This document discusses various networking devices and their functions: Layer 1 devices like repeaters and hubs extend signal distances or recreate signals on multiple ports. Layer 2 switches and bridges operate at the data link layer, using MAC addresses to determine packet forwarding. Layer 3 devices like routers use network layer addresses and can connect different network types. MPLS assigns "labels" to packets for efficient forwarding through label switching routers.

1. Connecting DevicesConnecting Devices
and Multi-Homed Machinesand Multi-Homed Machines

2. Layer 1 (Physical) DevicesLayer 1 (Physical) Devices
Repeater:Repeater:

Extends distances by repeating a signalExtends distances by repeating a signal

Any slight variations in the carrier wave for individualAny slight variations in the carrier wave for individual
bits is corrected when the carrier wave is reproducedbits is corrected when the carrier wave is reproduced
Hub:Hub:

As above, but re-creates the carrier wave on multipleAs above, but re-creates the carrier wave on multiple
portsports

Hubs do not decide whether or not to copy data, theyHubs do not decide whether or not to copy data, they
do it (necessary or unnecessary)do it (necessary or unnecessary)

All ports are part of the same collision domainAll ports are part of the same collision domain
With a hub as the centre of the star, any 2 hosts can haveWith a hub as the centre of the star, any 2 hosts can have
frame collisionsframe collisions

3. Layer 2 (Data Link) DevicesLayer 2 (Data Link) Devices
Layer 2 switch:Layer 2 switch:

Serves the same purpose as a hubServes the same purpose as a hub

Can dynamically determine if a frame should be placed on a portCan dynamically determine if a frame should be placed on a port
(and which one)(and which one)

The data link layer (e.g. MAC) address is used to make thisThe data link layer (e.g. MAC) address is used to make this
determinationdetermination

A table of MAC addresses and corresponding ports is built usingA table of MAC addresses and corresponding ports is built using
incoming framesincoming frames

Each LAN segment (port) becomes its own collision domainEach LAN segment (port) becomes its own collision domain
Only 2 hosts on the same LAN segment can have frame collisionsOnly 2 hosts on the same LAN segment can have frame collisions
Layer 2 bridgeLayer 2 bridge

As above, but the connected networks do not necessarily haveAs above, but the connected networks do not necessarily have
to be of the same typeto be of the same type

4. Layer 2/3 DevicesLayer 2/3 Devices
Broadband (Cable/DSL) or Wireless RouterBroadband (Cable/DSL) or Wireless Router

Primarily operates as a layer 2 switch / access pointPrimarily operates as a layer 2 switch / access point

However, theseHowever, these routersrouters often have additional features (some ofoften have additional features (some of
which are in layer 3):which are in layer 3):
Dynamic host configuration (DHCP) – IP address assignment forDynamic host configuration (DHCP) – IP address assignment for
network hosts, for examplenetwork hosts, for example
DNS capability (local or distributed) – Provides naming of hostsDNS capability (local or distributed) – Provides naming of hosts
inside the networkinside the network
IP masquerading – TheIP masquerading – The routerrouter can use one IP given by a broadbandcan use one IP given by a broadband
provider, but allow all of its hosts to use different IPs inside theprovider, but allow all of its hosts to use different IPs inside the
networknetwork
Layer 3 SwitchLayer 3 Switch

Uses layer 3 routing to determine a path for packetsUses layer 3 routing to determine a path for packets

Once a path is found, subsequent packets are switchedOnce a path is found, subsequent packets are switched
This switching typically occurs on layer 2This switching typically occurs on layer 2
These devices will be discussed in more detail laterThese devices will be discussed in more detail later

5. Layer 3 (Network) DevicesLayer 3 (Network) Devices
Layer 3 BridgeLayer 3 Bridge

A bridge that uses network layer addresses (e.g. IP) in itsA bridge that uses network layer addresses (e.g. IP) in its
forwarding database, instead of data link layer addresses (e.g.forwarding database, instead of data link layer addresses (e.g.
MAC)MAC)

This type of bridge more readily allows different types of networkThis type of bridge more readily allows different types of network
to be joined, since they need not share an address typeto be joined, since they need not share an address type

Cannot handle multiple paths effectively/efficiently: a host isCannot handle multiple paths effectively/efficiently: a host is
either on a given port or it is noteither on a given port or it is not
RouterRouter

Determines routes for each packet using network layerDetermines routes for each packet using network layer
addresses (e.g. IP)addresses (e.g. IP)

Can connect any type of network togetherCan connect any type of network together

Is capable of determining preferred paths where multiple pathsIs capable of determining preferred paths where multiple paths
existexist

6. RoutersRouters

7. What is a Router Made of?What is a Router Made of?
A router has many of the sameA router has many of the same
components as your computer:components as your computer:

CPUCPU

MemoryMemory

I/O Interfaces (mostly network interfaces)I/O Interfaces (mostly network interfaces)

Operating SystemOperating System

8. Routers Through HistoryRouters Through History
Gateways:Gateways:

A computer with installed software to forward packetsA computer with installed software to forward packets

These are obsolete, but were common in the early days ofThese are obsolete, but were common in the early days of
ARPANetARPANet
Routers:Routers:

