Discontiguous Networks (Free Preview)

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Discontiguous network is a network that has two or more subnetworks of a classful network joined together by different classful networks.



Figure Discontiguous network


In the figure above subnetwork and are joined together by network


What is the issue?

Discontiguous networks become an issue when we are using a classful routing protocol such as RIPv1 or IGRP.

Also, by default they do not work on RIPv2 and EIGRP due to auto summarization feature that is enabled. In order for RIPv2 and EIGRP to support discontiguous networks we must turn off auto summarization.


Since RIPv1 and IGRP are classful routing protocols, when a routing advertisements are sent across a major network boundary they summarize the advertised network to its major classful network address.


To clarify, in the figure below when R1 sends a routing advertisement to R2 about network, R1 summarizes it to major network because the advertisement is crossing the major network boundary

Similarly, when R2 sends a routing advertisement to R1 about network, R2 summarizes it to major network

The result of this is devices in subnetwork and cannot communicate with each other.



Figure Discontiguous network routing advertisement


On the contrary, OSPF by default supports discontiguous networks because it supports VLSM(Classless) and does not auto summarize.

Lesson tags: CCNA, Cisco, EIGRP, OSPF, RIP, Routing
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