OSPFv2 Enabler #1 – Answers

By certskills January 23, 2017 09:05


The first OSPF Enabler exercise asked you to configure three routers, specifically to add the OSPFv2 network commands. This post lists the answers – simple enough.

The Topology

It is probably best to have the exercise post on the screen at the same time you look at this post. If not, for convenience, here’s a copy of the topology figure for this exercise.

OSPF Enabler OSPF Topology 1


The Answers

I promised short, sweet, and to the point. Here are the answers!


Answers: Router R1


Answers: Router R2


Answers: Router R3


Comments and Explanations – R1

For R1, you needed to start by identifying the various classful networks (class A, B, or C networks) used on the interfaces. Then, to configure one network command to match each classful network, you need to take the default mask for each classful network and invert the mask to find the corresponding wildcard mask. For default masks, those are:

Default mask – Inverts to wildcard mask

Default mask – Inverts to wildcard mask

Default mask – Inverts to wildcard mask

For OSPF areas, in this case, all interfaces were in area 0, so all the network commands of course end with the area 0 keywords.


Comments and Explanations – R2

R2 requires the most thought. To get these commands totally correct per the requirements, you must:

  1. Begin with each interface’s IP address and subnet mask
  2. Calculate the subnet ID, which will be the first parameter in the network command
  3. Invert the subnet mask, which will be the second parameter in the network command
  4. The design places all interfaces in area 0, so the commands all end with area 0.

Because of how the exercise is worded, you should have four network commands, one matching the subnet off each of the four interfaces on R2.


Comments and Explanations – R3

R3 requires the least thought of the three requirements. To match a single IP address, use wildcard mask of For the number preceding the wildcard mask in the network command, use the interface IP address. The result: each network command matches an interface IP address, and only that address, enabling OSPFv2 on that interface.

Also, as with all other cases in this lab, all interfaces were in area 0, so all the network commands of course end with the area 0 keywords.

OSPFv2 Enabler #1
IOS DHCP Server Config
By certskills January 23, 2017 09:05
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  1. Mit June 13, 09:58

    Mr. Wendell,

    Do we really need to calculate the subnet ID for the R2’s configuration?

    For example,

    instead of writing
    ‘network area 0’

    cannot we write
    ‘network area 0’ ?

    Warmest Regards,

    Reply to this comment
    • certskills Author June 19, 07:17

      Hi Mit,
      If you typed ‘network area 0’ in OSPF config mode, IOS would accept it, but change it to ‘network area 0’. So the literal answer is that you could indeed use the version that includes However, you would be typing an incorrect version of the command, and letting IOS fix it for you. That’s not a good practice for learning for the exam, in case you then weren’t ready to choose the right answer on the exam. But literally, either is accepted by IOS.


      Reply to this comment
  2. Erison October 21, 17:47

    Hi Wendell,
    Could you provide any example for unsdestand this network : area 0, I don´t understand why the number 7.

    Reply to this comment
    • certskills Author October 23, 15:02

      Sure, no problem.
      First, note that the OSPF *network* command, with it’s wildcard mask, can be correctly configured in many ways. For the lab exercise, I made up different rules for R1, R2, and R3 – not because those rules are good for real networks, but as a way to let you practice all the ways you can configure the *network* command. With that context, the lab asks that for router R2, you should use wildcard masks such that the *network* command matches all hosts in the subnet of an interface.
      To figure out the wildcard mask to use:
      Look at the interface config
      Find the IP subnet mask ( in this case)
      subtract octet-by-octet from
      That’s the wildcard mask, which, when combined with the subnet ID of the subnet off that same interface, will match the IP addresses in the subnet.
      Hope this helps,

      Reply to this comment
  3. Ducino March 28, 13:28

    Mr Wendell
    Can you explain me why R2 configure for g0/4 is network area 0, not for subnet ID?
    Thanks you

    Reply to this comment
    • Wendell Odom March 31, 16:05

      Sure. It mostly has to do with the instructions, which are artificial, just to give us some exercise.
      For R2, the request is: each network command matches the addresses in one subnet.
      For R2’s G0/4 interface, to match all addresses in the subnet, you need to use the subnet ID of that subnet in the network command, and a wildcard mask that defines a range from the subnet ID to the end of the subnet’s range of addresses. network does that. The interface IP address is defined by “ip address”, so a little subnet math gives us subnet ID Subtract subnet mask from to get the wildcard mask to use ( The range of addresses then begin with .128 and ends w/ .191, matching the addresses in the subnet.

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