Understand the NAT Order of Operation

Introduction

This document describes that the order transactions are processed with NAT is based on the direction a packet travels inside or outside the network.

Prerequisites

Requirements

Cisco recommends that you have knowledge of this topic:

• Network Address Translation (NAT). For more information on NAT, see How NAT Works.

Components Used

The information in this document is based on the Cisco IOS® Software Release 12.2(27).

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Conventions

Refer to the Cisco Technical Tips Conventions for more information on document conventions.

Background Information

This document describes that the order in which transactions are processed with Network Address Translation (NAT) is based on whether a packet goes from the inside network to the outside network, or from the outside network to the inside network.

NAT Overview

In this table, when NAT performs the global to local, or local to global, translation is different in each flow.

NAT Configuration and Output

This example demonstrates how the order of operations can effect NAT. In this case, only NAT and routing are shown.

In the previous example, Router-A is configured to translate the inside local address 172.31.200.48 to 172.16.47.150, as shown in this configuration.

!
version 11.2
no service udp-small-servers
no service tcp-small-servers
!
hostname Router-A
!
enable password ww
!
ip nat inside source static 172.31.200.48 172.16.47.150

!--- This command creates a static NAT translation 
!--- between 172.31.200.48 and 172.16.47.150 

ip domain-name cisco.com
ip name-server 172.31.2.132
!
interface Ethernet0
 no ip address
 shutdown
!
interface Serial0
 ip address 172.16.47.161 255.255.255.240
 ip nat inside

!--- Configures Serial0 as the NAT inside interface

 no ip mroute-cache
 no ip route-cache
 no fair-queue
!
interface Serial1
 ip address 172.16.47.146 255.255.255.240
 ip nat outside

!--- Configures Serial1 as the NAT outside interface

 no ip mroute-cache
 no ip route-cache
!
no ip classless
ip route 0.0.0.0 0.0.0.0 172.16.47.145

!--- Configures a default route to 172.16.47.145


ip route 172.31.200.0 255.255.255.0 172.16.47.162
!
!
line con 0
 exec-timeout 0 0
line aux 0
line vty 0 4
 password ww
 login
!
end

The translation table indicates that the intended translation exists.

Router-A#show ip nat translation
Pro Inside global      Inside local       Outside local      Outside global
--- 172.16.47.150      172.31.200.48      ---                ---

This output is taken from Router-A with debug ip packet detail  and debug ip nat  enabled, and a ping issued from device 172.31.200.48 destined for 172.16.47.142.

Note: Debug commands generate a significant amount of output. Use them only when traffic on the IP network is low, so other activity on the system is not adversely affected. Before you issue debug  commands, refer to Important Information on Debug Commands.

IP: s=172.31.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=172.31.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1
IP: s=172.31.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.31.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=172.31.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1
IP: s=172.31.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.31.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=172.31.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1

Since there are no NAT debug messages in the previous output, the current static translation is not used, and the router does not have a route for the destination address (172.16.47.142) in its routing table. The result of the non-routable packet is an ICMP Unreachable Message, which is sent to the inside device, but Router-A has a default route of 172.16.47.145, so why is the route considered non-routable?

Router-A has no ip classlessconfigured, which means if a packet destined for a major network address (in this case, 172.16.0.0) for which subnets exist in the routing table, the router does not rely on the default route. In other words, if you issue the no ip classless command, this turns off the ability of the router to look for the route with the longest bit match. In order to change this behavior, you have to configure ip classless  on Router-A. The ip classless command is enabled by default on Cisco routers with Cisco IOS Software Releases 11.3 and later.

Router-A#configure terminal
Enter configuration commands, one per line.  End with CTRL/Z.
Router-A(config)#ip classless
Router-A(config)#end

Router-A#show ip nat translation
%SYS-5-CONFIG_I: Configured from console by console nat tr
Pro Inside global      Inside local       Outside local      Outside global
--- 172.16.47.150      172.31.200.48      ---                ---

When you repeat the same ping test as previously done, you see that the packet gets translated and the ping is successful.

