Config Museum: OSPFv2 Single Area Answers

By certskills August 22, 2014 09:05

As with all these #CCNA config museum pieces, the goal is to let you select some practice exercises to get in more reps and make configuration automatic.  Today’s post shows the answers; useful links below, with the answers below the fold!




The instructions give you some flexibility with the OSPF network commands, with the restriction that none of the commands match IP addresses outside the existing subnets. In other words, your network commands cannot match any IPv4 address that might be configured on new interfaces in the future. To do that, you could use a wildcard mask of on all network commands, matching only the specific interface IP address, or a mask that is the inverse of the subnet mask, matching all IPv4 addresses in the subnet. I’ll show a configuration that uses on R1 and R2, with the inverse masks on R3.

Next, the exercise asks you to set the OSPF router ID (RID), but to not use the OSPF router-id command. The only way to do that is to configure the IP address that is to be used as the RID on some interface. Loopback interfaces make the most sense for this purpose; the config shows interface loopback 1 on all three routers. You can use any subnet mask,but I used for the loopback interface on all three routers, just for consistency.

Figure 1 repeats the initial design, and table 1 lists a reminder of the initial IP address configuration.

Figure 1: Network Design

Table 1: IPv4 Addresses

Location IP/mask
R1 G0/0 /29
R1 G0/1 /28
R1 G0/4 23
R2 G0/0 /29
R2 G0/1 /28
R2 G0/4 /23
R3 G0/0 /28
R3 G0/1 /28
R3 G0/4 /28



Examples 1, 2, and 3 show the answers. Note that some of the parameters in your answers may differ – feel free to follow up if you’re wondering whether your answer met all the requirements or not.

Example 1: R1 OSPFv2 Configuration


Example 2: R2 OSPFv2 Configuration

Example 3: R3 IPv4 Address Configuration



Config Museum: OSPFv2 Single-area
Using Cisco IOS in the All-in-One VM
By certskills August 22, 2014 09:05
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  1. Rick December 3, 12:00

    While studying the OSPF single area configuration, I tried to replicate Wendell’s Sample Network,using Packet Tracer, listed as Figure 17-10 in his ICND1 book (An excellent book, by the way).
    For the most part, I was successful, but also very frustrated!
    When checking my neighbors on R3, I do not get the same results as in Wendell’s example. Since R3 sees the subnets in the LAN as directly connected via the switches for VLAN 341 and 342, it does not establish neighbors, and puts the routes in the routing table as directly connected. I thought maybe Wendell performed some type of networking magic, by changing the AD of the connected route, and attempted this on my own, still with no success.
    I am not sure if this is a type of network one would encounter in the real world, but I always try to absorb anything Wendell has printed, as it seems important to know.
    Does anyone know how to configure this network, and cause neighbor relationships to develop between the VLANs between R3 and R4?

    Many Thanks,

    Reply to this comment
  2. Jose July 14, 09:19

    Was there a particular reason you used x.x.x.248 on the loopbacks. I personally used for my masks on the loopback interfaces.

    Everything is working on my end, so I assume there might be a common practice?

    Reply to this comment
    • certskills Author July 14, 09:44

      Hi Jose,
      It’s an old habit of avoiding a /32 in case I needed to show in lab or in a book how OSPF still advertises the route as a host route. In a real network, I’d use a /32.

      Reply to this comment
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