How Split Horizon Affects RIP/IGRP Routing Updates when Secondary Addresses Are Involved

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Updated:August 18, 2005

Document ID:13712

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Introduction

Prerequisites

Requirements

Components Used

Conventions

Tables

Related Information

Introduction

A router configured with a primary IP address and secondary addresses on a given interface behave differently when you send updates out that interface depending on whether split horizon is enabled or disabled. This document provides tables that list the differences in the updates.

Note: Source interface is defined as the network interface on which the update is sent.

Split horizon	Update source	Update contents
Enabled	Primary	Subnets of primary (if known through non-source interfaces). Other major networks (including secondary network), known through non-source interface, summarized to major net boundary.
Enabled	Secondary	Subnets of secondary (if known through non-source interface). Other major networks (including primary network), known through non-source interface, summarized to major net boundary.
Disabled	Primary	All known subnets of primary. Other major networks (including secondary network), summarized to major net boundary.
Disabled	Secondary	All known subnets of secondary. Other major networks (including primary network), summarized to major net boundary.

Table 2: RIP updates with secondary address on same major network as primary

Split horizon	Update source	Update contents
Enabled	Primary	Subnets of primary/secondary (if known through non-source interfaces). Other major networks, known through non-source interface, summarized to major net boundary.
Enabled	Secondary	None - no updates sourced from secondary.
Disabled	Primary	All known subnets of primary/secondary. Other major networks summarized to major net boundary.
Disabled	Secondary	All known subnets of primary/secondary. Other major networks summarized to major net boundary.

Table 3: IGRP updates with secondary address on different major network than primary

Split horizon	Update source	Update contents
Enabled	Primary	Subnets of primary (if known through non-source interfaces). Other major networks (including secondary network), known through non-source interface, summarized to major net boundary.
Enabled	Secondary	Only subnets of secondary network.
Disabled	Primary	All known subnets of primary. Other major networks (including secondary network), summarized to major net boundary.
Disabled	Secondary	All known subnets of secondary. Other major networks (including primary network), summarized to major net boundary.

Table 4: IGRP updates with secondary address on same major network as primary

Split horizon	Update source	Update contents
Enabled	Primary	Subnets of primary/secondary (if known through non-source interfaces). Other major networks, known through non-source interface, summarized to major net boundary.
Enabled	Secondary	None - No updates source from secondary.
Disabled	Primary	All known subnets of primary/secondary. Other major networks summarized to major net boundary.
Disabled	Secondary	All known subnets of primary/secondary. Other major networks summarized to major net boundary.

Split horizon is enabled on each interface by default. In order to disable split horizon, use the no ip split-horizon interface subcommand as shown here: