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
 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











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
Overview
Routers
Switches, Bridges
Hub, Repeaters




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.
Ethernet Limitations
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
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.
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.
Factors that impact 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



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

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
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




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.
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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
Sending and receiving Ethernet frames
on a bus
1111
2222
3333
nnnn
Abbreviated
MAC
Addresses
3333 1111


When an Ethernet frame is sent out on the
“bus” all devices on the bus receive it.
What do they do with it?
Sending and receiving Ethernet frames
on a bus
Nope
1111
2222
Hey, that’s
me!
3333
Nope
nnnn
Abbreviated
MAC
Addresses
3333 1111



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
Sending and receiving Ethernet frames
on a bus
1111

2222
3333
nnnn
Abbreviated
MAC
Addresses
So, what happens when multiple computers
try to transmit at the same time?
Sending and receiving Ethernet frames
on a bus
1111
2222
3333
nnnn
X
Collision!
Abbreviated
MAC
Addresses
•
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 (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 54-3 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
Nope
1111
Notice the
location of
the DA!
2222
Hey, that’s
me!
3333
Nope
nnnn
Abbreviated
MAC
Addresses
3333 1111
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.
•
Sending and receiving Ethernet frames via a hub
3333 1111
1111
?
2222

5555
3333
4444

So, what does a hub
do when it receives
information?
Remember, a hub is
nothing more than a
multi-port repeater.
•
Hubs
Hub or
•
Hubs
3333 1111
1111
2222
Nope




5555
Nope


3333 For me!
4444 Nope
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.
•
Hubs
2222 1111
1111
2222
For me!
5555
Nope
3333 Nope
4444 Nope

Another disadvantage
with hubs is that is take up
unnecessary bandwidth
on other links.
Wasted
bandwidth
•
Sending and receiving Ethernet frames via a
switch
•
Switches
Source Address Table
Port Source MAC Add. Port Source MAC Add.
3333 1111

switch


1111
3333

Abbreviated
MAC
addresses
2222
4444

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.
•
No Destination Address in table, Flood
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
3333 1111

switch



1111

3333
Abbreviated
MAC
addresses
2222
4444
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.
•
Destination Address in table, Filter
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
1111 3333

switch


1111
3333

Abbreviated
MAC
addresses

2222
4444
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.
•
Destination Address in table, Filter
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
3333 1111
switch
1111 3333
1111
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?

Abbreviated
MAC
addresses

2222
4444
•
No Collisions in Switch, Buffering
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
9
4444
3333 1111
switch
3333 4444


1111
3333

Abbreviated
MAC
addresses
2222
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 there was another PC that
wanted to send to the same
destination.
•
Collision Domains
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
9
4444
3333 1111
Collision Domains
switch
3333 4444

1111

3333
Abbreviated
MAC
addresses
2222
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.
•
Other Information
Source Address Table
Port Source MAC Add. Port Source 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?


How many addresses can be
kept in the table?
–
1111
3333
Abbreviated
MAC
addresses

2222
4444
ARP Caches and ARP
Requests
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.
•
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
1
2222
1
3333
What happens here?
1111 3333

3333
1111 2222 5555
Notice the
Source
Address Table
has multiple
entries for port
#1.
•
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
1
2222
1
5555
What happens here?
1111 3333


3333
1111 2222 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.
•
Source Address Table
Port Source MAC Add. Port Source MAC Add.
1
1111
6
3333
1
2222
1
5555
What happens here?
1111 3333
Collision Domain
3333
1111 2222 5555
•
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.
•
(routing)
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. 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
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.
•
switch
Memory buffering
1111
3333
Abbreviated
MAC
addresses
2222
4444
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

Store-and-forward – The entire frame is received before
any forwarding takes place.
–
–

Cut-through – The frame is forwarded through the switch
before the entire frame is received.
–

The destination and source addresses are read and filters are
applied before the frame is forwarded.
CRC Check done
This mode decreases the latency of the transmission, but also
reduces error detection.
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.
–
–
–
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
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