A computer with specialized hardware and operating systemA computer with specialized hardware and operating system
designed for forwarding packetsdesigned for forwarding packets
Switching Routers:Switching Routers:

A computer with specialized hardware (switching fabric) thatA computer with specialized hardware (switching fabric) that
allows packets to be forwarded directly in hardwareallows packets to be forwarded directly in hardware

The specialized hardware is, in many respects, similar to that ofThe specialized hardware is, in many respects, similar to that of
a switch (e.g. ATM switch)a switch (e.g. ATM switch)

9. Router HardwareRouter Hardware
Input buffers (one for each network interface):Input buffers (one for each network interface):

Used to store incoming packets before they are processedUsed to store incoming packets before they are processed
Routing processor:Routing processor:

This is often software running on a CPU which:This is often software running on a CPU which:
Maintains and exchanges routing data with other routersMaintains and exchanges routing data with other routers
Controls the switching fabric to forward packetsControls the switching fabric to forward packets

With high-end routers, each network interface may have a localWith high-end routers, each network interface may have a local
routing processor (for forwarding) so that each can forward therouting processor (for forwarding) so that each can forward the
packets in its own input buffer independentlypackets in its own input buffer independently
Switching fabric:Switching fabric:

A network of connections between network interfaces (and theirA network of connections between network interfaces (and their
input and output buffers)input and output buffers)
Output buffers (one for each network interface):Output buffers (one for each network interface):

Used to store outgoing packets after they are processed, butUsed to store outgoing packets after they are processed, but
before the network is available for transmissionbefore the network is available for transmission

10. Routers: Network InterfacesRouters: Network Interfaces
Often, routers have modularized networkOften, routers have modularized network
interfacesinterfaces

One can add/remove/replace network interfaces asOne can add/remove/replace network interfaces as
needs changeneeds change

Some routers can accept network interface modulesSome routers can accept network interface modules
of different types (e.g. Ethernet, Token Ring)of different types (e.g. Ethernet, Token Ring)

Each network interface would have its own:Each network interface would have its own:
Input bufferInput buffer
Output bufferOutput buffer
Routing processor (in high-end routers)Routing processor (in high-end routers)

11. Routers: Input BuffersRouters: Input Buffers
The incoming packets of a network interface are placedThe incoming packets of a network interface are placed
in input buffersin input buffers

These are banks of very high speed memory for packet queuingThese are banks of very high speed memory for packet queuing
prior to processingprior to processing

The packet is stored here until the routing processor is availableThe packet is stored here until the routing processor is available
The network interface may have a routing processor,The network interface may have a routing processor,
which would:which would:

…… have a copy of the forwarding table (to prevent concurrenthave a copy of the forwarding table (to prevent concurrent
access)access)

…… lookup the destination address in this forwarding table, tolookup the destination address in this forwarding table, to
determine the correct output portdetermine the correct output port

…… configure the switching fabric to forward the packet to theconfigure the switching fabric to forward the packet to the
correct output buffercorrect output buffer
Low-end routers would share one routing processorLow-end routers would share one routing processor

12. Routers: Routing ProcessorsRouters: Routing Processors
Routing processors have two functions:Routing processors have two functions:
1.1. Maintain and exchange routing data withMaintain and exchange routing data with
other routers in the networkother routers in the network
Often this involves computing the forwardingOften this involves computing the forwarding
table from data received by other routerstable from data received by other routers
1.1. Use the forwarding table data toUse the forwarding table data to
configure the switching fabric to forwardconfigure the switching fabric to forward
the packet to the correct output portthe packet to the correct output port

13. Routers: Routing ProcessorsRouters: Routing Processors
A routing processor is software which executesA routing processor is software which executes
on a CPU:on a CPU:

Off-the-shelf CPUOff-the-shelf CPU
These are very inexpensiveThese are very inexpensive
However, the performance of these CPUs is low since theyHowever, the performance of these CPUs is low since they
are not optimized for the types of operations a routerare not optimized for the types of operations a router
typically needs to performtypically needs to perform

Application-Specific Integrated Circuit (ASIC)Application-Specific Integrated Circuit (ASIC)
These are expensive to design (time and money)These are expensive to design (time and money)
They are optimized for typical routing operationsThey are optimized for typical routing operations
High-end routers use these to achieve higher performanceHigh-end routers use these to achieve higher performance
levelslevels

14. Routers: Switching FabricRouters: Switching Fabric
Switching fabric’s job is to move packetsSwitching fabric’s job is to move packets
from the input buffer into the correct outputfrom the input buffer into the correct output
bufferbuffer

The routing processor determines the correctThe routing processor determines the correct
output port, using the forwarding tableoutput port, using the forwarding table

15. Routers: Switching FabricRouters: Switching Fabric
Switching fabric comes in 3 major types:Switching fabric comes in 3 major types:

In-memory switching fabric:In-memory switching fabric:
The packets are input into the routing processor’sThe packets are input into the routing processor’s
memory, and output into the correct output buffermemory, and output into the correct output buffer

