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, whetherthey are in the same building or even on the opposite side of the globe
Router : Network Layer Interface NETWORK LAYER ROUTER NETWORK LAYER DATA LINK LAYER DATA LINK LAYER PHYSICAL LAYER PHYSICAL LAYER X MEDIUM X MEDIUM
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 802.3 Sub-network IEEE 802.5 Sub-network Router PC-NFS Sub-network
Devices and Layers NETWORK LAYER Routers Layer 3 Switches DATA LINK LAYER Switches Layer 2 PHYSICAL LAYER Repeaters Layer 1
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
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 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