IP Routing - Computer Networks

Dear Aspirants,

We'll take up Routing today. This article is designed keeping in mind the level of questions asked in professional knowledge section for specialist officers in IT. Use comments section freely for discussions/queries/suggestions/motivation.

==>> This article is a part of IT Officer's PK Series

Routing

IP routing is the process of moving packets from one network to another network using routers. A routing protocol is a tool used by routers to find all the networks in the internetwork and to ensure that all routers have same routing tables.

Routing Tables

A routing table is a set of rules, in a table format, that is used to determine where data packets travelling over internet will be directed. All IP enabled devices including routers and switches, use routing tables. Typical fields in a routing table are as below:

Subnet Mask
Destination IP address
Next hop address
Flags
Interface

Static and Dynamic Routing

In Static Routing, router uses manually configured entries for directing the packets. It doesn't change with time.

In Dynamic Routing, routing tables are dynamically configured and automatically updated depending upon the network condition. It uses protocols like RIP, OSPF, BGP etc.

Routing Protocols

Routing Information Protocol

One of the oldest distance vector routing protocols which uses hop count as a routing metric
Table entries are updated using values from the neighbors.
Maintains timers to detect failed links
Used in first generation ARPANET

Open Shortest Path First (OSPF)

Widely used interior router protocol on TCP/IP.
Computes a route to all destination networks that incurs the least cost using Dijkstra's algorithm
Each router maintains a database
'Hello' packets sent after every 10 seconds and absence of the same for 40 seconds indicate failure of the neighbor. It is sent to check if the neighbor is up.
Uses encrypted authentication (MD5 Hash)

Border Gateway Protocol (BGP)

Widely used exterior router protocol for internet.
Allows router belong to different Autonomous systems to exchange routing information.
Four types of messages used: Open (open a neighbor connection), Update (transmit information about a single route), Keepalive (confirm the neighbor connection), Notification (notify error condition)
Uses port 179 for connections.