Bus-based switching fabric:Bus-based switching fabric:
The packets move along a shared bus (similar to aThe packets move along a shared bus (similar to a
network bus) to the correct output buffernetwork bus) to the correct output buffer

Crossbar switching fabric:Crossbar switching fabric:
The packets move along a grid of redundant busesThe packets move along a grid of redundant buses
If any bus fails, alternate paths exist so thatIf any bus fails, alternate paths exist so that
forwarding can continueforwarding can continue

16. Routers: Output BuffersRouters: Output Buffers
The switching fabric gets the packet to theThe switching fabric gets the packet to the
right output portright output port

However, that port’s network may not beHowever, that port’s network may not be
immediately availableimmediately available

The packets are stored in the output bufferThe packets are stored in the output buffer
until the network is availableuntil the network is available

17. Router PerformanceRouter Performance
Several methods to improve router performanceSeveral methods to improve router performance
have been discussed:have been discussed:

Use application-specific integrated circuitsUse application-specific integrated circuits
Optimized for routing operationsOptimized for routing operations
Include much routing functionality otherwise executed asInclude much routing functionality otherwise executed as
software (in memory)software (in memory)
Many routing functions can execute in parallel, adding newMany routing functions can execute in parallel, adding new
functionality without decreasing throughputfunctionality without decreasing throughput

Use efficient switching fabricUse efficient switching fabric
Bus or crossbar-based switching fabrics reduce the need forBus or crossbar-based switching fabrics reduce the need for
in-memory processingin-memory processing

18. MPLSMPLS
Multi-Protocol Label SwitchingMulti-Protocol Label Switching

19. MPLSMPLS
MPLS is another way to improve routerMPLS is another way to improve router
performanceperformance
Label switching tries to leverage some of theLabel switching tries to leverage some of the
performance of virtual circuit switched networksperformance of virtual circuit switched networks
(e.g. ATM)(e.g. ATM)
Packets are assigned a label upon entering anPackets are assigned a label upon entering an
MPLS networkMPLS network
This label is used (instead of the IP address) forThis label is used (instead of the IP address) for
making forwarding decisionsmaking forwarding decisions

20. MPLS LabelsMPLS Labels
An MPLS label is an arbitrary valueAn MPLS label is an arbitrary value

This value is typically a numeric identifierThis value is typically a numeric identifier
However, labels could also be the frequency (i.e. colour) ofHowever, labels could also be the frequency (i.e. colour) of
light used in multi-mode optical fibrelight used in multi-mode optical fibre

The label can change from one label-switching routerThe label can change from one label-switching router
(LSR) to the next(LSR) to the next

The label must only be unique for the sending andThe label must only be unique for the sending and
receiving routerreceiving router
IP addresses, in contrast, are usually unique across theIP addresses, in contrast, are usually unique across the
networknetwork

A value could even be chosen to help the routingA value could even be chosen to help the routing
processor choose the correct output portprocessor choose the correct output port

21. MPLS: SimplifiedMPLS: Simplified
MPLS LSR
MPLS LSR MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
Web Server
E-Mail Server
User

22. MPLS: SimplifiedMPLS: Simplified
MPLS LSR
MPLS LSR MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
Web Server
E-Mail Server
User
• Here, the label is shown as colour
• Notice the simplicity of the router’s job:
• Red: Up
• Blue: Right

23. MPLS: SimplifiedMPLS: Simplified
MPLS LSR
MPLS LSR MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
Web Server
E-Mail Server
User
• Notice that two labels can be directed
down the same link

24. MPLS: Label ValuesMPLS: Label Values
MPLS LSR
MPLS LSR MPLS LSR
MPLS LSR
MPLS LSR
MPLS LSR
Web Server
E-Mail Server
User
• Notice that label values are not
globally unique
• Each pair of routers agrees on a label
7
15
31
7
31
47

25. MPLS PacketsMPLS Packets
MPLS adds a small pre-header to the start of any IPv4MPLS adds a small pre-header to the start of any IPv4
(or IPv6, IPX, etc.) packet(or IPv6, IPX, etc.) packet

In other words, between the data link and network headersIn other words, between the data link and network headers
Label
20 bits
Class of Service3 bits
Stack1 bit
Hop Limit8 bits
The label value
The QoS class of the packet attached (e.g. discardable?)
Is there a stack of labels?
The hop limit, copied from/to the IP header

26. MPLS and ATMMPLS and ATM
LSRs can be ATM-enabledLSRs can be ATM-enabled

An LSR can forward a packet (as cells) through anAn LSR can forward a packet (as cells) through an
ATM networkATM network
This can be for any number of hops through the ATMThis can be for any number of hops through the ATM
networknetwork

In this situation the source and destination ATMIn this situation the source and destination ATM
switches must be LSRsswitches must be LSRs
Other switches in between can be normal ATM switches,Other switches in between can be normal ATM switches,
howeverhowever
The source LSR will use AAL segmentation to send the cellsThe source LSR will use AAL segmentation to send the cells
on the ATM network using a VPI/VCI for the destination LSRon the ATM network using a VPI/VCI for the destination LSR
The destination LSR will extract the packet and continueThe destination LSR will extract the packet and continue
transmission using MPLStransmission using MPLS