Chapter 5
Intro to Routing & Switching
 Upon
completion of this chapter, you should
be able to:








Describe the operation of the Ethernet sublayers.
Identify the major fields of the Ethernet frame.
Describe the purpose and characteristics of the
Ethernet MAC address.
Describe the purpose of ARP.
Explain how ARP requests impact network and
host performance.
Explain basic switching concepts.
Compare fixed configuration and modular
switches.
Configure a Layer 3 switch.
5.1.1
 Most
widely used LAN technology
 What
2 layers does it operate at?

Data link & physical
 What

are the 2 sublayers of the data link?
LLC & MAC
 IEEE
802.2
 Helps communicate with network layer
 Adds control info
 Performs it in software
 Encapsulates


Adds MAC addresses
Adds error detection to frame
 Media

Access Control
Placing the frames on the media
 Ethernet



the data
is a logical bus; physical star
Signal passes to all
Can send whenever
Can be collisions

Ethernet is contention-based/non-deterministic
Data contends or shares for a spot on the media
 Doesn’t know when it’ll get access to it


Listens for signal on media
No signal = transmit data
 Transmit at same time= collision


Devices do not keep track of whose turn it is

More collisions= less throughput

SOLUTION: CSMA/CD & CSMA/CA
 Listens

 If

for silence
Silence= transmit data
devices transmit at same time, collision
Backoff random time, listen for silence,
retransmit
 Collision
detection not a problem much
anymore

Using switches & full-duplex, this is not a
problem anymore
 Wireless
 Listen
for silence
 Sends Ready to Send message to AP
 Gets a Clear to Send message from AP
 Sends data
 Used
to identify frame
 Each host has a unique address

Burned into NIC hardware
 48-bits
/ 12 hex digits / 6 bytes
 1st 24 bits OUI; 2nd 24 bits serial #
 How

many bits is a MAC address?
48 bits
 What
is the OUI in this MAC?
A2:07:CC:F6:AD:32

A2:07:CC
 What
does a wireless network use to avoid
collisions?

CSMA/CA
 What
sublayer places the frames on the
media?

MAC
 The

OUI is how many bytes?
3
 Ethernet
shares access to the media. It
contends for the media and does not take
turns transmitting. Not taking turns means
the network is…

Nondeterministic
 Is
the MAC address found in hardware or
software?

Software
End of Day One
5.1.2
 Min.

>
frame size is 64 bytes; max is 1522 bytes
Data is 46-1500 bytes
64 bytes is considered a collision fragment
& will be dropped

5.1.2.4
 What
important addresses are encapsulated
into a frame?

Source & destination MAC
 What

Timing/synchronization
 What

does the preamble used for?
is the minimum frame size?
64 bytes
 What
happens if a frame is less than 64
bytes?

Considered a fragment & dropped
 How

many bits in a MAC address?
48 bits
 How

12
 The

first 6 hex digits are what?
The OUI
 The

many hex digits?
OUI would then be the 1st ____ bytes.
3
 What
sublayer is used to communicate with
the upper layers?

LLC
5.1.3
 IP
Address:
 32 bits, 4 octets


8 bits in each octet
11111111.10101010.11001100.00100101
 Written

192.101.28.36
 Value

in decimal
in each octet from 0-255
That’s a total of 256 numbers.
 Add
up the values of the binary 1’s
128
64
32
16
8
4
2
1
1
0
0
1
1
1
0
0

156
 11100101

229
 Ipconfig
/all
 Hexadecimal (Base 16)
 0-9, A-F (10-15)

16 total #’s
 Handouts
 Begin

0xA4
with a 0x
 One
to one communication
 One
to all in a network
 Dest. MAC address will be all F’s
 DHCP & ARP use broadcasts
 One
to a group in a network
 Remote gaming or video conference
 Dest. IP will be 224.0.0.0 -239.255.255.255
 Dest. MAC will begin with 01-00-5E
 5.1.3.6
 What
kind of message gets sent from one PC
to a group within a network?

Multicast
 Identify
each as unicast, multicast, or
broadcast:
5.1.4

MAC address
 Burned into NIC (DOES NOT CHANGE)
 Similar to the name of a person
 Physical address

IP address
 Similar to the address of a person
 Based on where the host is actually located
 Logical address

Both the physical MAC & logical IP addresses are
required for a computer to communicate just
like both the name and address of a person are
required to send a letter
 Destination
IP NEVER changes!
 Source & Destination MAC changes at each
router interface
 5.1.4.3
Wireshark
 How

do you change your MAC address?
Get a new NIC
T
or F. The destination IP address changes
during transmission.

False
 What
happens to the source & destination
MAC addresses as you go from router to
router across the Internet?

