Troubleshoot passive interfaces

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

If you are preparing for the CCNP routing and switching CCDP certification you will have to clear the exam number 300-101 route. The exam is a 120 minute exam. It consists of about 45 to 65 questions. The questions are mostly objective type and you will have to choose the right answers.

There are six main topics under this exam one such topic is the layer 3 technologies. The topic “troubleshoot passive interface” is a part of this topic. We will try to look into all those aspects of this topic that is important from the exam point of view. We hope that this chapter will help you to prepare better for the exam and get a better score.

We will first discuss how the passive interface is configured. The steps are mentioned below:

  1. As a first step you will have to enable the privileged EXEC mode. You will have to enter the password if that is required.
  2. As the second step you will have to enter the global configuration mode.
  3. Step three you will have to configure the EIGRP process and then enter the router configuration mode.
  4. Now set all the interfaces as passive by default.
  5. Now you will need to activate only those interfaces that will need adjacencies.
  6. Now specify the list of networks that will be advertised by the routing protocols.
  7. Now you will have to exit the router configuration mode and you will have to enter the privileged EXEC mode.
  8. Now using the show eigrp interfaces command check if the interfaces on the network has actually been set to passive or not.
  9. As a last step you will have to verify if the interface that you have enabled is active or not.

The main advantage of the passive interface is that it will simplify the configuration of the distribution devices. This is done by allowing all the interfaces to be set to a passive interface by default. You will often notice that large enterprise networks will often have more than two hundred interfaces across its many distribution devices. If you want the routing information from these devices then you will have to look at manual configuration of the passive interface command. This has to be done on interfaces where the adjacencies were not desired at the first place.

You can use a routing protocol like OSPF (open shortest path first) in order to support the interfaces and to redistribute the interfaces that are connected. One more thing that you must remember as a network administrator is that you will have to configure a routing protocol on all the interfaces and then manually have to set most of them as passive. You can use the passive –interface default command to ensure that all the interfaces are set to passive by default. After this in individual interfaces where you need adjacencies you can use the no passive interface command. The passive interface is often used in ISPs too. ISP is also a network that is used in large enterprises.

The passive interface is a command that you can use in order to control the advertisement of the routing information. This command will help you to enable the suppression of the routing updates over some of the interfaces but it will allow the updates to be exchanged in a normal manner over the other interfaces. The passive interface is a command that will restrict the outgoing advertisements only.

However, you must know that when the passive interface command is used with the EIGRP (enhanced interior gateway routing protocol) it will suppress the exchange of the hello packets between the two routers. This will result in the loss of the neighbour relationship between them. It will not only stop the routing updates that are being advertised but also the incoming routing updates.

You must also know how the configuration is required in order to allow the suppression of the routing updates that are outgoing. It will allow the incoming routing updates to be learned normally that come from the neighbours. You will notice that with the EIGRP running on a network the passive interface command will stop both the incoming and the outgoing updates.

This is because the effect of the command is such that it will stop the router from sending and receiving the hello packets. If there is no output of the passive interface command then it means that the EIGRP is not only suppressing the hellos that are outbound but it is also ignoring the inbound hellos at the same time. It can also be the case if two routers are not neighbours anymore.

You must also remember as you troubleshoot passive interface that when the passive interface command is used in EIGRP then the router will not be able to form neighbour adjacencies on the interface. It will also not be able to send or receive any routing updates. You can do something if you want to suppress the outgoing routing updates then you can use the distribute list command. It will ensure that the inbound updates continue to be received. It will also ensure that the routers continue to be neighbours as before.

Some of the commands that you can often use while troubleshooting are as follows:

  1. Show ip eigrp interfaces – it is used to get a quick view of all the local interfaces that are running on eigrp.
  2. Show ip eigrp neighbours – this command will ensure that the EIGRP adjacencies are working as expected.
  3. Show ip eigrp topology – it is a very useful command that is used to see the routes that are marked as passive and also the routes that are marked as active.
  4. Debug ip routing – this is a command that is used to send console messages for all the major events that are apparent in the routing table.
  5. Debug eigrp packets - this is a powerful command that is used in troubleshooting. It allows the administrator to have a look at the EIGRP packets that have been received and sent.
  6. Debug ip eigrp – this command concentrates more on the events that occurred with the EIGRP.

We hope that this discussion on troubleshoot passive interface seemed useful to you and it will help you to get a better score in the CCNP exam. We wish you all the best for the exam.

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