IPv4 Routing Basic

Introduction

An entreprize network is an instance of an autonomous systems. An autonomous system or AS, as it is also commonly known, is a connected group of one or more IP prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy.

In the evolution phase of networking, the AS considered the existence of a single routing protocol, however as networks have evolved, it is possible to support multiple routing protocols that inter-operate through the injection of routes from one protocol to another.

Routing Policy : A routing policy can be understood to be a set of rules that determine how traffic is managed within an autonomous system, to which a single, or multiple operator(s) must adhere to.

Router : A router determines the forwarding path via which packets are to be sent the route to a given destination. The decisions governing the route that should be taken can vary depending on the routing protocol in use, ultimately relying on metrics of each protocol to make decisions in relation to varying factors such as bandwidth and hop count.

Routing Table:

Every router prepares a table known as routing table that stores the forwarding information base (FIB) about the packets. Routers forward packets based on routing tables and a forwarding information base (FIB), and maintain at least one routing table and one FIB.

 

The decisions governing the route that should be taken can vary depending on the routing protocol in use, ultimately relying on metrics of each protocol to make decisions in relation to varying factors such as bandwidth and hop count Destination/Mask : Used to find the network address (IP and Mask are ANDed)

The protocol (Proto) field, indicates the protocol through which routes are learned.

The preference (Pre) specifies the preference value that is associated with the protocol, and is used to decide which protocol is applied to the routing table where two protocols offer similar routes.

A cost value represents the metric that is used to distinguish when multiple routes to the same destination have the same preference, the route with the lowest cost is selected as the optimal route.

A next-hop value indicates the IP address of the next network layer device or gateway that an IP packet passes through.

Default preference level of routing algoritm

Route Direct OSPF Static RIP
Preference 0 10 60 100

The one with lower value has the higher preference.

Routing Decision

 

Updated: November 14, 2019 — 5:40 pm

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