CCIE Exam: NAT Order of Operation

In preparation of our CCNP exam, we want to make sure we cover the various concepts that we could see on our Cisco CCNP exam. So to assist you, below we will discuss NAT Order of Operation.

Introduction

This document illustrates that the order in which transactions are processed using Network Address Translation (NAT) is based on whether a packet is going 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 the following 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

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

NAT Overview

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

NAT Configuration and Output

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

In the above example, Router-A is configured to translate the inside local address 171.68.200.48 to 172.16.47.150, as shown in the configuration below.
  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

The following 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 issuing debug commands, please see Important 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 output above, you know that the existing static translation is not being 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, which is sent to the inside device.

However, 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, issuing the no ip classless command turns off the router's ability to look for the route with the longest bit match. 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 IOS Version 11.3 and above.
  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

Repeating the same ping test as before, we 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 above example shows that when a packet is traversing 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.

I hope you found this article to be of use and it helps you prepare for your Cisco CCNP certification. Achieving your CCNP certification is much more than just memorizing Cisco exam material. It is having the real world knowledge to configure your Cisco equipment and be able to methodically troubleshoot Cisco issues. So I encourage you to continue in your studies for your CCNP exam certification.