Router

Router

Router

• A router is an electronic device that interconnects two or more computer networks

• Works at Layer 3, Network Layer in an intelligent manner

• Can connect different network segments, whether they are in the same building or even on the opposite side of the globe

Router : Network Layer Interface

X MEDIUM X MEDIUM

PHYSICAL LAYER

PHYSICALLAYER

DATA LINKLAYER

DATA LINKLAYER

NETWORKLAYER

ROUTERNETWORK

LAYER

Router

• Works in LAN, WAN environments • Allows access to resources by

selecting the best path • Can interconnect different

networks • Changes packet size and format to

match the requirements of the destination network

Internetworking with a Router

IEEE 802.3Sub-network IEEE 802.5

Sub-network

PC-NFSSub-network

Router

Devices and Layers

PHYSICAL LAYER

DATA LINKLAYER

NETWORKLAYER

Switches

Routers

Repeaters

Sw

itch

es

Layer 1

Layer 2

Layer 3

Difference Between Routers, Switches and

Hubs• Hubs

– Simply provides the mechanical and electrical connections between the nodes

• Switches– Examine the data packet for the

destination address– Do not alter the data packets

• Routers– Examine and alter the data packets– Perform protocol conversion

Delivery, Forwarding & Routing

• Delivery– Refers to the way a packet is handled

by the underlying networks under the control of the network layer.

Delivery, Forwarding & Routing

• Forwarding– Refers to the way a packet is

delivered to the next station.

Delivery, Forwarding & Routing

• Routing– Refers to the way routing tables are

created to help in forwarding.

Functions of a Router

• Two primary functions: • Determine the 'best path' • Share details of routes

with other routers

Router has

• Routing Table - a database which keeps track of the routes to networks and the associated costs

• Routing Protocol - uses a special algorithm to route data across a network eg RIP

Format of Routing Table

Mask Network Address

Next-hop Address

Interface

Flags

Reference Count

Use

Format of Routing Table

• Mask– This field defines the mask applied for

the entry.

Format of Routing Table

• Network Address– Defines the network address to which

the packet is finally delivered.

Format of Routing Table

• Next-hop Address– Defines the address of the next-hop

router to which the packet is delivered.

Format of Routing Table

• Interface– Shows the name of the interface.

Format of Routing Table

• Flags– U(up): If this flag is not present, it

means that the router is down.– G(gateway): Destination is in another

network.– H(host-specific): Indicates that the

entry in the Network Address field is a host-specific address.

Flags Contd.

– D(added by redirection): Indicates that the routing information for this destination has been added to the host routing table by a redirection message from ICMP.

– M(modified by redirection): Indicates that the routing information for this destination has been modified by a redirection message from ICMP.

Format of Routing Table

• Reference Count– Gives the number of users of this

route at the moment.

Format of Routing Table

• Use– Shows the number of packets

transmitted through this router for the corresponding destination.

Types of Routing Tables

• Static Routing Table –– Routes are manually configured by a

network administrator – Cannot update automatically when

there is a change in the internet

Types of Routing Tables

• Dynamic Routing Table– Adjusts automatically to changes in

network topology– Uses one of the dynamic routing

protocols such as RIP

RIP

• Stands for Routing Information Protocol

• RIP implementation considerations:– We are dealing with routers and

networks(links)– Routers have routing tables; networks

do not

RIP Implementation Considerations Contd.

– The destination in a routing table is a network.• This means the first column defines a

network address– Metric used is very simple; distance

defined as no. of links to reach destination

RIP Implementation Considerations Contd.

– Infinity is defined as 16• Which means no route can have more

than 15 hops– Next-node column defines the

address of the router to which the packet is to be sent to reach its destination.

Router Hardware

• Similarity with a PC:– A CPU– A memory– Ports & interfaces

• Dissimilarity:– Routers are diskless

Router Motherboard

Router CPU

• 50 MHz CPUs are generally used for small offices & homes.

• For more powerful purposes, processors from Motorola, Silicon Graphics, etc. are used.

Router Memory

• RAM/DRAM• NVRAM• Flash Memory• ROM

RAM/DRAM

• Stands for random access memory/dynamic random access memory

• Used by the router’s central processor to do its work

• Cisco’s smallest router ships with a minimum of 16MB of DRAM

NVRAM

• Stands for nonvolatile RAM– retains information after losing power

• stores a copy of the router’s configuration file– enables the router to restart in its

proper configuration in case of accidental shutdown

Flash memory

• Originally developed by Intel• also nonvolatile• differs from NVRAM in that it can

be erased and reprogrammed as needed

• used to store one or more copies of the IOS software– Helps in upgrading IOS on all routers

ROM

• Used to hold a so-called bootstrapprogram

– which is a file that can be used to boot to a minimum configuration state after a catastrophe

Router Ports

• Port means a physical connection through which I/O can pass– a serial port, for example