Routing A broad Perspective

Routing is…What fans do at football gamesWhat pigs do for trufflesSend corps of infantrymen into full scale,

disorganized retreat.

Uh! So what is Routeing?

Something that Brits use, to distinguish between what happens in Networks from what happened to them in New Orleans in 1814.

What ISO and CCITT use in their documentsAnd Yes, it is…

Routeing is…

The process of determining, selecting the best outgoing path that a packet has to take in a Internetwork.

But, with due apologies to British Standards

Institute, lets stick to Routing!

OSI Routing Scheme

A set of Routing Protocols Allows end systems and intermediate

systems to collect and distribute information necessary to determine routes

Routing Information base Table containing all routing relevant

connectivityRouting Algorithms

Uses information in routing information base to derive routes between end systems.

OSI Routing Architecture

For the Next 10 min….

Design Goals of Routing AlgorithmsAlgorithm TypesRouting MetricsDynamic Routing Algorithms

Design Goals

OptimalitySimplicity and low OverheadRobustness and stabilityRapid ConvergenceFlexibility

Algorithm types

Static Vs DynamicSingle Path Vs Multiple PathHost Intelligent Vs Router

IntelligentIntradomain Vs Interdomain

Routing Metrics

Path LengthReliabilityDelayBandwidthLoadCommunication cost

Static AlgorithmsShortest Path AlgorithmFloodingFlow based Routing

Dynamic AlgorithmsDistance Vector RoutingLink State RoutingHierarchical Routing

Distance Vector Routing Bellman Ford/ Ford Fulkerson Algorithm - Original

ARPANET routing algorithm AppleTalk and CISCO routers use improves distance vector

protocols Each router knows the id of every other router in the

network. Each router maintains a vector with an entry for every

destination that contains: The cost to reach the destination from this router. The direct link that is on that least cost path.

Each router, periodically sends its vector to his direct neighbors.

Upon receiving a vector a router updates the local vector based on the direct link’s cost and the received vector.

Link State Routing

Discover the neighbors and learn the network addresses

Measure the delay or cost to each of the neighbors

Construct a packet telling all it has just learnt

Send this packet to all other routersCompute the shortest path to every

other router

Link State Routing contd..

Each router knows the id of every other router in the network.

Each router maintains a topology map of the whole network.

Each router, periodically floods its link state updates (with its direct connectivity information).

Upon receiving a vector, a router updates the local topology map and re-calculates shortest paths.

Increasing Network Size

Router memory requirements increase.More CPU time for look up and more

bandwidth needed to send status reports.Routing table size increases with increase in

network size.

Hierarchical Routing

Routers divided into Regions.Regions > Clusters > Zones > Groups >Internal structure of a region known only to

routers within that region.Different networks do not need to know the

topological structure of other ones.

Example of Hierarchical Routing

Penalty for Hierarchical Routing

Path length may increase. But this increase is sufficiently small and usually acceptable.

The optimum number of levels for an N router subnet is ln N, with a total of e ln N entries per router.

Internet Control Protocols

Routing Information ProtocolOpen Shortest Path FirstInterior Gateway Routing ProtocolExterior Gateway protocolBorder Gateway Protocol

Routing Information Protocol

The protocol is limited to networks whose longest path is 15 hops.

Inappropriate for larger networks.Uses fixed metrics to compare alternative routes. Not appropriate for routing based on dynamic

parameters.Cannot operate within a hierarchical structure.

Open Shortest Path First

OSPF is a link-state routing protocol that calls for the sending of link-state advertisements to all other routers within the same hierarchical area.

As OSPF routers accumulate link-state information, they use the SPF algorithm to calculate the shortest path to each node using Dijkstra’s Shortest Path Algorithm .

Can operate in a hierarchy.Supports multi-path routing and type of service

based routing.

Interior Gateway Routing Protocol

A distance-vector routing protocol.Replaced RIP

Network SizeSingle Metric

IGRP uses five criteria to determine the best path: the link's speed, delay, packet size, loading and reliability.

The weighting factors for each of these metrics can be fixed up by Network Administrators.

Permits multi-path routing.

Exterior Gateway Protocol

First exterior routing protocol to gain widespread accepted on the internet.

Dynamic routing protocol with a very simple design used for communication of reachability information.

It does not use metrics and therefore cannot make intelligent routing decisions

Border Gateway ProtocolInternet standard for inter-domain exterior routing.BGP is used to exchange routing information between

Internet service providers (ISP). External / Interior BGPExtremely robust and scalableSupports classless inter-domain routing.BGP-4, the latest version, lets administrators configure cost

metrics based on policy statements.Routers using BGP, exchange routing information using

TCP connection.

BGP contd.

BGP communicates with autonomous (local) networks using Internal BGP (IBGP) since it doesn't work well with IGP. The routers inside the autonomous network thus maintain two routing tables: one for the interior gateway protocol and one for IBGP.

Policies are manually configured into each BGP router.Never put Iraq on a route starting at Pentagon.Traffic starting and ending at IBM should not transit

Microsoft.

Questions ?