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IP Addressing Services

NAT Order of Operation

Document ID: 6209

Updated: Sep 30, 2008

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Introduction

This document illustrates that the order in which transactions are processed using 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.

Prerequisites

Requirements

Readers of this document should have knowledge of this topic:

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

Components Used

This document is not restricted to specific software and hardware versions.

Note: The information in this document is based on the Software Version, Cisco IOS® Software Release 12.2(27)

Conventions

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

NAT Overview

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

Inside-to-Outside Outside-to-Inside
  • If IPSec then check input access list
  • decryption - for CET (Cisco Encryption Technology) or IPSec
  • check input access list
  • check input rate limits
  • input accounting
  • redirect to web cache
  • policy routing
  • routing
  • NAT inside to outside (local to global translation)
  • crypto (check map and mark for encryption)
  • check output access list
  • inspect (Context-based Access Control (CBAC))
  • TCP intercept
  • encryption
  • Queueing
  • If IPSec then check input access list
  • decryption - for CET or IPSec
  • check input access list
  • check input rate limits
  • input accounting
  • redirect to web cache
  • NAT outside to inside (global to local translation)
  • policy routing
  • routing
  • crypto (check map and mark for encryption)
  • check output access list
  • inspect CBAC
  • TCP intercept
  • encryption
  • Queueing

NAT Configuration and Output

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

5a-1.gif

In the previous example, Router-A is configured to translate the inside local address 171.68.200.48 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 171.68.200.48 172.16.47.150

!--- This command creates a static NAT translation   
!--- between 171.68.200.48 and 172.16.47.150 

ip domain-name cisco.com
ip name-server 171.69.2.132
!
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 171.68.200.0 255.255.255.0 172.16.47.162
!
!
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      171.68.200.48      ---                ---

This output is taken from Router-A with debug ip packet detail and debug ip nat enabled, and a ping issued from device 171.68.200.48 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 toImportant Information on Debug Commands.

IP: s=171.68.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=171.68.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1
IP: s=171.68.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=171.68.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=171.68.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1
IP: s=171.68.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=171.68.200.48 (Serial0), d=172.16.47.142, len 100, unroutable
    ICMP type=8, code=0
IP: s=172.16.47.161 (local), d=171.68.200.48 (Serial0), len 56, sending
    ICMP type=3, code=1

Since there are no NAT debug messages in the previous output, you know that the existing static translation is not used and that 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 leavingcisco.com, 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 classless configured, 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      171.68.200.48      ---                ---

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 171.68.200.48

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 
171.68.200.48 are getting translated by NAT. 

Router-A#
*Mar 28 03:34:28: IP: tableid=0, s=171.68.200.48 (Serial0), d=172.16.47.142 
(Serial1), routed via RIB
*Mar 28 03:34:28: NAT: s=171.68.200.48->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->171.68.200.48 [160]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=171.68.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=171.68.200.48 (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=171.68.200.48->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->171.68.200.48 [161]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=171.68.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=171.68.200.48 (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=171.68.200.48->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->171.68.200.48 [162]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=171.68.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=171.68.200.48 (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=171.68.200.48->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->171.68.200.48 [163]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=171.68.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=171.68.200.48 (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=171.68.200.48->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->171.68.200.48 [164]
*Mar 28 03:34:28: IP: tableid=0, s=172.16.47.142 (Serial1), d=171.68.200.48 
(Serial0), routed via RIB
*Mar 28 03:34:28: IP: s=172.16.47.142 (Serial1), d=171.68.200.48 (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

Updated: Sep 30, 2008
Document ID: 6209