ch. 5 – switching concepts ccna 3 version 3.0. rick graziani [email protected] overview –...

Ch. 5 – Switching Concepts

CCNA 3 version 3.0

Rick Graziani [email protected] 2

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 microsegmentation • 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

• 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

• 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.

RoutersSwitches, BridgesHub, Repeaters


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

• 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 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

• 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


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

• 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

• 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

• 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

• 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

• 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 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

• 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 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

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


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?

1111 2222 3333 nnnn Abbreviated MAC Addresses

11113333



• 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


11113333

Nope

Nope

Hey, that’s me!



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




Collision!


X


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)

Access Methods


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 (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 access allows 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 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.


11113333

Nope

Nope

Hey, that’s me!

Notice the location of the DA!

CSMA/CD and Collisions•


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.

1111 2222

3333 4444

5555

?

11113333

•



Hub or

•



• 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

3333 4444

5555

11113333

Nope

Nope

Nope

For me!

•



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

1111 2222

3333 4444

5555

11112222

Nope

Nope

Nope

For me!

Wasted bandwidth

•


Sending and receiving Ethernet frames via a switch•


Sending and receiving Ethernet frames via a switch

Source Address TablePort Source MAC Add. Port Source 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.

switch

1111

2222

3333

4444

Abbreviated MAC addresses

11113333

•


No Destination Address in table, Flood


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.

switch

1111

2222

3333

4444


11113333

•


Destination Address in table, Filter


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.

switch

1111

2222

3333

4444


33331111

•


Destination Address in table, Filter


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?

switch

1111

2222

3333

4444


11113333

33331111

•


No Collisions in Switch, Buffering


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.

switch

1111

2222

3333

4444


11113333

44443333

•


Collision Domains


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.

switch

1111

2222

3333

4444


11113333

44443333

Collision Domains

•


Other Information


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

2222

3333

4444


•


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

Receive Packet

Learn source address or refresh aging timer

Is the destination a broadcast, multicast or unknown unicast?

Are the source and destination on the same interface?

Forward unicast to correct port

Flood Packet

Filter Packet

Yes

Yes

No

No


What happens here?

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

33331111

3333

1111


Add. 1 1111 6 3333 1 2222 1 3333

2222 5555

•


What happens here?

• 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?

33331111

3333

1111


Add. 1 1111 6 3333 1 2222 1 5555

2222 5555

•


What happens here?

33331111

3333

1111


Add. 1 1111 6 3333 1 2222 1 5555

2222 5555

Collision Domain

•


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

• 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

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

•


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

switch

1111

2222

3333

4444


• 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

• 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

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-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 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

• 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

• 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)

• 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 (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)

• 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 segments)

switch

1111

2222

3333

4444


• 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

•


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.

All Sw itched Netw ork - Tw o Netw orks Two S ubnets S evera l C o llis ion D om ains

O ne per sw itch port O ne B roadcast D om ain

Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

Sw itch 2

172.30.1.25255.255.255.0

172.30.2.14255.255.255.0

172.30.1.27255.255.255.0

172.30.2.16255.255.255.0172.30.2.12

255.255.255.0

• ARP Request

•


Switches and broadcast domains

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

• 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”.

•



• 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.


Communication between switches and workstation

Hubs to VLANsPart 1

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

•


Using Hubs

• Layer 1 devices

• Inexpensive

• In one port, out the others

• One collision domain

• One broadcast domain

•


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

Hub 1

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

172.30.1.24255.255.255.0

Single Hub O ne N etwork (IP N etwork Address - usua lly) O ne C ollis ion D om ain O ne B roadcast D om ain

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.

Hub 1

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.2.21255.255.255.0

172.30.2.22255.255.255.0

Single Hub - Tw o subnets Two subnets O ne C ollis ion D om ain O ne B roadcast D om ain

Single Hub

Note: Different color hosts refer to different subnets.

•


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

All Hubs O ne N etwork A ddress O ne C ollis ion D om ain O ne B roadcast D om ain

Hub 1

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Hub 2

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

Multiple Hubs•


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

•


Sw itch and Hub Netw ork O ne N etwork S evera l C o llis ion D om ains

O ne per sw itch port O ne for the entire H ub

O ne B roadcast D om ain

Hub

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Sw itch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

Switches create multiple parallel paths

Two parallel paths: (complete SAT tables)

• Data traffic from 172.30.1.24 to 172.30.1.25


•





Hub

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Sw itch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

Collision!

Hubs do not create multiple parallel paths

As opposed to the Hub:



•





Hub

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Sw itch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

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 paths•





Hub

172.30.1.21255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Sw itch

172.30.1.24255.255.255.0

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

Frames buffered

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.

Switches create multiple parallel paths•


Other Switching Features

Review

• Asymmetric ports: 10 Mbps and 100 Mbps

• Full-duplex ports

• Cut-through versus Store-and-Forward switching

•


All Sw itched Netw ork O ne N etwork S evera l C o llis ion D om ains


Sw itch 1172.30.1.21

255.255.255.0

172.30.1.22255.255.255.0

172.30.1.23255.255.255.0

Sw itch 2

172.30.1.25255.255.255.0

172.30.1.26255.255.255.0

172.30.1.27255.255.255.0

172.30.1.28255.255.255.0172.30.1.24

255.255.255.0

• 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.

