Testing and Troubleshooting NAT

Cisco's NAT gives you some serious power—and without too much effort, because the configurations are really pretty simple. But we all know nothing’s perfect, so in case something goes wrong, you can figure out some of the more common causes by going through this list of possible snags:

1.Check the dynamic pools—are they composed of the right scope of addresses?

2.Check to see if any dynamic pools overlap.

3.Check to see if the addresses used for static mapping and those in the dynamic pools overlap.

4.Ensure that your access lists specify the correct addresses for translation.

5.Make sure there aren't any addresses left out that need to be there, and ensure none are included that shouldn't be.

6.Check to make sure that you've got both the inside and outside interfaces delimited properly.

One thing to keep in mind is that one of the most common problems with a new NAT configuration isn't specific to NAT at all—it usually involves a routing blooper. So, make sure that because you're changing a source or destination address in a packet, your router knows what to do with the new address after the translation!

Supposedly the sky’s the limit regarding the number of mappings the NAT table can hold. In reality, however, it comes down to things like memory and CPU or the boundaries set in place by the scope of available addresses or ports that do, in fact, cause there to be limitations placed on the number of entries possible. You see, each NAT mapping devours about 160 bytes of memory. And sometimes—but not very often—the amount of entries has to be limited for the sake of performance or because of policy restrictions. In situations like these, just use the ip nat translation max-entries command for help.

Another handy command for troubleshooting is show ip nat statistics. Deploying this gives you a summary of the NAT configuration, and it will count the number of active translation types. Also counted are hits to an existing mapping, as well any misses—the latter will result in an attempt to create a mapping. This command will also reveal expired translations. If you want to check into dynamic pools, their types, the total available addresses, how many addresses have been allocated and how many failed, plus the number of translations that have occurred, just use the pool (refcount) command.

And did you know you can manually clear dynamic NAT entries from the NAT table? Doing this can come in pretty handy if you need to get rid of a specific rotten entry without sitting around waiting for the time-out to expire. A manual clear also is really useful when you want to clear the whole NAT table to reconfigure a pool of addresses.

You also need to know that the Cisco IOS software just won’t allow you to change or delete address pools if any of that pool’s addresses are mapped in the NAT table. The clear ip nat translations command clears entries—you can indicate a single entry via the global and local address and through TCP and UDP translations (including ports), or you can just type in an asterisk (*) to wipe out the entire table. But know that if you do that, only dynamic entries will be cleared because this command does not remove static entries.

Oh, and there's more—any outside device’s packet destination address that happens to be responding to any inside device is known as the IG address. This means that the initial mapping has to be held in the NAT table so that all packets arriving from a specific connection get translated consistently. Holding entries in the NAT table also cuts down on repeated look ups happening each time the same machine sends packets to the same outside destinations on a regular basis.

Here’s what I mean: When an entry is placed into the NAT table the first time, a timer begins ticking; the duration of that timer is known as the translation timeout. Each time a packet for a given entry translates through the router, the timer gets reset. If the timer expires, the entry will be unceremoniously removed from the NAT table and the dynamically assigned address will then be returned to the pool. Cisco’s default translation timeout is 86,400 seconds (24 hours), but you can change that with the command ip nat translation timeout.

Before we move on to the configuration section and actually use the commands I just talked about, let’s go through a couple of NAT examples and see if you can figure out the configuration that needs to be used. To start, look at Figure 11.4 and ask yourself two things: Where would you implement NAT in this design, and what type of NAT would you configure?

In Figure 11.4, the NAT configuration would be placed on the corporate router and the configuration would be dynamic NAT with overload (PAT). In this NAT example, what type of NAT is being used?

The above command uses dynamic NAT. The pool in the command gives the answer away, plus there is more than one address in the pool, which means we probably are not using PAT. In the next NAT example, we’ll use Figure 11.5 to see if we can figure out the configuration needed.

The example in Figure 11.5 shows a border router that needs to be configured with NAT and will allow the use of six public IP addresses, through 114. However, on the inside network, you have 63 hosts that use the private addresses of through 126. What would your NAT configuration be on the border router?

 Two different answers would work here, but the following would be my first choice:
The command ip nat pool Todd netmask sets the pool name as Todd and creates a dynamic pool of addresses for the NAT to use address Instead of the netmask command, you can use the prefix-length 29 statement. (And I know what you’re thinking, but no, you cannot do this on router interfaces as well.) The second answer would end up with the exact same result of having only as your inside global, but you can type this in and have it work too: ip nat pool Todd netmask This is a waste because the second through sixth addresses would only be used if there was a conflict with a TCP port number.

If you do not understand the second line where the access-list is set, please see, “Security.”

The command ip nat inside source list 1 pool Todd overload sets the dynamic pool to use Port Address Translation (PAT) by using the overload command.

Be sure to add the ip nat inside and ip nat outside statements on the appropriate interfaces.

Simple Verification of NAT

Once you have configured the type of NAT you are going to use, typically overload (PAT), you need to be able to verify the configuration.

To see basic IP address translation information, use the following command:

Router# show ip nat translation

When looking at the IP NAT translations, you may see many translations from the same host to the same host at the destination. This is typical of many connections to the Web.

In addition, you can verify your NAT configuration with the debug ip nat command. This output will show the sending address, the translation, and the destination address on each debug line:

Router# debug ip nat

How do you clear your NAT entries from the translation table? Use the clear ip nat translation command. To clear all entries from the NAT table, use an asterisk (*) at the end of the command.

PAT (Overloading) Configuration

This last example shows how to configure inside global address overloading. This is the typical NAT that we would use today. It is rare that we would use static or dynamic NAT unless we were statically mapping a server, for example.
The nice thing about PAT is that the only differences between this configuration and the previous dynamic NAT configuration is that our pool of addresses has shrunk to only one IP address and at the end of our ip nat inside source command we included the overload command.

Notice in the example that the one IP address that is in the pool for us to use is the IP address of the outside interface. This is perfect if you are configuring NAT Overload for yourself at home or for a small office that only has one IP from your ISP. You could, however, use an additional address such as if you had the address available to you. This could be helpful in a very large implementation where you may have so many internal users that you have to have more than one overloaded IP address on the outside.