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

Chapter 3 Part 1
Switching and Bridging
Networking
CS 3470, Section 1
Why do you need to switch?


So far, we’ve seen how to create small
networks
Switching enables us to build very large
networks
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 2.5.
http://en.wikipedia.org/wiki/File:Linksys48portswitch.jpg
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
4
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
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
Types of Switching

Circuit 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
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
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
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
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
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.
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
Bridges

Link layer device




transparent


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
hosts are unaware of presence of bridges
plug-and-play, self-learning

bridges do not need to be configured
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
collision
domain
LAN segment
collision
domain
bridge
LAN segment
LAN (IP network)
= hub
= host
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
Backbone configuration
Recommended !
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
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
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
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
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?
22
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
23