Abstract: Routing is the process of selecting best paths in a network. In the past, the term routing was also used to mean forwarding network traffic among networks. However this latter function is much better described as simply forwarding. Routing is performed for many kinds of networks, including the telephone network (circuit switching), electronic data networks (such as the Internet), and transportation networks. In packet switching networks, routing directs packet forwarding (the transit of logically addressed network packets from their source toward their ultimate destination) through intermediate nodes. Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls, or switches. General-purpose computers can also forward packets and perform routing, though they are not specialized hardware and may suffer from limited performance. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations. Thus, constructing routing tables, which are held in the router's memory, is very important for efficient routing. Most routing algorithms use only one network path at a time. Multipath routing techniques enable the use of multiple alternative paths.

Fig 1: Routing.

Keywords: Routers, Border Gateway Protocol (BGP), Source, Destination, default route, on demand route, IP addresses.

I.INTRODUCTION (Default Routing)

A default route is the next hop a packet should take from a node within a datagram network if there is no specific route available for the destination of that

packet. A device (computer or router) may have no static route or dynamically learned route to a particular destination within the network. A static route may be absent because the network administrator never defined one, or because the software does not support defining one (e.g., end stations often do not support static routes). Learned routes might be absent if the device has no ability to learn them (also commonly true of end stations), if it has not had time to learn one, or if the routing protocol explicitly bars it from learning one. If there is no static route or learned route, the Network Layer checks for a default route.

Default routes are also useful on a smaller scale, within single autonomous systems. The same benefits of decreased memory and processor utilization can be gained in smaller networks, although the benefits decrease as the number of routes decreases.

Fig 2: Default routing

II. The role of the default routeDefault routes serve an important function. Consider the Internet and all of its subnetworks. Maintaining a list of all possible destinations is well beyond the ability of most routers, and certainly beyond the ability of the end stations. Imagine the memory required to store thousands of IP addresses, each four octets long, together with the route information for how to reach that network (not to mention the processor time and bandwidth required to run the protocol that would keep them all up-to-date)! And if the company has a single link to the Internet, all of that information is largely useless because all of those networks can be reached by passing the packets to the same next-hop router: the one

Default Routing AndOn Demand Routing

owned by the ISP providing Internet access. It makes far more sense for this router to have detailed information about all of the networks within the corporate network, so it can forward inbound packets correctly, but a single default route (for outbound traffic) representing all networks out in the Internet.

III. Default router in IP

Although the concept of the default route can occur in any packet network, most modern packet networks are based on IP, so our discussion will focus on that protocol.

In IP, the default route is also known as the default gateway or the gateway of last resort. The term gateway is an old one and reflects the roots of the IP protocol. In the earliest days of internetworking, the device that interconnected the various networks in the Internet were known as gateways. However, that term came to have a new meaning due to its use by the ISO and the term router became more common.

The default route is represented using a special IP address (0.0.0.0) and subnet mask (0.0.0.0). This address is placed in the routing table along with the address of the local router to be used (i.e., the default gateway). In IP, routers and end stations use a simple rule when selecting a route from the routing table: the best match wins. So if a routing table contained entries for 192.14.17.0, 192.14.0.0, 192.0.0.0, and 0.0.0.0, and a packet arrived destined for 192.14.17.23, the route associated with 192.14.17.0 would be used because it has the greatest match with the destination. If a packet arrived destined for 192.14.192.134, however, the route associated with 192.14.0.0 would be used. By extension, if a packet arrived destined for 120.12.143.167, the default route of 0.0.0.0 would be used. This route matches every IP address not matched by any other entry in the routing table, making it the default.

IV. Implementing the default routing.

As with any entry in the IP routing table, the default route can be placed there statically, or learned dynamically. It is typically entered statically by a network administrator in a router that connects by a single path to the broader internetwork. For example, if there was a large regional branch location that had a single connection back to the data centre and the rest of the corporate network, the router at the branch could be configured with a static default route pointing back to the router in the data centre. A router connected to an ISP could be configured with a static default route pointing to the ISP's router.

If there are multiple subnetworks behind the router with the statically configured default route, these other routers could learn about the default route using the routing protocol. In this case, the router with a statically configured default route in known as the originating

router. In our example of the router connecting the regional branch to the data centre, this router could teach any routers within the regional branch that it is the "go to" router if they receive packets bound for locations outside the regional branch. The router connecting a corporation to the Internet could teach the routers within the corporation that it is the "go to" router for any traffic intended for networks outside the corporation (e.g., out in the Internet).

When originating the default route, it is important to ensure that the originating router be as close as possible to the exit point of the network. Otherwise, routing loops can result.

Fig 3: Implementing the default routing

V. INTRODUCTION (ODR)On-Demand Routing (ODR) is an enhancement to Cisco Discovery Protocol (CDP), a protocol used to discover other Cisco devices on either broadcast or non-broadcast media. With the help of CDP, it is possible to find the device type, the IP address, the Cisco IOS® version running on the neighbor Cisco device, the capabilities of the neighbor device, and so on. In Cisco IOS software release 11.2, ODR was added to CDP to advertise the connected IP prefix of a stub router via CDP. This feature takes an extra five bytes for each network or subnet, four bytes for the IP address, and one byte to advertise the subnet mask along with the IP. ODR is able to carry Variable Length Subnet Mask (VLSM) information.The routes are created when required. The source has to discover a route to the destination. The source and intermediate nodes have to maintain a route as long as it is used. Routes have to be repaired in case of topology changes.