Ping Response on device 172.31.200.48

D:\>ping 172.16.47.142
Pinging 172.16.47.142 with 32 bytes of data:

Reply from 172.16.47.142: bytes=32 time=10ms TTL=255
Reply from 172.16.47.142: bytes=32 time<10ms TTL=255
Reply from 172.16.47.142: bytes=32 time<10ms TTL=255
Reply from 172.16.47.142: bytes=32 time<10ms TTL=255

Ping statistics for 172.16.47.142:
    Packets: Sent = 4, Received = 4, Lost = 0 (0%)
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum =  10ms, Average =  2ms

Debug messages on Router A indicating that the packets generated by device 
172.31.200.48 are getting translated by NAT. 

Router-A#
*Mar 28 03:34:28: IP: tableid=0, s=172.31.200.48 (Serial0), d=172.16.47.142 
(Serial1), routed via RIB
*Mar 28 03:34:28: NAT: s=172.31.200.48->172.16.47.150, d=172.16.47.142 [160]
*Mar 28 03:34:28: IP: s=172.16.47.150 (Serial0), d=172.16.47.142 (Serial1),
 g=172.16.47.145, len 100, forward
*Mar 28 03:34:28: ICMP type=8, code=0
*Mar 28 03:34:28: NAT*: s=172.16.47.142, d=172.16.47.150->172.31.200.48 [160]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=172.31.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=172.31.200.48 (Serial0), 
g=172.16.47.162, len 100, forward
*Mar 28 03:34:28: ICMP type=0, code=0
*Mar 28 03:34:28: NAT*: s=172.31.200.48->172.16.47.150, d=172.16.47.142 [161]
*Mar 28 03:34:28: NAT*: s=172.16.47.142, d=172.16.47.150->172.31.200.48 [161]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=172.31.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=172.31.200.48 (Serial0), 
g=172.16.47.162, len 100, forward
*Mar 28 03:34:28: ICMP type=0, code=0
*Mar 28 03:34:28: NAT*: s=172.31.200.48->172.16.47.150, d=172.16.47.142 [162]
*Mar 28 03:34:28: NAT*: s=172.16.47.142, d=172.16.47.150->172.31.200.48 [162]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=172.31.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=172.31.200.48 (Serial0), 
g=172.16.47.162, len 100, forward
*Mar 28 03:34:28: ICMP type=0, code=0
*Mar 28 03:34:28: NAT*: s=172.31.200.48->172.16.47.150, d=172.16.47.142 [163]
*Mar 28 03:34:28: NAT*: s=172.16.47.142, d=172.16.47.150->172.31.200.48 [163]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=172.31.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=172.31.200.48 (Serial0), 
g=172.16.47.162, len 100, forward
*Mar 28 03:34:28: ICMP type=0, code=0
*Mar 28 03:34:28: NAT*: s=172.31.200.48->172.16.47.150, d=172.16.47.142 [164]
*Mar 28 03:34:28: NAT*: s=172.16.47.142, d=172.16.47.150->172.31.200.48 [164]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=172.31.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=172.31.200.48 (Serial0), 
g=172.16.47.162, len 100, forward
*Mar 28 03:34:28: ICMP type=0, code=0

Router-A#undebug all
All possible debugging has been turned off

The previous example shows that when a packet traverses inside to outside, a NAT router checks its routing table for a route to the outside address before it continues to translate the packet. Therefore, it is important that the NAT router has a valid route for the outside network. The route to the destination network must be known through an interface that is defined as NAT outside in the router configuration.

It is important to note that the return packets are translated before they are routed. Therefore, the NAT router must also have a valid route for the Inside local address in its routing table.

Related Information

• Configuring Network Address Translation
• Verifying NAT Operation and Basic NAT Troubleshooting
• NAT: Local and Global Definitions
• How Does Multicast NAT Work on Cisco Routers?
• NAT Support Page
• Cisco Technical Support & Downloads