Answers: IPv6 Special Addresses 1

By certskills May 7, 2016 09:10

This puzzle starts with a partially-configured network, and your job is to figure out which parts of the IPv6 configuration has already been configured. Think of it as an exercise to make sure you really understand each config command, rather than just thinking of groups of commands. 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?

To begin, open another window to the lab post, just so you can see the show ipv6 interface command output more easily. Pick any example command, and you will notice:

  1. On the 2nd line, a Link Local address (begins with FE80)
  2. A few lines below, a heading “Global Unicast Addresses”
  3. On the next line, one global unicast address

Now look hard at the last four quartets of each interface’s global unicast address versus the link local address. For each interface, those values are the same. When the router is configured with a global unicast address, it creates a matching link local address, copying the second half (last four quartets) from the global unicast address.

Next, scan Examples 1 through 5 in the lab post for the lines that list the global unicast addresses. Some list a notation of “EUI-64”, and some do not. Then compare that output to the topology figure. It just so happens that the initial configuration for this lab has some ipv6 address commands that use the eui-64 keyword, and some do not. (As a reminder, this keyword tells the router to derive the second half of the IPv6 address, rather than it being configured in the ipv6 address command.)

So, some conclusions you can reach from this analysis are:

  • Each interface listed in the lab’s Examples 1 through 5 have an ipv6 address command that configures an address.
  • These interfaces configure a prefix, along with the eui-64 parameter:
    • R2 G0/1
    • R2 G0/2
    • R3 G0/1
  • These interfaces configure the entire address, and do not include an eui-64 parameter:
    • R1 G0/1
    • R3 G0/2

Frankly, the above analysis is enough to know what was configured. The rest of the analysis explains the rest of the output, and also explains how you can know that OSPF has not yet been enabled.

Each router interface in the lab lists several supported multicast addresses. An interface needs either the ipv6 address command configured or an ipv6 enable command configured to enable IPv6 on the interface. Once enabled, the interface supports the following multicast IPv6 addresses, which happen to be listed on every interface in the lab:

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

Next, each interface lists either one or two solicited node multicast addresses. These addresses begin with:

  • FF02::1:FF

The solicited node multicast address then ends with the same 6 hex digits as any unicast address on the interface (including link local addresses).

Note that IOS enables the FF02::1, FF02::2, and the appropriate solicited node multicast addresses automatically, without extra configuration, so there is no matching configuration to predict.

Finally, the lab stated that the intent was to use OSPFv3. However, OSPFv3 uses the following multicast addresses, which would be enabled on any interface on which OSPFv3 had been enabled:

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

Because these two multicast addresses are not listed on any of the interfaces shown in the lab, you can predict that OSPFv3 had not yet been configured.

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


IPv6 Special Addresses 1
IPv4 Static Routes 1
By certskills May 7, 2016 09:10
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  1. eldar June 10, 09:16

    It is very good lab in terms of predict the configuration of router…

    Reply to this comment
  2. Fred July 11, 01:04

    “Now look hard at the last four quartets of each interface’s global unicast address versus the link local address. For each interface, those values are the same.”

    Only for the ones configured with the eui-64 parameter right ?

    Reply to this comment
  3. Chris B July 25, 16:04

    One thing i encountered putting this into packet tracer was I realized you have to enable the interfaces beforehand, otherwise the router will leave the interface in a tentative state and not generate any of the multicast groups or the solicited node address. Thanks for the exercise, I just finished Chapter 30 ICND1 and this really helps!

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