Skip to main content

1.1.5 Verifying PAT configuration / 1.1.6 Troubleshooting NAT and PAT configuration


1.1.5 Verifying PAT configuration
Once NAT is configured, use the clear and show commands to verify that it is operating as expected.
By default, dynamic address translations will time out from the NAT translation table after a period of non-use. When port translation is not configured, translation entries time out after 24 hours, unless the timers are reconfigured with the ip nat translation timeouttimeout_ seconds command from global configuration mode. Clear the entries before the timeout by using one of the commands in Figure .
Translation information may be displayed by performing one of the tasks in EXEC mode .
Alternatively, use the show run command and look for NAT, access list, interface, or pool commands with the required values.


1.1.6 Troubleshooting NAT and PAT configuration
When IP connectivity problems in a NAT environment exist, it is often difficult to determine the cause of the problem. Many times NAT is mistakenly blamed, when in reality there is an underlying problem.
When trying to determine the cause of an IP connectivity problem, it helps to rule out NAT. Use the following steps to determine whether NAT is operating as expected:
  1. Based on the configuration, clearly define what NAT is supposed to achieve.
  2. Verify that correct translations exist in the translation table.
  3. Verify the translation is occurring by using show and debug commands.
  4. Review in detail what is happening to the packet and verify that routers have the correct routing information to move the packet along.
Use the debug ip nat command to verify the operation of the NAT feature by displaying information about every packet that is translated by the router. The debug ip natdetailed command generates a description of each packet considered for translation. This command also outputs information about certain errors or exception conditions, such as the failure to allocate a global address.
Figure shows a sample debug ip nat output. In this example, the first two lines of the debugging output show that a Domain Name System (DNS) request and reply were produced. The remaining lines show the debugging output of a Telnet connection from a host on the inside of the network to a host on the outside of the network.
Decode the debug output by using the following key points:
  • The asterisk next to NAT indicates that the translation is occurring in the fast-switched path. The first packet in a conversation will always go through the slow path, which means this first packet is process-switched. The remaining packets will go through the fast-switched path if a cache entry exists.
  • s = a.b.c.d is the source address.
  • Source address a.b.c.d is translated to w.x.y.z.
  • d = e.f.g.h is the destination address.
The value in brackets is the IP identification number. This information may be useful for debugging. This is useful, for example, because it enables correlation with other packet traces from protocol analyzers. 

Comments

Post a Comment

Popular posts from this blog

OSI layers / Peer-to-peer communications / TCP/IP model

OSI layers 2.3.4 This page discusses the seven layers of the OSI model. The OSI reference model is a framework that is used to understand how information travels throughout a network. The OSI reference model explains how packets travel through the various layers to another device on a network, even if the sender and destination have different types of network media. In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. - Dividing the network into seven layers provides the following advantages: • It breaks network communication into smaller, more manageable parts. • It standardizes network components to allow multiple vendor development and support. • It allows different types of network hardware and software to communicate with each other. • It prevents changes in one layer from affecting other layers. • It divides network communication into smaller parts to make learning it easier to understand. In the foll...
Post a Comment
Read more

PC Basic...

• Backplane – A backplane is an electronic circuit board containing circuitry and sockets into which additional electronic devices on other circuit boards or cards can be plugged; in a computer, generally synonymous with or part of the motherboard. • Network interface card (NIC) – An expansion board inserted into a computer so that the computer can be connected to a network. • Video card – A board that plugs into a PC to give it display capabilities. • Audio card – An expansion board that enables a computer to manipulate and output sounds. • Parallel port – An interface capable of transferring more than one bit simultaneously that is used to connect external devices such as printers. • Serial port – An interface that can be used for serial communication in which only one bit is transmitted at a time. • Mouse port – A port used to connect a mouse to a PC. • USB port – A Universal Serial Bus connector. A USB port connects devices such as a mouse or printer to the computer ...
Post a Comment
Read more

Windowing

Windowing 11.1.5 This page will explain how windows are used to transmit data. Data packets must be delivered to the recipient in the same order in which they were transmitted to have a reliable, connection-oriented data transfer. The protocol fails if any data packets are lost, damaged, duplicated, or received in a different order. An easy solution is to have a recipient acknowledge the receipt of each packet before the next packet is sent. If a sender had to wait for an ACK after each packet was sent, throughput would be low. Therefore, most connection-oriented, reliable protocols allow multiple packets to be sent before an ACK is received. The time interval after the sender transmits a data packet and before the sender processes any ACKs is used to transmit more data. The number of data packets the sender can transmit before it receives an ACK is known as the window size, or window. TCP uses expectational ACKs. This means that the ACK number refers to the next packet that is...
Post a Comment
Read more