What is IP Routing

IP Routing describes concepts including the lookup process of router and packet forwarding. It analyzes and contrast strategies for routing and routing protocols. IP Routing is a set of protocols that sends packets from one network host to another different remote network host. IP Routing also determines the path from source to destination that data follows to travel across multiple networks.

IP Routing

Packet Forwarding

Packet Forwarding

Through switching and path determination it is accomplished by routers. In path determination how the router determines which path to use when forwarding a packet. For the determination of the best path, the router searches for a network address in its routing table that matches the destination IP address of the Packets.

This search contains three path determinations results:

Directly connected network: If the packet destination IP address on a network belongs to a device connected directly to one of the router’s interfaces, that packet is directly forwarded to that device.

Remote network: The packet is forwarded to another router if the packet destination IP address belongs to a remote network. When forwarding packets to other routers then the remote network can be reached.

No route determined: The packet is discarded when the packet destination IP address doesn’t belong to a remotely or directly connected network as well as and the router doesn’t have a default route. The router sends an Unreachable ICMP (Internet Control Message Protocol) to the packet source IP address.

Methods of Routing

Directly connected routes: When the IP address is activated on the interface then entered in the routing table automatically.

Static routes: Configured and entered in the routing table manually by the

Network administrator if the exit interface is active for the static route.

Dynamic routes: They learned from the routers through sharing routes with those routers that use the same routing protocol.

In most cases, the number of networks, network topology complexity, and the need to adjust automatically changes for the network require the use of the dynamic routing protocol. There are several advantages of dynamic routing over static routing, however, in today's network, static routing is still used. The network use combinations of both static and dynamic routing.

Features of Static Routing and Dynamic Routing

Static vs Dynamic Routing Features

The below figure shows IP routing protocol timelines along with a chart that helps you memorize the various ways to classify routing protocols.

Classification and Evolution of Routing Protocols

Routing Protocols Evolution

Routing Protocols Classification

The highlighted parts (RIP, RIPv2, EIGRP, and OSPF) in the above picture are the focus of the CCNA exam and according to the routing protocols characteristics these can be classified into different groups.

v EGP and IGP
v Link-State or Distance Vector
v Classless or Classful

EGP and IGP

Under a common administration AS (Autonomous System) is a collection of routers that presents a clearly defined, common routing policy to the Internet. An ISP and the internal network of large companies are an example of the autonomous system. A network only having its ISP is an autonomous system, while most company networks are not autonomous systems. Because the concept of an autonomous system is based on the Internet, two types of routing protocols are required:

v IGP (Interior Gateway Protocols): Used for Inter-AS routing, the routing inside an AS (Autonomous System).

v EGP (Exterior Gateway Protocols): Used for Inter-AS routing, the routing between an AS (Autonomous System).

Distance Vector Routing Protocols

As a vector of direction and distance, the routes are advertised, that is the means of distance vector. Hop count is defined as the distance, and direction is the exit interface or next-hop router. Route determination for the best-path distance vector protocols typically uses the algorithm of Bellman-Ford. Periodically some distance vector protocols send the complete routing tables to all connected neighbors. These routing updates become enormous, causing significant traffic on the links in the large networks.

Although to maintain the reachable of networks database the Bellman-ford algorithm accumulates enough knowledge, and to know the exact internetwork topology the algorithm doesn’t allow a router. However, the router knows only received information from its neighbors. In the path to the final destination, the distance vector uses routers as signposts. The router knows only about a metric or the distance of a remote network to reach that network and to use which interface or path to get there. Distance vector protocols don’t have a network topology map.

Distance vector protocol work efficient in situations where

v The network doesn’t require a hierarchical design and the network should be flat and simple.
v Troubleshooting and configuration of Link-State protocols the Administrator doesn’t have enough knowledge of it.
v Specific network types are being implemented such as hub and spoke networks.
v No concern about worst-case convergence time in a network.

Link-State Routing Protocols

Link-State Routing Protocols Diagram

If a router is configured with a link-state routing protocol instead of a distance vector routing protocol operation can create the topology or a complete view of the network by information gathering from all connected routers. The Link-State routing protocols have a complete network topology map. All link-state routers use an identical map of the network so; from source to destination, signposts are not necessary.

The link-state information is used by the link-state router for topology map creation and best path selection to all destination networks in the topology. Periodic updates are not used by routers to all connected neighbors in the link-state routing protocol. When there is a change in the topology only a link-state update is sent after the network convergence.

Link-State routing protocol work efficient in situations where

v Hierarchical network design usually occurs in large networks.
v For the implantation of link-state routing protocol, the Administrator has a piece of good knowledge.
v Network fast convergence is crucial.

Classful Routing Protocols

Classful Routing Protocols Diagram

In Classful Routing, the information of the subnet mask is not sent in routing updates. The RIP (Routing Information Protocol) was the first routing protocol that was Classful. This was at a time on the classes’ base: Class A, B, and C, the addresses of the network were allocated. The Subnet mask was not sent in the routing updates because based on the first octet of the network address the network mask could be determined.

In today’s network, the Classful addresses can still be used, but in all situations, they cannot be used because they don’t include a subnet mask. When a network is subnetted with more than one subnet mask then, Classful routing protocols cannot be used. It means VLSM (Variable-length Subnet Masking) is not supported in classful routing protocols.

The inability to support supernets and discontiguous networks, it’s the limitation that exists in classful routing protocols. However,

(Routing Information Protocol version 1) and IGRP (Interior Gateway Protocol) are included in classful routing protocols.

Classless Routing Protocols

Classless Routing Protocols Diagram

The routing updates of the classless routing protocols contain the information of the subnet mask as compared to classful. Based on the classes, today’s networks are no longer allocated and the determination of subnet mask cannot be the value of the first octet of the network address.

Classless routing protocols support VLSM (Variable-length Subnet Masking), supernets, and discontiguous networks; due to this support the classless routing protocols are required in today’s most of the networks. RIPv2 (Routing Information Protocol version 2), EIGRP (Enhanced IGRP), OSPF (Open Shortest Path First), IS-IS (Intermediate System-to-Intermediate System), and BGP (Border Gateway Protocol).

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