Post on 19-Nov-2014
description
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