15-441 Computer Networking
Lecture 8 – Switching and Bridging
Peter Steenkiste
Departments of Computer Science and
Electrical and Computer Engineering
15-441 Networking, Spring 2008
http://www.cs.cmu.edu/~dga/15-441/S08
1
Scale
yak yak…

What breaks when we keep adding people to the same
wire?
2
2
Scale
yak yak…

What breaks when we keep adding people to the same
wire?

Only solution: split up the people onto multiple wires
» But how can they talk to each other?
3
3
Problem: How to Connect LANs?
yak yak…

When should these boxes forward packets between
wires?

How do you specify a destination?

How does your packet find its way?
4
4
Outline

Bridging

Internetworks
» Methods for packet forwarding
5
5
Building Larger LANs: Bridges


Extend reach of a single shared medium
Connect two or more “segments” by copying data frames between
them
» Only copy data when needed  key difference from repeaters/hubs
» Reduce collision domain compared with single LAN
» Separate segments can send at once  much greater bandwidth

Challenge: learning which packets to copy across links
LAN 1
LAN 2
6
6
Transparent Bridges

Design goals:
» Self-configuring without hardware
or software changes
» Bridge do not impact the operation
of the individual LANs

Three parts to making bridges transparent:
1) Forwarding frames
2) Learning addresses/host locations
3) Spanning tree algorithm
7
7
Frame Forwarding
Bridge
2
MAC
Address
A21032C9A591
99A323C90842
8711C98900AA
301B2369011C
695519001190
Port
Age
1
2
2
36
2
3
16
3

A machine with MAC Address lies in the
direction of number port of the bridge

For every packet, the bridge “looks up”
the entry for the packets destination
MAC address and forwards the packet
on that port.
01
15
11
1
» Other packets are broadcast – why?

Timer is used to flush old
8 entries
8
Learning Bridges

Manually filling in bridge tables?
» Time consuming, error-prone

Keep track of source address of packets arriving on
every link, showing what segment hosts are on
» Fill in the forwarding table based on this information
host
host
host
host
host
host
host
host
Bridge
host
host
host
host
9
9
Spanning Tree Bridges

More complex topologies can provide
redundancy.
» But can also create loops.
What is the problem with loops?
 Solution: spanning tree

host
host
host
Bridge
host
host
host
host
host
host
Bridge
host
host
host
10
10
Ethernet Switches

Bridges make it possible to increase LAN
capacity.
» Packets are no longer broadcasted - they are only
forwarded on selected links
» Adds a switching flavor to the broadcast LAN

Ethernet switch is a special case of a bridge:
each bridge port is connected to a single host.
» Can make the link full duplex (really simple protocol!)
» Simplifies the protocol and hardware used (only two
stations on the link) – no longer full CSMA/CD
» Can have different port speeds on the same switch
– Unlike in a hub, packets can be stored
– An alternative is to use cut through switching
16
16
Ethernet Evolution
Ethernet or 802.3
Early Implementations
WAN
B/R
LAN
Conc..
A Local Area Network
•
MAC addressing, non-routable
•
BUS or Logical Bus topology
•
Collision Domain, CSMA/CD
•
Bridges and Repeaters for distance/capacity extension
•
1-10Mbps: coax, twisted pair (10BaseT)
Current Implementations
CSMA - Carrier Sense
Multiple Access
•
Switched solution
•
Little use for collision domains
HUB
•
80% of traffic leaves the LAN
Switch
•
Servers, routers 10 x station speed
•
10/100/1000 Mbps, 10gig coming: Copper, Fiber
•
95% of new LANs are Ethernet
Router
Ethernet
•
Server
WAN
CD - Collision Detection
17
17
Ethernet in a campus network
Server farm
Server
Today: x 10
Server
Server
Gigabit Ethernet links
Switch/router
Server
Ethernet
switch
100 Mbps links
Hub
10 Mbps links
Department A
Gigabit Ethernet links
Ethernet
switch
100 Mbps links
Server
Switch/router
Ethernet
switch
100 Mbps links
Server
Hub
Hub
10 Mbps links
10 Mbps links
Department C
Department B
18
18
Problem: Bridging Weaknesses

Doesn’t handle incompatible LAN technologies
» Is interoperable within 802.* standard

How well does it scale?
19
19
Outline

Bridging

Internetworks
» Methods for packet forwarding
20
20
What is an Internetwork?