VI. On-Demand Routing Protocolsa. Ad hoc On-demand Distance Vector Routing

Includes route discovery and route maintenance.Minimizes the number of broadcasts by creating routes on-demand. Uses only symmetric links because the route reply packet follows the reverse path of route

request packet. Uses hello messages to know its neighbors and to ensure symmetric links.

Fig 4: Example of Ad hoc On-demand Distance Vector Routing

Route Maintenance: If the source node moves, it reinitiates the route discovery. If intermediate node moves, its upstream node sends a RREP to the source. The source restarts the route discovery.

b. Dynamic Source Routing Protocol

A node maintains route caches containing the routes it knows. Include route discovery and route maintenance.Route discovery: The source sends a broadcast packet which contains source address, destination address, request id and path. If a host saw the packet before, discards it. Otherwise, the route looks up its route caches to look for a route to destination, If not find, appends its address into the packet, rebroadcast, If finds a route in its route cache, sends a route reply packet, which is sent to the source by route cache or the route discovery.

Fig 5: Example Dynamic Source Routing Protocol

How to send a reply packet? If the destination has a route to the source in its route cache, use it. Else if symmetric links are supported, use the reverse of route record. Else if symmetric links are not supported, the destination initiates route discovery to source.Route maintenance: Whenever a node transmits a data packet, a route reply, or a route error, it must verify that

the next hop correctly receives the packet. If not, the node must send a route error to the node responsible for generating this route header. The source restart the route discovery.

Add entries into route cache: The Source and destination in route discovery. Intermediate hosts in route discovery. The hosts receiving any broadcast

c. Temporally Ordered Routing Algorithm

Creating Routes: query/reply QRY packet is flooded through network UPD packet propagates back if route exist

Maintaining Routes: link-reversal UPD packets re-orient the route structure

Erasing Routes CLR packet is flood through network to erase

invalid routes

Fig 6: Example of Temporally Ordered Routing Algorithm

TORA: Height metricEach node contains a quintuple

• Logical time of a link failure

• Unique ID of the node that defined the new reference level

• Reflection indicator bit

• A propagation ordering parameter, height

• Unique ID of the node

Route Maintenance and Erasing: No reaction necessary if all nodes still have downstream links. A new reference level is defined if a node loses its last downstream link. Synchronized clock is important, accomplished via GPS or algorithm such as Network

Time Protocol.CLR packet to be flooded to clear the invalid packet.

Fig 7: Example of Link Failure

Fig 8: Example of Re-establishment route.

d. Associativity Based Routing

Each route keeps a associativity table. A high value of associativity tick indicates a low state of node mobility. A route is selected based on associativity states of nodes, finds the high value of associativity tick (low mobility routes).Associativity table: All nodes generate periodic beacons. When a neighbor node receives a beacon, it increases its associativity tick with respect to the sending node in associativity table. Associativity ticks are reset when the neighbors of a node or the node itself move out of proximity.Route Discovery: The source broadcast a QRY message. Each intermediate node appends its address and associativity ticks to QRY, The destination can examine the associativity ticks to select route. If the multiple paths have the same overall degree of stability, select the minimum number of hopsRoute Erasing: If the the route is no longer desired, the source may not be aware of any route node changes because partial reconstruction. The source node initiates a route delete (RD) broadcast to erase the invalid route.

VII. Overview:

On Demand

AODV

DSR TORA

ABR SSR

Over all complex-

ity

Medium

Medium

High High High

Overhead Low Medium

Medium

High High

LoopFree Yes Yes Yes Yes Yes

Becoming require-

ment

No No No Yes Yes

Multiple route

support

No Yes Yes No No

Routes maintain

Route table

Route cache

Route table

Route table

Route table

Route configure

metho-dology

Erase route; notify source

Erase route; notify source

Link reversal

route repair

Localize broad-

cast

Erase route; notify source

Routing metric

Freshest and

shortest path

Shortest path

Shortest path

Associative and shortest

path

Associative and stability

Table 1: Overview of all the On-Demand Routing Protocols

ConclusionDefault Routing Is configured for unknown destinations. Generally used on internet as destinations are unknown. It is configured at end points. Default Routing is last preferred routing. In On-Demand Routing the routes are created when required. The source and intermediate nodes have to maintain a route as long as it is used. The source has to discover a route to the destination.

Acknowledgement:

I will like to thank our Prof.. for giving the chance to explore oneself.

References:[1].http:// www.hill2dot0.com /wiki/ index.php? title = Default_route [2].http:// www.cisco.com /c/en/us/tech/ ip /on-demand- routing- odr / index.html [3].http:// www.slideshare.net / kishoreeekumarrr / multi- static-routng-default-routing-29830267?v = default&b =& from_search =2 [4].https://cs.wmich.edu/wsn/cs691_sp03/adhocrouting.ppt[5].http:// kiki.ee.ntu.edu.tw / mmnet1 / adhoc /