1. Chapter 3 Part 1 Switching and Bridging
Networking
CS 3470, Section 1

2. Why do you need to switch?
So far, we’ve seen how to create small networks
Switching enables us to build very large networks

3. What is a switch?
A mechanism that allows us to interconnect links to form a large network
A multi-input, multi-output device which transfers packets from an input to one or more outputs
Adds the star topology to the point-to-point link, bus (Ethernet), and ring (802.5 and FDDI) topologies **
** Picture copyright Cisco Systems, license GNU

4. Advantages of Switching
We can interconnect switches to connect many hosts to the same network
Can use point-to-point links
Adding a new host to the network by connecting it to a switch does not necessarily reduce network performance for other attached hosts

5. The University of Adelaide, School of Computer Science
1 October 2017
Switching
How does the switch decide which output port to place each packet on?
It looks at the header of the packet for an identifier that it uses to make the decision
Two common approaches
Datagram or Connectionless approach
Virtual circuit or Connection-oriented approach
Also called forwarding
Chapter 2 — Instructions: Language of the Computer

6. The University of Adelaide, School of Computer Science
1 October 2017
Forwarding
Assumptions
Each host has a globally unique address
There is some way to identify the input and output ports of each switch
We can use numbers
We can use names
Chapter 2 — Instructions: Language of the Computer

7. Types of Switching Circuit Switching Packet switching
Most commonly used in telephone networks
Physical connection is set up from one end to the other
Data is transmitted,
Connection is torn down
Packet switching
Data networks, uses datagrams
Enables packets to be sent from one host to another without a direct or dedicated physical connection.
Asymmetric routing common
Asymmetric routing occurs when packets take one route from sender to receiver, but take a different route from receiver to sender.

8. Circuit Switching
Circuit switching dedicates fixed amount of bandwidth from one end to another
Example: A DS3 from LA to Cedar Falls consumes a full DS3 from end to end whether it is in use or not

9. Circuit Switching
Virtual Circuit Switching requires set up and tear down of the circuit after each use.
No one else can use any of the bandwidth during an active session—even if it is idle!
Known as a connection-oriented approach

10. Packet Switching Differs from circuit switching (obviously)
Data link layer — single broadcast and collision domain
Data is sent across links when space is available
Known as a connectionless (or datagram) approach

11. Datagram / Connectionless Approach
The University of Adelaide, School of Computer Science
1 October 2017
Datagram / Connectionless Approach
Key Idea
Every packet contains enough information to enable any switch to decide how to get it to destination
Every packet contains the complete destination address
Chapter 2 — Instructions: Language of the Computer

12. Packet Switching Bad example:
We can have a switch with 100 attached clients at 100Mbps linked through 10Mbps to another switch with 100 clients at 100Mbps
Obviously not the best solution because there's contention at the interconnect
Bandwidth is not reserved from one end to another.

13. Interconnecting with Hubs
Hub – Repeats incoming signals out of all outgoing ports (like a repeater)
Backbone hub interconnects LAN segments
Extends max distance between nodes
But individual segment collision domains become one large collision domain

14. Bridges Link layer device transparent plug-and-play, self-learning
stores and forwards Ethernet frames
examines frame header and selectively forwards frame based on MAC dest address
when frame is to be forwarded on segment, uses CSMA/CD to access segment
transparent
hosts are unaware of presence of bridges
plug-and-play, self-learning
bridges do not need to be configured

15. Bridges: Traffic Isolation
Bridge installation breaks LAN into LAN segments
Bridges filter packets:
Same-LAN-segment frames not usually forwarded onto other LAN segments
Segments become separate collision domains
bridge
collision
domain
= hub
= host
LAN segment
LAN segment
LAN (IP network)

16. Interconnection without Backbone
Not recommended for two reasons:
Single point of failure at Computer Science hub
All traffic between EE and SE must path over CS segment

17. Backbone configuration
Recommended !

18. Self Learning A bridge has a bridge table Entry in bridge table:
(Node LAN Address, Bridge Interface, Time Stamp)
Stale entries in table dropped (TTL can be 60 min)
Bridges learn which hosts can be reached through which interfaces
When frame received, bridge “learns” location of sender: incoming LAN segment
Records sender/location pair in bridge table

19. Filtering/Forwarding
When bridge receives a frame:
index bridge table using MAC dest address
if entry found for destination { if dest on segment from which frame arrived drop the frame
else forward the frame on interface indicated }
else flood
forward on all but the interface
on which the frame arrived

20. Bridge Example
Suppose C sends frame to D and D replies back with frame to C.
Bridge receives frame from C
Notes in bridge table that C is on interface 1
Because D is not in table, bridge sends frame into interfaces 2 and 3
Frame received by D

21. Bridge Learning: Example
D generates frame for C, sends
Bridge receives frame
Notes in bridge table that D is on interface 2
Bridge knows C is on interface 1, so selectively forwards frame to interface 1

22. Your Turn
Assume fresh (empty) address/port table.
Through which bridge interfaces are the following messages sent?
1. H sends to F, and F replies back to H
2. Next, A sends to F, and F replies back to A
What does the end table look like?

23. Bridge Learning: Drawback
Previous strategy works fine until a LAN has a loop in it
Possible bad failure case – frames could loop forever without getting to final destination!
How could this happen?
In a large network, some administrator could add a bridge that closes a loop without realizing it
Could also be built in on purpose to provide redundancy
So single link failure does not bring down whole network