This document provides a summary of key concepts about switches and the Address Resolution Protocol (ARP) at the link layer of computer networks: - Switches operate at the link layer, selectively forwarding frames to the correct outgoing link based on the frame's MAC address. Switches learn MAC addresses and their associated ports through a self-learning process. - ARP is used to dynamically map IP addresses to MAC addresses when two hosts are on the same local area network (LAN). When a host wants to send to an IP address, it first broadcasts an ARP request. The host with that IP responds with its MAC address. - For hosts on different LANs, the sending host uses ARP to get the MAC address

1. Chapter 5
Link Layer
Computer Networking: A
Top Down Approach
4th edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2007.

2. Last Lecture
Link Layer Switches

3. Todays’ Lecture
Address Resolution Protocol
(ARP)

4. Switch
 Link-layer Device
Store, forward Ethernet frames
 Examine incoming frame’s MAC address
 Selectively forward frame to the outgoing link
 Switch itself is transparent to the nodes
 Nodes are unaware of presence of switches
 Plug-and-Play
 Switches do not need to be configured
 Two important functions:
 Filtering

• Determines whether a frame should be forwarded to
some interface or should be dropped.

Forwarding
• Determine the interface to which a frame should be
directed

5. Switch Table

Q: How does switch know that
A’ reachable via interface 4,
B’ reachable via interface 5?
A
C’
B
 A: Each switch has a switch
table, each entry:
 MAC address of a host
 Interface that leads
towards the host
 Time at which the entry
for the node as placed in
the table
 How is the table created?
6
1
5
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)

6. Switch: self-learning
 switch
learns which hosts
can be reached through
which interfaces


Source: A
Dest: A’
A A A’
C’
when frame received,
switch “learns” location of
sender: incoming LAN
segment
records sender/location
pair in switch table
B
1
6
5
2
3
4
C
B’
A’
MAC addr interface TTL
A
1
60
Switch table
(initially empty)

7. Self-Learning,Forwarding:Example
Source: A
Dest: A’
A A A’
C’
B
 frame destination
unknown: flood
6
A A’
1
2
4
5
 destination A
location known:
selective send
C
A’ A
B’
3
A’
MAC addr interface TTL
A
A’
1
4
60
60
Switch table
(initially empty)

8. Interconnecting Switches
 switches can be connected together
S4
S1
S2
A
B
S3
C
F
D
E
I
G
H
 Q: sending from A to G - how does S1 know to
forward frame destined to G via S4 and S3?
 A: self learning! (works exactly the same as in
single-switch case!)

9. Switch: Advantages
 Elimination of Collisions:
 Switches
buffer frames and never transmit
more than one frame on any segment at one
time
 Modern switches are Full Duplex
 Heterogeneous Links
 Switch isolates one link from another
 Different links can operate at different
speeds and over different media
Difference between Hubs and Switches?
Difference between Switches and Routers?
(Home Assignment)

10. MAC Addresses
 32-bit IP address:

network-layer address
used to get datagram to destination IP subnet
 MAC (or LAN or physical or Ethernet) address:
 A node also has a link layer address

• Nodes adapter

6 Bytes (48 bit) MAC address (for most LANs)
• Expressed in Hexadecimal notation
 No two adapters have the same MAC address
 IEEE manages the MAC address space
 MAC address does not change no matter where the adapter
goes
 MAC address: like NIC number
IP address: like postal address

11. MAC Addresses
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98

12. ARP: Address Resolution Protocol
 How does the sending node A with IP
222.222.222.220 determine the MAC address for the
destination node B with IP address say
222.222.222.222?
 Job of Address Resolution Protocol (ARP)
 ARP module in the sending node
• Takes IP address on the same LAN as input
• Return corresponding MAC address
 Each node has an ARP table


Contains mapping of IP addresses to MAC addresses
The ARP table contains a time-to-live (TTL) value
• Indicates when each mapping will be deleted from the table
• Typical expiration time is 20 minutes

13. ARP: Same LAN (network)
 A wants to send datagram
to B, and B’s MAC address
not in A’s ARP table.
 A broadcasts ARP query
packet, containing B's IP
address
 Destination MAC
address = FF-FF-FF-FFFF-FF
 All machines on LAN
receive ARP query
 B receives ARP packet,
replies to A with its (B's)
MAC address

Frame sent to A’s MAC
address (unicast)
 A caches (saves) IP-to-
MAC address pair in its
ARP table until information
becomes old (times out)
 ARP is “plug-and-play”:
 nodes create their ARP
tables without
intervention from
network administrator

14. Addressing: Routing to another LAN
Send datagram from A to B via Router (R)
assume A knows B’s IP address
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
111.111.111.111
E6-E9-00-17-BB-4B
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
111.111.111.112
R
222.222.222.221
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
 Router has two interfaces, two IP addresses, two ARP
modules and two adapters

15.  A creates IP datagram with destination IP of B
 A uses ARP to get MAC address of R (111.111.111.110)
 A creates link-layer frame with R's MAC address as dest,





frame contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from frame, sees its destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram and sends to
B
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
E6-E9-00-17-BB-4B
111.111.111.111
222.222.222.220
111.111.111.110
111.111.111.112
CC-49-DE-D0-AB-7D
222.222.222.221
1A-23-F9-CD-06-9B
R
222.222.222.222
B
49-BD-D2-C7-56-2A

16. Announcements
 Lab Final on Wednesday, 1st January 2014
Every
one has to be present at 2pm
Objective paper: 25 min at 2pm
Hands-on Exam: 45 min
For Hands-on Exam: Class divided into
two groups
On the Spot Marking of Hands-on
Exam

17. Announcements
 Research Presentations/Demos
 Thursday
2nd January, 2014
• 12pm-1pm and 3pm-5pm
 Friday 3rd January, 2014
• 11am-1pm and 2pm- 4pm
 Bring hardcopy of your allocated Paper and
report

Viva/Report for programming project
 Paper Showing on 15th January, 2014 after
exam of Software Engineering at 1pm.

Venue will be notified via email