Routing

Routing Protocols:RIP, OSPF, and BGP

Amitava Nag

Assistant Professor

[email protected]

April 11, 2012

Amitava Nag (Academy of Technology) Routing Protocols April 11, 2012 1 / 23

What is routing?


What is routing?

Finding a path to a destination address


What is routing?


Routing is performed for many kinds of networks, including thetelephone network (Circuit switching) , electronic data networks (suchas the Internet).


What is routing?



In computer networking a Routing table, or Routing Information Base(RIB), is a data structure in the form of a table-like object


What is routing?




A Routing table is stored either in a router or a networked computerthat lists the routes to particular network destinations.


What is routing?




A Routing table is stored either in a router or a networked computerthat lists the routes to particular network destinations.

The routing table contains information about the topology of thenetwork immediately around it.


Common Fields Of Routing Table


IP Routing Basics


Static or Dynamic


Static or Dynamic

1 Static routing - YOU are defining the way packets are going


Static or Dynamic


Entries are configured manually


Static or Dynamic


Entries are configured manuallyFixes routes at boot time


Static or Dynamic


Entries are configured manuallyFixes routes at boot timeUseful only for simplest cases


Static or Dynamic



2 Dynamic Routing - A routing protocol is ”trying” to find the best wayto a given destination


Static or Dynamic




entries are learned by routing protocols


Static or Dynamic




entries are learned by routing protocolsTable initialized at boot time


Static or Dynamic




entries are learned by routing protocolsTable initialized at boot timeNecessary in large internets


INTRA- AND INTERDOMAIN ROUTING



An Autonomous system(AS) is a group of networks and routers underthe authority of a single administration.




Routing inside an autonomous system is referred to as intradomainrouting.




Routing inside an autonomous system is referred to as intradomainrouting.

Routing between autonomous systems is referred to as interdomainrouting.


Autonomous systems


Popular routing protocols


Intradomain (Intra-AS) Routing



also known as Interior Gateway Protocols (IGP)




most common Intradomain routing protocols:





RIP: Routing Information Protocol





RIP: Routing Information ProtocolOSPF: Open Shortest Path First





RIP: Routing Information ProtocolOSPF: Open Shortest Path FirstIGRP: Interior Gateway Routing Protocol (Cisco proprietary)


DISTANCE VECTOR ROUTING

In distance vector routing,




The least cost route between any two nodes(routers) is the route withminimum distance.





Each router maintains a vector (table) of minimum distances to everynode






After powering-up each router only knows about directly attachednetworks







Routing table is sent periodically to all neighbour-routers








Received updates are examined, changes are adopted in own routingtable









Metric information (originally) is number of hops










Use ”Bellman-Ford” algorithm










Use ”Bellman-Ford” algorithm

Examples: RIP, RIPv2, IGRP (Cisco)


Initialization of tables in distance vector routing


Updating in distance vector routing


Distance vector routing tables


Two-node instability


Count to infinity

In this example we see what would happen if network link between router xand router A crashes. Immediately, router A has no more informationabout this net. What would happen if router B sends a routing updatenow?Now router A receives a routing update from router B including reachability information about router x. Because router A has no informationabout router x, A adds this information in his routing table and continuoussending his normal routing updates to router B, hereby increasing the hopcount by 1.Either router A or router B has information about the router x, bothrouter will increase the hop count by 1 every routing update. Count toinfinity occurs. Now Update packets are caught in a routing loop.


Example: Two-node instability


Example: Two-node instability


Split Horizon


Split Horizon

A router will not send information about routes through an interfaceover which the router has learned about those routes - Exactly THISis split horizon


Split Horizon


Idea: ”Don’t tell neighbor of routes that you learned from thisneighbor”


Split Horizon



That’s what humans (almost) always do: Don’t tell me what I’ve toldyou !


Split Horizon



That’s what humans (almost) always do: Don’t tell me what I’ve toldyou !Cannot 100


Split Horizon with Poison Reverse



Declare learned routes as unreachable




”Bad news is better than no news at all”




”Bad news is better than no news at all”

Stops potential loops due to corrupted routing updates


Example: Split Horizon With Poison Reverse


Link State Routing


Link State Routing

Each node in the domain has the entire topology of the domain


Link State Routing


the node can use Dijkstras algorithm to build a routing table


Link State Routing


the node can use Dijkstras algorithm to build a routing table

link-state: each router exchanges information about itsneighborhood with all routers in the network when there is a change


Concept of Link State Routing


The End


Engineering

Routing