They change (router port substituted)
 What

LLC sublayer
 What

is a layer 2 address?
MAC address
 What

is IEEE 802.3?
Ethernet/MAC sublayer
 What

is IEEE 802.2?
is a layer 3 address?
IP address
5.2.1
 ARP


table of IP/MACs
Added from communication
Added from ARP requests
 You
have the dest. IP, not the MAC
 ARP request is all F’s where?

Destination MAC
 How

Broadcast
 Who

is an ARP request sent?
replies to the ARP request?
Only one with matching dest. IP
 Windows
Arp –a
 Cisco

router
Router# show ip arp
 If
you want to access a remote server, and
you don’t have the destination MAC, what
gets substituted for it?

The default gateway’s MAC address
A
router has ports with MAC addresses. How
do you view the router’s ARP table?

Router#sh ip arp
 What

does ARP find? What do you know?
Finds the dest. MAC; you know the dest. IP
5.2.2
 Broadcasts

It’s a broadcast. If many devices started at same
time, there’d be a flood of ARP requests which
would cause a reduction in performance for a
short period of time.
 Security


ARP poisoning (or spoofing)
Attacker forges MAC address to have frames
delivered to different computer
 Solution:
Use a switch
 Segments
network into smaller collision
domains
 Replies go to one device only
 Implement security too
5.3.1
 Full


duplex, Half duplex, Auto
Must match setting of device
Half duplex uses CSMA/CD to avoid collisions
 Cisco
uses cut-through switching

As soon as destination MAC is read, it forwards
the frame

Fast-forward


Lowest latency; in and out
Fragment-free



Store first 64 bytes before forwarding
Most errors & collision happen there (runts)
If it makes it through, should be error-free
 Complete
5.3.1.7
2
switches connect to each other. One port is
100Mbps and the other is 1000Mbps. What
speed will that connection operate at?

100Mbps
 What
feature will allow you to use a straightthrough cable to connect two switches
together?

Auto-MDIX
 Your
switch port is connected to a hub with 3
computers on it. How many MAC addresses
will be in the table for that port?

3
 Switch



it!
5.3.1.9
Do it at least 4 times
Different scenario each time
 Lab
together
 Draw
network from MAC address table
5.3.2

Fixed Configuration Switch



Modular


Connected by special cable to act as one switch
Fixed may be stackable
PoE (Power over Ethernet)



Add-in cards/ports
Stackable



As is; no add-ons or changing
A 24 port switch will always be 24 ports
Delivers power to a device, like an IP phone
We use this in school
More $$, faster forwarding rate
 Which
type of switch can you add a card to
add more Ethernet ports or add fiber ports?

Modular
 You
bought a 24 port switch and cannot add
more ports to it. What kind of switch did you
buy?

Fixed
 You
have the switch above and need more
ports. You then buy more 24 port switches
and connect them with a special cable. What
kind of switch do you now have?

Fixed & stackable
5.3.3
 Looks

Combined with router functions
 Adds


like a switch
in router functions
Knows which IP addresses are out each port too
Fast
 Make


an Ethernet port a routed port
To connect to the ISP
Configure a port
 What
have we configured with an IP on a
switch?


IP for remote management
This is similar
 5.3.3.5
 Configuring
a Layer 3 Switch
 What

MAC addresses
 What

address(es) does a layer 2 switch read?
address(es) does a layer 3 switch read?
IP & MAC
 You
want to connect your Layer 3 switch to
your ISP instead of using a regular router.
What must you configure one of the ports as?

Routed port
 Which
command enables the routing function
on a switch port?

No switchport
 Complete
 Take
the study guide handout
the quiz on netacad.com
 Jeopardy
review
In this chapter, you learned:
 Ethernet is the most widely used LAN technology
used today.
 Ethernet standards define both the Layer 2
protocols and the Layer 1 technologies.
 The Ethernet frame structure adds headers and
trailers around the Layer 3 PDU to encapsulate
the message being sent.
 As an implementation of the IEEE 802.2/3
standards, the Ethernet frame provides MAC
addressing and error checking.
 Using switches in the local network has reduced
the probability of frame collisions in half-duplex
links.
The Layer 2 addressing provided by Ethernet
supports unicast, multicast, and broadcast
communications.
 Ethernet uses the ARP to determine the MAC
addresses of destinations and map them against
known IP addresses.
 Each node on a network has both a MAC address
and an IP address.
 The ARP protocol resolves IPv4 addresses to MAC
addresses and maintains a table of mappings.
 A Layer 2 switch builds a MAC address table that
it uses to make forwarding decisions.

 Layer
3 switches are also capable of
performing Layer 3 routing functions,
reducing the need for dedicated routers on a
LAN.
 Layer 3 switches have specialized switching
hardware so they can typically route data as
quickly as they can switch.
Chapter 5
Intro to Routing & Switching