Ip Routing Principles

IP Routing Principles

IP routing Principles • Direct Routing:- if the Datagram is routed locally i.e if the destination is on the same subnet as the originator. Indirect Routing :- if the use of aForwarding devices such as router is invoked i.e the destination is remote .

IP Routing • A Datagram that is sent may invoke both Direct and indirect routing • The last router in the path must use direct routing to deliver the packet to the destination host. • A framed Packet will be accepted by a router only if Data link identifier in destination address Contains

Routing • Identification of router’s interface or • A broadcast identification • Router Strips off the frame and passes the enclosed packet to the network layer. • At network layer the destination IP address is examined

Routing Basics • If the destination network indicated by the address in the IP header is directly attached to the router , it will forward the packets directly to the destination station, after encapsulation. • If the destination network indicated by the address in the IP header is not directly attached to the router , it must use the services of another router to forward the packets and let the router determine the next Hop.

Router Basics • For routing the Router will do a route table look up • At minimum , each address of the network the router can reach. • A pointer to the destination • The pointer will indicate the destination network directly connected to the router.

Router Basics • The address of another router on a directly connected network • A router one hop closer to the the destination is called a next hop router.

Router Table Updation • Route Table acquires information in two ways • i) Information may be added manually , by means of static route entries or • Automatically by one of the several systems of automatic information discovery and sharing known as dynamic routing protocols.

Static – Dynamic Routing • Static / Non – Adoptive • Choice of route is computed in advance, off line , and downloaded to the router when network is booted. Dynamic / Adaptive • Routing decisions change to reflect the hanges in topology and / or traffic.

Static Routing • Routes to destination are set up Manually. • Network reach ability is not dependent on the existence and state of the network. • Route may be up or down but static routes will remain in the routing tables and traffic would still be sent towards the route. • Not suitable for large networks.

Default Routing • When a router receives a packet and its table does not contain the network number indicated in the packet , it is forwarded to default router. • If there is no routes or default route at any stage , the router will send a control message ( through ICMP ) to the originating station.

Default Routing • Refers to “ last resort “ outlet. • Easiest form of Routing for a domain connected to a single exit point. • Default router is indicated as 0.0.0.0 with no subnet mask. • Routes are learnt via an internal and external routing protocols.

Dynamic Routing • If a router is down , its entry will be deleted from the routing table and traffic to that will not be forwarded. • Used to enable routers to build their own routing tables and make appropriate decisions.

Route Table

Router lookup • The router will match the most specific address it can in the descending order of specificity as below :• A host address • A subnet • A group of Subnets • A major network Number • A default Number

Routing Protocol • It is a language a router speaks with other routers to share information about the reach ability and status of the network. • Provides mechanisms for sharing routing information. • Allows the routers to communicate with other routers to update and maintain routing table.

Routing Protocol • Routing protocol messages do not carry end user traffic from network to network . • Routing protocol uses the routed protocol to pass information between routers. • Rip and OSPF are Routing protocols.

Convergence • The process of bringing all routes table to a state of consistency is called convergence . • The time it takes to share information acress an internet work and for all routers to calculate best path in convergence time. • When an internet work is in unconverged state , it may cause routing errors.

Metrics • A metric is a variable assigned to routes as a means of ranking them. • Different routing protocols use different and sometimes multiples metrics. o RIP defines the “ best “ route as one with minimum number of hops. o IGRP defines the “ best “ route on a combination of lowest bandwidth along the route and the total delay of the route.

Metrics • • • • • •

Hop Count Bandwidth Load Delay Reliability Cost

Metrics • Hop Count • A hop Count metric simply count router hops.

Metrics • Bandwidth • A bandwidth metric would choose a higher bandwidth over a lower bandwidth.

Metrics • Load • The metric reflects the amount of traffic utilizing the links along the path. • The best path is the one with the lowest load. • Unlike hop count and bandwidth , the load on a route changes and therefore the metric will change and is called route flapping. • Route flaps have adverse effect on routers CPU and the overall stability of the network.

Metrics • Delay • Delay is the measure of time a packet takes to traverse a route. • A protocol using delay as metric would choose the path with the least delay as the best path.

Metrics • Reliability • Reliability measures the likelihood that the link will fail in some way. • Variable reliability metrics are • i) Number of times link has failed or • Ii) Number of errors it has received within a certain time period • The path with highest reliability would be selected as the best.

Metrics • Cost • Cost may be defined by any policy or link characteristic or may reflect the arbitrary judgment of the administrator. • The Term “ Cost “ is often used as generic term. • E.g RIP chooses the lowest – cost path based on hop count. • Another generic term is “ Shortest “ e.g RIP chooses the shortest path based on the hop count.

Interior- Exterior Protocols • Interior Gateway protocols • Routing protocol which run with an Autonomous system are IGPs. • Distance Vector and Link state protocols are IGPs. Exterior Gateway Protocols • Routing protocols that route between Autonomous system are EGPs. • IGPs discover paths between Networks. • EGPs discovers paths between autonomous Sysyems.

Administrative Distances • Metrics are assigned to the routes to determine the most preferred route. • RIP Metric is hop count. • IGRP uses bandwidth and delay • OSPF uses COST • EIGRP uses composite metrics. • Diversity of metrics poses problems in routers running more than one routing protocol.

Administrative Distances • The administrative Distance of various protocols is as below: • Connected Interface - 0 • Static Route -1 • EIGRP summary route – 5 • External BGP - 20 • EIGRP - 90 • IGRP - 100

Administrative Distance • • • • •

OSPF IS – IS RIP EGP Unknown

- 110 - 115 - 120 - 140 - 255