Answers: Multi-area OSPF 1

certskills
By certskills April 20, 2016 09:05

This latest lab sets up the requirements for OSPFv2 configuration in a multi-area design. It also shows yet another case of the fact that you must master IPv4 subnetting to be ready for more advanced labs. As always, go try the lab first. Then you can check whether you interpreted the lab requirements like I did by comparing your answers. And if you did this lab mentally or on paper, without doing it in VIRL or on lab gear, make sure to pay close attention to your matching logic in your OSPF network commands.

 

Answers

Figure 1: Multi-area OSPF Topology

 

Example 1: Core1 Config

 

Example 2: Core2 Config

 

Example 3: Branch1 Config

 

Example 4: Branch2 Config

 

Commentary

For this lab you are tasked with configuring the four different routers with OSPF and placing each of the interfaces into their respective areas by using network commands. If you have mastered how to configure OSPF network commands, the lab is relatively basic, and all you have to do is work through the details. If you are still unsure about the OSPF network command, this lab gives you several chances to practice. When creating your configuration, keep these key points in mind:

  • The OSPF network command’s wildcard mask works like an ACL wildcard mask
  • If more than one network command exists, the OSPF process places an interface into a particular OSPF area based on the first network command to match the interface

So, a multi-area OSPF configuration is not that much more complex than a single area configuration, but it may cause more problems if you get the configuration incorrect.

For this review, begin with router Core1 (found in Example 1). Per the requirements, Core1 should use process ID 10 with the router ID 1.1.1.1 explicitly configured, so the config begins with the router ospf 10 command followed by the router-id 1.1.1.1 command. Core1 then needs two network statements, because the requirements ask for wildcard masks that cause each network command to match all the addresses in one subnet. In the case of Core1, it connects to two subnets, with this result:

  • Core1 G0/1, uses subnet 100.100.100.0 with mask 255.255.255.252, so the OSPF configuration should use network 100.100.100.0 0.0.0.3 area 0
  • Core1 G0/2 uses subnet 100.100.100.128 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.128 0.0.0.63 area 10.

Note that Core1’s two network commands place those interfaces in two different areas, making Core1’s configuration be multi-area.

Core2’s configuration follows the exact same concepts, but with different details. In particular, for the network commands:

  • Core2 G0/1, uses subnet 100.100.100.0 with mask 255.255.255.252, so the OSPF configuration should use network 100.100.100.0 0.0.0.3 area 0
  • Core2 G0/2 uses subnet 100.100.100.192 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.192 0.0.0.63 area 20.

Like Core1, Core2 also serves as an ABR because it connects to two different areas.

For the two branch routers, both have two interfaces, but each router is an internal router: Branch1 internal to area 10, and Branch2 internal to area 20. Working through the logic for the various network commands on routers Branch1 and Branch2:

  • Branch1 G0/1 uses subnet 100.100.100.128 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.128 0.0.0.63 area 10
  • Branch1 G0/2 uses subnet 100.100.101.0 with mask 255.255.255.128, so the OSPF configuration should use network 100.100.101.0 0.0.0.127 area 10
  • Branch2 G0/1 uses subnet 100.100.100.192 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.192 0.0.0.63 area 20
  • Branch2 G0/2 uses subnet 100.100.101.128 with mask 255.255.255.128, so the OSPF configuration should use network 100.100.101.128 0.0.0.127 area 10.

 

Remote DHCP Server 1
Answers: Remote DHCP Server 1
certskills
By certskills April 20, 2016 09:05
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4 Comments

  1. Erjol September 7, 04:59

    Hi, I think the second network command statement for core2 should be in area 20.

    thanks for all your exercises.

    Reply to this comment
  2. Eugene January 17, 07:50

    Hello, I think this statement for Branch2: G0/2 network 100.100.101.128 0.0.0.127 area 10 (last line) should be in area 20.
    Thanks for all your labs.

    Reply to this comment
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