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

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)
Hub
Switch


to isolate traffic between segments.
to achieve more bandwidth per user by
creating smaller collision domains.
•
Why segment LANs? (Layer 2
switch
segments)
Collision Domains

1111
3333
Abbreviated
MAC
addresses

2222
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.
•
Broadcast domains
172.30.1.21
255.255.255.0
172.30.2.10
255.255.255.0
Switch 1
172.30.1.23
255.255.255.0
172.30.2.12
255.255.255.0
Switched
Network - Two Networks
•All
ARP
Request
Ÿ Two Subnets
Ÿ
Ÿ
Several Collision Domains
Ÿ One per switch port
One Broadcast Domain


Switch 2
172.30.2.16
255.255.255.0
172.30.1.25
255.255.255.0
172.30.2.14
255.255.255.0
172.30.1.27
255.255.255.0
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.
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 domains
•
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
Review
 Asymmetric ports: 10 Mbps and 100 Mbps
 Full-duplex ports
 Cut-through versus Store-and-Forward
switching
•
Other Switching Features
172.30.1.21
255.255.255.0
172.30.1.22
255.255.255.0
Switch 1
172.30.1.23
255.255.255.0
172.30.1.24
255.255.255.0
All Switched Network
Ÿ One Network
Ÿ Several Collision Domains
Ÿ One per switch port
Ÿ One Broadcast Domain
Switch 2
172.30.1.28
255.255.255.0
172.30.1.25
255.255.255.0
172.30.1.26
255.255.255.0
172.30.1.27
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.
•
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.
•
Switched Network with Multiple Subnets
ARP Request
172.30.1.21
255.255.255.0
172.30.2.10
255.255.255.0
Switch 1
172.30.1.23
255.255.255.0
172.30.2.12
255.255.255.0
All Switched Network - Two Networks
Ÿ Two Subnets
Ÿ Several Collision Domains
Ÿ One per switch port
Ÿ One Broadcast Domain
•
•
•
•
Switch 2
172.30.2.16
255.255.255.0
172.30.1.25
255.255.255.0
172.30.2.14
255.255.255.0
172.30.1.27
255.255.255.0
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
ARP Request
172.30.1.21
255.255.255.0
172.30.2.10
255.255.255.0
Switch 1
172.30.1.23
255.255.255.0
172.30.2.12
255.255.255.0
All Switched Network - Two Networks
Ÿ Two Subnets
Ÿ Several Collision Domains
Ÿ One per switch port
Ÿ One Broadcast Domain
•
•
•
Switch 2
172.30.2.16
255.255.255.0
172.30.1.25
255.255.255.0
172.30.2.14
255.255.255.0
172.30.1.27
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.
•
One Solution: Physically separate the subnets
172.30.1.21
255.255.255.0
172.30.1.23
255.255.255.0
Switch 1
172.30.1.25
255.255.255.0
Two Switched Networks
Ÿ Two Subnets
Ÿ Several Collision Domains
Ÿ One per switch port
Ÿ Two Broadcast Domain
172.30.1.26
255.255.255.0
Switch 2
172.30.2.16
255.255.255.0
172.30.2.10
255.255.255.0
172.30.2.12
255.255.255.0
172.30.2.14
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?
•
Another Solution: Use a Router
172.30.1.21
255.255.255.0
172.30.1.23
255.255.255.0
172.30.1.1
255.255.255.0
Switch 1
172.30.2.1
255.255.255.0
Router
172.30.1.25
255.255.255.0
172.30.1.26
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Several Collision Domains
Ÿ One per switch port
Ÿ Communication between subnets
•
Switch 2
172.30.2.16
255.255.255.0
172.30.2.10
255.255.255.0
172.30.2.12
255.255.255.0
172.30.2.14
255.255.255.0
Two separate broadcast domains, because the router will
not forward the layer 2 broadcasts such as ARP Requests.
•
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.
•
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
Router
172.30.1.1
172.30.2.1 sec
255.255.255.0
ARP Request
Secondary addresses
can be used when the
router does not support
sub-interfaces which will
be discussed later.
172.30.1.21
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
•
•
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Communication between subnets
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)
interface e 0
ip address 172.30.1.1 255.255.255.0
ip address 172.30.2.1 255.255.255.0 secondary
172.30.1.21
255.255.255.0
Router
172.30.1.1
172.30.2.1 sec
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Advantages
Communication
between
 UsefulŸwhen
there are limited
Ethernetsubnets
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)
interface e 0
ip address 172.30.1.1 255.255.255.0
ip address 172.30.2.1 255.255.255.0 secondary
Router
172.30.1.1
172.30.2.1 sec
255.255.255.0
ARP Request
172.30.1.21
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Communication between subnets

