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Classify unicast routing protocol. Explain exterior routing protocol in brief.

Mumbai university > Electronics and telecommunication Engineering > Sem 6 > Computer Communication and Telecom Network

Marks: 10

Years: May 2016

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• Unicast routing protocol is classified into interior protocols and exterior protocols.

• The popular interior protocols are RIP (Routing Information Protocol) and OSPF (Open Shortest Path First) whereas the exterior protocol used popularly is BGP (Border Gateway Protocol).

• RIP and OSPF are used to upgrade the routing tables inside an Autonomous System (AS) and BGP is used for upgrading the routing tables for the routers which join the AS together.

Exterior routing protocol:

• BGP is an exterior routing protocol. It is a unicast routing protocol.

• It is based on the routing method called path vector routing. This principle is used because the distance vector routing and link state routing do not prove to be good candidates for inter-autonomous system routing.

o Path vector routing:

• Table below shows the example of a path routing table. Each entry in the routing table contains the destination network, the next router and the path to reach the destination.

Network Next router Path
N01 R01 AS 12, AS 21, AS 56
N02 R08 AS 20, AS 57, AS 06
. . . . . . . . .

o BGP operation:

• Routing involves two basic activities: determination of optimal routing paths and the transport of information groups (typically called packets) through an inter-network.

• The transport of packets through an inter-network is relatively straightforward. Path determination on the other hand can be very complex. One protocol that addresses the task of path determination in today’s networks is the Border Gateway Protocol (BGP). BGP performs inter-domain routing in Transmission-Control Protocol/Internet Protocol (TCP/IP) networks.

• BGP is an exterior gateway protocol (BGP) which means that it performs routing between multiple autonomous systems or domains and exchanges routing and reachability information with other BGP systems.

• BGP was developed to replace its predecessor, the now obsolete Exterior Gateway Protocol (EGP), as the standard exterior gateway routing protocol used in the global internet.

• BGP performs three types of routing- inter autonomous system routing, intra autonomous routing and pass-through autonomous system routing.

• Inter autonomous system routing occurs between two or more BGP routers in different autonomous systems. Peer routers in these systems use BGP to maintain a consistent view of the internetwork topology.

• BGP neighbours communicating between autonomous systems must reside on the same physical network. The internet serves as an example of an entity that uses this type of routing because it is comprised of autonomous systems or administrative domains. Many of these domains represent the various institutions, corporations and entities that make up the internet. BGP is frequently used to provide path determination to provide optimal routing within the internet.

• Intra autonomous system routing occurs between two or more BGP routers located within the same autonomous system. Peer routers within the same autonomous system use BGP to maintain a consistent view of the system topology.

• BGP also is used to determine which router will serve as the connection point for specific external autonomous systems. The internet provides an example of inter autonomous system routing. An organization such as a university could make use of BGP to provide optimal routing within its own administrative domain or autonomous system. The BGP protocol can provide both inter and intra autonomous system routing services.

• Pass-through autonomous system routing occurs between two or more BGP peer routers that exchange traffic across an autonomous system that does not run BGP. In a pass-through autonomous system environment, the BGP traffic did not originate within the autonomous system in question and is not destined for node in the autonomous system.

• Figure below illustrates a pass-through autonomous system environment: BGP pairs with another intra-autonomous system routing protocol.

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o BGP solves the count to infinity problem

• The BGP is basically a distance vector protocol but it is very much different from the most other protocols such as RIP (Routing Information Protocol).

• Instead of maintaining just the cost of each destination, each BGP router keeps track of the path used.

• Similarly instead of periodically giving each neighbour its estimated cost to each possible destination, each BGP router tells its neighbour the exact path that is using.

enter image description here

• Figure shows a set of BGP routers and table shows the information that router F receives from its neighbours about D.

• BGP can solve the count to infinity problem easily. This can be explained as follows: Suppose that the router G in figure crashes or if the line FG becomes faulty, then router F receives routes from the remaining three neighbours i.e. B, I and E.

• As shown in the table these routes are BCD, IFGCD and EFGCD.

• Looking at these routes, router F immediately understands that the routes IFGCD and EFGCD are useless because they pass through F itself.

• So it decides to choose FBCD path as a new route. This avoids the count to infinity problem.

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