernet switch latency l.jpg

    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 l.jpg

    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.

    switch

    1111

    3333

    Abbreviated MAC addresses

    2222

    4444


    Two switching methods l.jpg

    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.

    • 1900 and 2800 series switches this is configurable, otherwise depends on the model of the switch.


    Cut through l.jpg

    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 through50 l.jpg

    Cut-through

    Cut-through

    • Fragment-free – Fragment-free switching filters out collision fragments before forwarding begins.

      • Collision fragments are the majority of packet errors.

      • 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 methods51 l.jpg

    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 l.jpg

    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


    How switches learn addresses l.jpg

    How switches learn addresses

    • 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.

    “Learning bridges” or Learning switches”


    Filter or flood switch l.jpg

    Filter or Flood (Switch)

    • 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 flood switch55 l.jpg

    Filter or Flood (Switch)

    • 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 l.jpg

    Why segment LANs? (Layer 2 segments)

    • First is to isolate traffic between segments.

    • The second reason is to achieve more bandwidth per user by creating smaller collision domains.

    Hub

    Switch


    Why segment lans layer 2 segments57 l.jpg

    Why segment LANs? (Layer 2 segments)

    switch

    • 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.

    Collision Domains

    1111

    3333

    Abbreviated MAC addresses

    2222

    4444


    Broadcast domains l.jpg

    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


    Switches and broadcast domains l.jpg

    Switches and broadcast domains

    • 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”.

    These are logical not physical representations of what happens to these frames.


    Switches and broadcast domains60 l.jpg

    Switches and 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.


    Switches and broadcast domains61 l.jpg

    Switches and broadcast domains


    Communication between switches and workstation l.jpg

    Communication between switches and workstation


    Hubs to vlans part 1 l.jpg

    Hubs to VLANsPart 1

    (Part 2 will be discussed when we cover VLANs.)


    Using hubs l.jpg

    Using Hubs

    • Layer 1 devices

    • Inexpensive

    • In one port, out the others

    • One collision domain

    • One broadcast domain


    Single hub l.jpg

    Single Hub

    This is fine for small workgroups, but does not scale well for larger workgroups or heavy traffic.


    Single hub66 l.jpg

    Single Hub

    • What if the computers were on two different subnets?

    • Could they communicate within their own subnet? Yes

    • Between subnets? No, need a router. The sending host will check the destination IP address with its own IP address and subnet mask. The AND operation will determine that it is on a different subnet and cannot be reached without sending the packet to a default gateway (router). This is even though they are on the same physical network.

    Note: Different color hosts refer to different subnets.


    Multiple hubs l.jpg

    Multiple Hubs

    • Same issues as before, with more of an impact on the network.


    Using switches l.jpg

    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


    Switches create multiple parallel paths l.jpg

    Switches create multiple parallel paths

    Two parallel paths: (complete SAT tables)

    • Data traffic from 172.30.1.24 to 172.30.1.25

    • Data traffic from 172.30.1.26 to 172.30.1.2


    Hubs do not create multiple parallel paths l.jpg

    Hubs do not create multiple parallel paths

    As opposed to the Hub:

    • Data traffic from 172.30.1.21 to 172.30.1.22

    • Data traffic from 172.30.1.23 to 172.30.1.24

    Collision!


    Switches create multiple parallel paths71 l.jpg

    Switches create multiple parallel paths

    Collisions and Switches:

    What happens when two devices on a switch, send data to another device on the switch?

    172.30.1.24 to 172.30.1.25 and 172.30.1.26 to 172.30.1.25


    Switches create multiple parallel paths72 l.jpg

    Switches create multiple parallel paths

    The switch keeps the frames in buffer memory, and queues the traffic for the host 172.30.1.25.

    This means that the sending hosts do not know about the collisions and do not have to re-send the frames.

    Frames buffered


    Other switching features l.jpg

    Other Switching Features

    Review

    • Asymmetric ports: 10 Mbps and 100 Mbps

    • Full-duplex ports

    • Cut-through versus Store-and-Forward switching


    Slide74 l.jpg

    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 l.jpg

    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.


    Slide76 l.jpg

    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?


    Slide77 l.jpg

    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.


    Slide78 l.jpg

    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?


    Slide79 l.jpg

    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 l.jpg

    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.


    Slide81 l.jpg

    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.


    Slide82 l.jpg

    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.


    Slide83 l.jpg

    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

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

    ARP Request


    Slide84 l.jpg

    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


    Slide85 l.jpg

    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.


    Slide86 l.jpg

    Still one broadcast domain

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

    ARP Request


    Introducing vlans l.jpg

    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 and Inter-VLAN Routing in later chapters.


    Slide88 l.jpg

    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.


    Slide89 l.jpg

    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.


    Slide90 l.jpg

    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.


    Slide91 l.jpg

    With VLANs – Broadcast Control

    Switch Port: VLAN ID

    ARP Request


    Slide92 l.jpg

    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.


    Slide93 l.jpg

    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.


    Slide94 l.jpg

    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.


    Ch 4 switching concepts95 l.jpg

    Ch. 4 – Switching Concepts

    CCNA 3 version 3.0


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