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)
interface e 0
ip address 172.30.1.1 255.255.255.0
ip address 172.30.2.1 255.255.255.0 secondary
172.30.1.21
255.255.255.0
Router
172.30.1.1
172.30.2.1 sec
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Communication between subnets
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
•
Interface for each subnet
172.30.1.1 E0
255.255.255.0
172.30.1.21
255.255.255.0
E1 172.30.2.1
Router
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Communication between subnets
• 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.
•
Still one broadcast domain
172.30.1.1
255.255.255.0
Router
172.30.2.1
255.255.255.0
ARP Request
172.30.1.21
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
Routed Networks
Ÿ Two Subnets
Ÿ Communication between subnets

Still the same problem of the switch forwarding
broadcast traffic to all devices on all subnets.
•
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.
•
Layer 2 Broadcast Segmentation
Switch Port: VLAN ID
ARP Request
172.30.1.21
255.255.255.0
VLAN 1
Switch 1
172.30.2.12
255.255.255.0
VLAN 2
172.30.2.10
255.255.255.0
VLAN 2
172.30.1.23
255.255.255.0
VLAN 1
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
Two VLANs
Ÿ Two Subnets
• 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.
•
Layer 2 Broadcast Segmentation
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
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.
•
Without VLANs – No Broadcast Control
ARP Request
172.30.1.21
255.255.255.0
Switch 1
172.30.2.12
255.255.255.0
172.30.2.10
255.255.255.0
172.30.1.23
255.255.255.0
No VLANs
Ÿ Same as a single VLAN
Ÿ Two Subnets
• Without VLANs, the ARP Request would be seen by all hosts.
• Again, consuming unnecessary network bandwidth and host processing
cycles.
•
With VLANs – Broadcast Control
Switch Port: VLAN ID
ARP Request
172.30.1.21
255.255.255.0
VLAN 1
Switch 1
172.30.2.12
255.255.255.0
VLAN 2
172.30.2.10
255.255.255.0
VLAN 2
172.30.1.23
255.255.255.0
VLAN 1
Two VLANs
Ÿ Two Subnets
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
•
Inter-VLAN Traffic
Switch Port: VLAN ID
172.30.1.21
255.255.255.0
VLAN 1
Switch 1
172.30.2.12
255.255.255.0
VLAN 2
172.30.2.10
255.255.255.0
VLAN 2
172.30.1.23
255.255.255.0
VLAN 1
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
Two
VLANs
1. Remember that
VLAN
IDs (numbers) are assigned to the switch port
and not to theŸ host.
Two(Port-centric
Subnets 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
Switch Port: VLAN ID
To 172.30.2.12
172.30.1.21
255.255.255.0
VLAN 1
Switch 1
172.30.2.12
255.255.255.0
VLAN 2
172.30.2.10
255.255.255.0
VLAN 2
172.30.1.23
255.255.255.0
VLAN 1
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
Two VLANs
Ÿ Two Subnets
•
•
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.
•
Inter-VLAN Routing needs a Router
172.30.1.1
255.255.255.0
(VLAN 1)
Router
172.30.2.1
255.255.255.0
(VLAN 2)
1 2 3 4 5 6 . Port
1 2 1 2 2 1 . VLAN
• 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 Concepts
CCNA 3
Cabrillo College