Answers: Special IPv6 Addresses 2

By certskills August 24, 2016 09:10

Ready to play detective? Go back and check out the lab post, and take your best shot at predicting the configuration. Then come back here and check your thoughts versus the answer. The theme: interpreting the unicast and multicast addresses listed by the show ipv6 interface command.


Figure 1: Three Router Topology


Analysis that Lead to the Answers

This lab takes a much different approach than the other Config Lab posts. How did you do?

The show ipv6 interface commands on routers R1, R2, and R3 showed you some addresses. However, it did not list some addresses you may be used to seeing: global unicast addresses. In this case, the initial configurations did not include any ipv6 address interface subcommands to configure any global unicast addresses. Basically, the lab state for this lab post is that IPv6 is enabled, with OSPF enabled as well, but without the global unicast addresses configured.

So, what configuration appears to have already been configured?

First, each router interface (or in the case of router R1, a couple of subinterfaces as well) had an ipv6 enable command configured. How can you determine this? Well, the output on each interface states “no global unicast address is configured”, which means that there is no ipv6 address command configured on the interface. At the same time, IPv6 is clearly enabled on the interface per many parts of the command output.

The ipv6 enable command:

  • Enables IPv6 on the interface
  • Causes the router to calculate its link local (unicast) address on that interface
  • Causes the router to calculate the solicited node multicast address associated with that link local address

Because of an interface’s ipv6 enable command, you should see each interface with a link local address (begins with FE80). You should also see a matching solicited node multicast address, which begins FF02::1:FF, followed by the same last 6 hex digits as the link local address.

That ipv6 enable command also enables the use of two multicast addresses, again found on each interface/subinterface in examples 2 through 7 in the lab post:

  • FF02::1 – All nodes
  • FF02::2 – All routers

Note that if the configuration had happened to have included an ipv6 address command to configure a global unicast address, that command would also have triggered the addition of these two multicast addresses. However, the output states “no global unicast address is configured”, which tells us that the ipv6 address command was not configured.

Finally, each interface also has OSPFv3 enabled, although the output does not tell you enough details about the specifics of the commands used by OSPF. In this case, the interface subcommand happened to be ipv6 ospf 10 area 0.  As a result, each interface also showed the router listening for two additional IPv6 multicast addresses to support OSPF:

  • FF02::5 – All OSPF routers
  • FF02::6 – All OSPF DRs and BDRs

The following section shows the relevant IPv6 configuration that existed at the beginning of this lab.


The IPv6 Configuration that Existed at the Beginning of the Lab

Examples 1, 2, and 3 show the additional IPv6 configuration that existed in the configuration at the beginning of this lab.

Example 1: R1 Config


Example 2: R2 Config


Example 3: R3 Config


SNMPv3 1
Answers: SNMPv3 1
By certskills August 24, 2016 09:10
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  1. Papus December 20, 09:45

    Thank you for this exercise, very instructive! I have noticed though that the global configuration ipv6 router OSPF x was escaped. I believe the loopback setting has been used for same purpose. Hope I clearly understand that…

    Reply to this comment
    • CCENTSkills January 18, 10:27

      Hi Papus,
      You’re welcome!
      I’m not quite understanding your comment. I’m not understanding what you mean by “escaped” in this context. Sure it’s my problem! Feel free to follow up and spell it out for me if you like.

      Reply to this comment
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