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Internet Routing and Routing Protocols

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Helsinki Central Railway Station, Helsinki, Finland - Hsi-Pin Ma)

- Overview

A routing protocol specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network. 

The purpose of routing protocols is to learn of available routes that exist on the enterprise network, build routing tables and make routing decisions. Some of the most common routing protocols include RIP, IGRP, EIGRP, OSPF, IS-IS and BGP.

Routing algorithms determine the specific choice of route. Each router has a priori knowledge only of networks attached to it directly. 

A routing protocol shares this information first among immediate neighbors, and then throughout the network. This way, routers gain knowledge of the topology of the network.


- Types of Routing

Routing is a process which is performed by layer 3 (or network layer) devices in order to deliver the packet by choosing an optimal path from one network to another. 

There are 3 types of routing:

  • Static routing – Static routing is a process in which we have to manually add routes in routing table.
  • Default Routing – This is the method where the router is configured to send all packets towards a single router (next hop). It doesn’t matter to which network the packet belongs, it is forwarded out to router which is configured for default routing. It is generally used with stub routers. A stub router is a router which has only one route to reach all other networks.
  • Dynamic Routing – Dynamic routing makes automatic adjustment of the routes according to the current state of the route in the routing table. Dynamic routing uses protocols to discover network destinations and the routes to reach it. RIP and OSPF are the best examples of dynamic routing protocol. Automatic adjustment will be made to reach the network destination if one route goes down. A dynamic protocol have following features: The routers should have the same dynamic protocol running in order to exchange routes. When a router finds a change in the topology then router advertises it to all other routers.


- Major Classes of Internet Routing Protocols

Although there are many types of routing protocols, three major classes are in widespread use on IP networks:


- Major Internet Routing Protocols

Internet routing protocols are algorithms that determine how packets of data travel from source to destination. Some common routing protocols include:

  • Routing Information Protocol (RIP): RIP is one of the oldest distance vector routing protocols that uses hop count as a routing metric. RIP limits the number of hops allowed in the path from source to destination to prevent routing loops.
  • Border Gateway Protocol (BGP): A high-level protocol that determines routing based on other factors such as weight and local preferences. Network administrators can bypass automatic routing decisions made by protocols.
  • Open Shortest Path First (OSPF): The link-state IGP uses the Shortest Path First (SPF) algorithm to calculate the shortest path spanning tree.
  • Enhanced Interior Gateway Routing Protocol (EIGRP): Popular choice for routing within campus networks. Many network engineers consider EIGRP to be the best choice of routing protocol for private networks.
  • Interior Gateway Protocol (IGP): IGP was established to overcome the shortcomings of RIP and is more suitable for large networks. The IGP automatically updates itself when routing changes occur within a particular network.
  • Exterior Gateway Protocol (EGP): From the mid-1980s to the mid-1990s, it was used to connect autonomous systems on the Internet. EGP is replaced by BGP.



- Core Router (Internet Backbone Router) 

A core router is a router designed to operate in the Internet backbone, or core. The core router that resides within the middle of the network rather than at its periphery. The routers that make up the backbone of the Internet are core routers. 

To fulfill this role, a core router must be able to support multiple telecommunications interfaces of the highest speed in use in the core Internet and must be able to forward IP packets at full speed on all of them. It must also support the routing protocols being used in the core. A core router is distinct from an edge router.


- Edge Router

Edge routers sit at the edge of a backbone network and connect to core routers. Edge routers are gateways that accept inbound traffic into your network. Edge routers work to secure the network edge and protect the core by characterizing and securing IP traffic from other edge routers as well as core routers. They differ from core routers in that core routers forward packets between routers to manage traffic and prevent packet loss, often using multiplexing. 

Edge routers also referred to as access or branch routers, are specialized routers that act as gateways at the network edge. They enable connectivity between an enterprise network and an external network. They are typically used at the wide area network (WAN) or the Internet.

An edge router should support edge interconnect protocols: IPv4, IPv6, ISO, MPLS. It should also support routing protocols: Static routes, OSPF, OSPF-TE, OSPFv3, IS-IS, BGP.



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