Multiple incompatible LANs can be physically
connected by layer 3 switches called routers
The connected networks are called an internetwork
» The “Internet” is one (very big & successful) example of an
internetwork
host
host ...
host
host
host ...
LAN 1
host
LAN 2
router
WAN
router
WAN
router
LAN 1 and LAN 2 might be completely different,
totally incompatible LANs (e.g., Ethernet and ATM)
21
21
Logical Structure of Internet
host
router
router
host
router
router
router
router
» Ad hoc interconnection of networks
– No particular topology
– Vastly different router & link capacities
» Send packets from source to destination by hopping through
networks
– Router connect one network to another
– Different paths to destination may exist
22
22
Internet Protocol (IP)

Hour Glass Model
» Create abstraction layer
that hides underlying
technology from network
application software
Network applications
SMTP HTTP RTP...
TCP UDP…
» Make as minimal as
possible
» Allows range of current &
future technologies
email WWW phone...
IP
ethernet PPP…
CSMA async sonet...
Network technology
copper fiber radio...
» Can support many
different types of
applications
23
23
Problem: Internetwork Design
host
host ...
host
host
host ...
LAN 1
LAN 2
router

host
WAN
router
WAN
router
How do I designate a distant host?
» Addressing / naming

How do I send information to a distant host?
» What gets sent?
» What route should it take?

Must support:
» Heterogeneity LAN technologies
» Scalability  ensure ability to grow to worldwide scale
24
24
Getting to a Destination

How do you get driving directions?

Intersectionsrouters

Roadslinks/networks

Roads change slowly
25
25
Forwarding Packets

Table of virtual circuits
»Connection routed through network to
set up state
»Packets forwarded using connection
state

Source routing
»Packet carries path

Table of global addresses (IP)
»Routers keep next hop for destination
»Packets carry destination address
26
26
Simplified Virtual Circuits
Example
Packet
Sender
5
5
2
1
R1
4
3
2
1
R2
4
conn 5  4
3
5
conn 5  3
2
1
R3
4
3
Receiver
5
conn 5  3
28
28
Virtual Circuits

Advantages
» Efficient lookup (simple table lookup)
» Can reserve bandwidth at connection setup
» Easier for hardware implementations

Disadvantages
» Still need to route connection setup request
» More complex failure recovery – must
recreate connection state

Typical use  fast router
implementations
» ATM – combined with fix sized cells
» MPLS – tag switching for IP networks
29
29
Source Routing Example
Packet
R2, R3, R
R1, R2, R3, R
2
Sender
1
R1
4
2
3
1
R2
3
4
R3, R
2
1
R3
4
3
Receiver
R
31
31
Source Routing

Advantages
»Switches can be very simple and
fast

Disadvantages
»Variable (unbounded) header size
»Sources must know or discover
topology (e.g., failures)

Typical uses
»Ad-hoc networks (DSR)
»Machine room networks (Myrinet)
32
32
Global Address Example
Packet
R
Sender
R
2
1
R1
4
R3
3
2
1
R2
3 R4
4
R
2
1
R3
4
R3
3
Receiver
R
34
34
Global Addresses

Advantages
»Stateless – simple error recovery

Disadvantages
»Every switch knows about every
destination
– Potentially large tables
»All packets to destination take same
route
»Need routing protocol to fill table
35
35
Comparison
Source Routing
Global Addresses
Virtual Circuits
Header Size
Worst
OK – Large address
Best
Router Table Size
None
Number of hosts
(prefixes)
Number of circuits
Forward Overhead
Best
Prefix matching
(Worst)
Pretty Good
Setup Overhead
None
None
Connection Setup
Tell all routers
Tell all routers and
Tear down circuit
and re-route
Error Recovery
Tell all hosts
36
36