OSPFv2 Enabler #3 – Answers

Wendell
By Wendell March 6, 2017 12:51

This latest OSPF Enabler exercise asked you to configure four routers in a three-area design. As usual, the exercise asks for different styles of OSPFv2 network commands, for no other reason than to give you a variety of exercise. This post lists the answers – take a look.

The Topology

It is probably best to have the exercise post in another tab/window while looking at this post, but if not, here’s a copy of the topology figure for this exercise.

OSPF Enabler OSPF Topology

 

The Answers

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

Answers: Router R1

 

Answers: Router R2

 

Answers: Router R3

 

Answers: Router R4

 

Comments and Explanations – R1

R1 requires more thought than for routers R2 and R3. 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 a network command per interface, or in this case, five commands. Each matches the addresses in the subnet off each of the three interfaces on R1.

Note that R1 is an area border router (ABR), with interfaces in three different areas. So, you would need at least three different network commands regardless of the rules for this exercise. So, for this exercise, I asked you to match addresses by subnet, so that you would need one network command per interface.

 

Comments and Explanations – R2

For R2, 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 255.0.0.0 – Inverts to wildcard mask 0.255.255.255

Default mask 255.255.0.0 – Inverts to wildcard mask 0.0.255.255

Default mask 255.255.255.0 – Inverts to wildcard mask 0.0.0.255

For OSPF areas, in this case, all interfaces were in area 123, so both the network commands of course end with the area 123 keywords.

 

Comments and Explanations – R3

R3 requires the least thought of the three requirements. To match a single IP address, use wildcard mask of 0.0.0.0. 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.

 

Comments and Explanations – R4

R4, like R1, requires more thought. Following the same process as with R1, you end up with two network commands, one per interface, with each matching the range of IP addresses in the subnets connected to those interfaces.

The only possibly tricky part with R4 is due to the subnet mask of 255.255.240.0 on interface g0/2. The inverse of that subnet mask is 0.0.15.255, which looks a little counter-intuitive, but is correct. Additionally, the interface IP address of 172.17.20.4 with mask 255.255.240.0 is in subnet 172.17.16.0, which again looks a little strange at first glance. So, the network 172.17.16.0 0.0.15.255 area 14 command requires that you be pretty comfortable with your math.

 

OSPFv2 Enabler #3
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Wendell
By Wendell March 6, 2017 12:51
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