Other Switching Features•


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.

•


All Switched Network - Two Networks Two Subnets Several Collision Domains

One per switch port One Broadcast Domain

Switch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

Switch 2

172.30.1.25255.255.255.0

172.30.2.14255.255.255.0

172.30.1.27255.255.255.0

172.30.2.16255.255.255.0172.30.2.12

255.255.255.0

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?

Switched Network with Multiple Subnets•


All Sw itched Netw ork - Tw o Netw orks Two S ubnets S evera l C o llis ion D om ains


Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

Sw itch 2

172.30.1.25255.255.255.0

172.30.2.14255.255.255.0

172.30.1.27255.255.255.0

172.30.2.16255.255.255.0172.30.2.12

255.255.255.0

• 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.

ARP Request

Switched Network with Multiple Subnets•


Tw o Sw itched Netw orks Two S ubnets S evera l C o llis ion D om ains

O ne per sw itch port Two B roadcast D om ain

Sw itch 1172.30.1.21

255.255.255.0

172.30.1.23255.255.255.0

172.30.1.25255.255.255.0

Sw itch 2

172.30.2.10255.255.255.0

172.30.2.12255.255.255.0

172.30.2.14255.255.255.0

172.30.2.16255.255.255.0172.30.1.26

255.255.255.0

• But still no data can travel between the subnets.

• How can we get the data to travel between the two subnets?

One Solution: Physically separate the subnets•


Routed Netw orks Two S ubnets S evera l C o llis ion D om ains

O ne per sw itch port C om m unica tion between subnets

Sw itch 1172.30.1.21

255.255.255.0

172.30.1.23255.255.255.0

172.30.1.25255.255.255.0

Sw itch 2

172.30.2.10255.255.255.0

172.30.2.12255.255.255.0

172.30.2.14255.255.255.0

172.30.2.16255.255.255.0172.30.1.26

255.255.255.0

Router

172.30.1.1255.255.255.0

172.30.2.1255.255.255.0

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

Another Solution: Use a Router•


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.

•


Routed Netw orks Two Subnets C om m unication between subnets

Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1172.30.2.1 sec255.255.255.0

interface e 0

ip address 172.30.1.1 255.255.255.0

ip address 172.30.2.1 255.255.255.0 secondary

• 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.

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

ARP Request

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

•



Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1172.30.2.1 sec255.255.255.0

interface e 0

ip address 172.30.1.1 255.255.255.0


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.

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



Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1172.30.2.1 sec255.255.255.0

interface e 0

ip address 172.30.1.1 255.255.255.0


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

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

ARP Request

•


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


Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1172.30.2.1 sec255.255.255.0

interface e 0

ip address 172.30.1.1 255.255.255.0


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


Routed Netw orks Two S ubnets C om m unica tion between subnets

Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1255.255.255.0

172.30.2.1255.255.255.0

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.

Interface for each subnet•


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

Still one broadcast domain

Routed Netw orks Two S ubnets C om m unica tion between subnets

Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

Router

172.30.1.1255.255.255.0

172.30.2.1255.255.255.0

ARP Request

•


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.

•


Tw o VLANs Two S ubnets

Sw itch 1172.30.1.21

255.255.255.0VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Switch Port: VLAN ID

• 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.

ARP Request

Layer 2 Broadcast Segmentation

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

•


1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

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.

Layer 2 Broadcast Segmentation•


No VLANs Sam e as a s ingle VLAN Two Subnets

Sw itch 1172.30.1.21

255.255.255.0

172.30.2.10255.255.255.0

172.30.1.23255.255.255.0

172.30.2.12255.255.255.0

• Without VLANs, the ARP Request would be seen by all hosts.

• Again, consuming unnecessary network bandwidth and host processing cycles.

ARP Request

Without VLANs – No Broadcast Control•



Sw itch 1172.30.1.21

255.255.255.0VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Switch Port: VLAN IDARP Request

With VLANs – Broadcast Control

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

•



Sw itch 1172.30.1.21

255.255.255.0VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Switch Port: VLAN ID

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

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.

Inter-VLAN Traffic•


• 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.


Sw itch 1172.30.1.21

255.255.255.0VLAN 1

172.30.2.10255.255.255.0

VLAN 2

172.30.1.23255.255.255.0

VLAN 1

172.30.2.12255.255.255.0

VLAN 2

Switch Port: VLAN IDTo 172.30.2.12

1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

Inter-VLAN Traffic•


1 2 3 4 5 6 .1 2 1 2 2 1 .

PortVLAN

Router

172.30.1.1255.255.255.0

(VLAN 1)

172.30.2.1255.255.255.0

(VLAN 2)

• 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.

Inter-VLAN Routing needs a Router•

Ch. 4 – Switching Concepts

CCNA 3 version 3.0

Rick Graziani

Cabrillo College

ch. 5 – switching concepts ccna 3 version 3.0. rick graziani [email protected] overview –...

Documents

switches switches

switches slide

routers switches

rick graziani

network segmentation

network latency

network performance

overview ethernet networks