Cisco Cisco Email Security Appliance X1050 Information Guide

ESA Default IP Address Determination with
Multiple Interfaces on the Same Subnet

Document ID: 118575

Contributed by Stephan Fiebrandt and Enrico Werner, Cisco TAC
Engineers.
Oct 14, 2014

Contents

Introduction
AsyncOS Default Interface Determination
AsyncOS Versions 8.5.5 and Later
AsyncOS Version 8.0.0 Only
All Other AsyncOS Versions
Exceptions

Introduction

This document describes the Cisco Email Security Appliance (ESA) default interface determination process if
there are multiple interfaces configured on the same subnet.

AsyncOS Default Interface Determination

This section describes how AsyncOS selects a default IP address (AUTO) when there are multiple IP
addresses configured on the same subnet.

Note: If multiple IP addresses are present on the same subnet, they must be configured on the same interface.

AsyncOS Versions 8.5.5 and Later

If your system runs AsyncOS Version 8.5.5 or later, and there are multiple IP addresses configured within the
same subnet as the default gateway, the IP address with the lowest numeric value is chosen.

For example, these IP addresses might be configured:

10.10.10.2 /24

•

10.10.10.30 /24

•

10.10.10.100 /24

•

10.10.10.105 /24

•

In this case, 2 is lower than 30, 100, and 105, so it is selected as the default source IP address unless
configured otherwise. (Refer to the Exceptions section for more information.)

AsyncOS Version 8.0.0 Only

If your system runs AsyncOS Version 8.0.0, the default interface is selected based on the order that the IP
interfaces appear under Network > IP Interfaces on the GUI, or the order in which they appear in the output
of the ifconfig CLI command output. The first interface in the list that resides on the subnet in question is