In this lab, you have to configure global unicast addresses, which does not require a lot of thinking. The harder part is the contrived part that lets you review some modified EUI-64 math, making you manipulate hexadecimal values to determine how to configure the link local addresses. This serves as good exercise to make sure you understand how to configure IPv6 addresses. Check out your answers here; the lab requirements are at this post.

## Commentary

First, configuring the IPv6 global unicast addresses, without using EUI-64, requires little thought. Just configure the ipv6 address address/prefix-length interface subcommand, with the entire address configured. Note that the two examples show the same IPv6 addresses originally listed in Table 1 of the original lab exercise post.

When you configure those global unicast addresses, the router also calculates and creates an associated link local address, using modified EUI-64 rules. The EUI-64 process takes the MAC address (12 hex digits), inserts FFFE in the middle, and inverts the 7th bit. For example, if the MAC address 1234.5678.1234 was used, this first split would result in 1234.56 and 78.1234. Then the 16 byte string ‘FF FE’ is placed between them which results in 1234.56FF.FE78.1234. Finally, the 7th bit counting from the left, started at 1, is inverted (if it was 1 it would be come 0 and vise versa); this results in 1034.56FF.FE78.1234.

Table 1 of this lab re-lists some information from Table 2 in the original lab post. It shows the router, interface, and the last 16 hex digits (unabbreviated) of the desired link local address. In that form, those last 16 hex digits make it obvious where the FFFE sits. All you have to do to figure out what MAC address to use is to remove the FFFE, and then invert that 7th bit.

 Rx Int. Unabbreviated 2nd Half of Link Local Resulting MAC Address R1 G0/1 2E25:DBFF:FEEF:A3DF 2C25:DBEF:A3DF R1 G0/2 A83D:FEFF:FEEA:1353 AA3D:FEEA:1353 R2 G0/1 9CAB:28FF:FEDF:AB24 9EAB:28DF:AB24 R2 G0/2 BE81:BAFF:FE82:8362 BC81:BA82:8362

For this lab there were 4 different global unicast IPv6 addresses given. You should see those addresses configured in the two answer examples. Additionally, by configuring the MAC addresses per Table 3, the router should calculate the link local addresses as requested in the original lab.

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1. Em3xus October 2, 20:30

and you can use “show ipv6 interface brief” to verify the link local address has created successfully based on the MAC addresses used ðŸ™‚

2. sanchez October 15, 09:08

Dear wendell

I had a question and curious about UCS and it’s configuration , and what levels is it ? ccna , ccnp or ccie ??

3. Rui Gomes May 25, 12:14

Is anyone with problems with this exercise on packet tracer? I realise that if the 7th bit is a 1, the created link-local address won’t invert it, like it does when the 7th bit is a 0.

I don’t know why that would be the case. The bit is inverted regardless.

• CCENTSkills June 6, 09:46

Rui,
I agree with Anezhad – the 7th bit is inverted regardless of its original value.

• Chris B July 26, 16:01

Packet Tracer seems to indeed have this issue, and after searching around the Internet I have found various responses to this. Some people responded that you only flip the 7th bit when it’s a 0 (which isn’t a flip but actually always setting it to 1). You can search and find long RFC-based discussion on the reasons for the 7th bit flip, the history of it, the practical reasons for doing it, and the arguments about what actually happens. Long story short, flip the bit and ignore Packet Tracer for the test!

4. rob July 20, 18:33

How did the resulting unicast address become 1034.56FF.FE78.1234 from the address 1234.56FF.FE78.1234?

5. David February 16, 13:44

Hi Wendell,

Where is 2c25.dbef.a3df coming from?

Thank you

• CCENTSkills March 7, 09:42

Hi David,
Sorry for the delay in replying.
That MAC address (and the others in this exercise) are there for convenience – for you and for me. Here’s the longer story.
In real life, all interfaces that use MACs have a MAC assigned to it by the device. However, for the purposes of the lab, I needed to let you know the value of the MAC address. I could have put a comment line there to tell you the interface’s MAC address. Or I could have given show command output to list the MAC address. However, note that there’s no config command that configures the pre-defined MAC address of the interface.

For convenience, I just overrode the interface’s assigned (aka burned in) MAC address by configuring it to use a different MAC address value. I just chose different legal MAC addresses for variety, so that the math practice didn’t use a similar MAC address each time. And you can then do the same in your own lab, changing interface MAC addresses to suit your needs.
Hope this helps…
Wendell

6. winsen December 12, 00:39

Hi Wendell,

Is each router and switch interface has its own mac-address? I thought one device only can have one mac address

Thank you, Winsen

• CCENTSkills January 18, 10:18

Winsen,
Sorry for the delay. You may not even remember what you asked, but…
I’m a little unsure what in this post you’re looking at when asking this question, so I’m probably not understanding the context of what you’re asking. But your questions, without context:
1) Does each device interface have its own (unique) MAC address? Yes.
2) Not sure of your final statement, but by design, MACs should be unique, at least within a subnet. If using only the MACs that manufacturers assign to devices/interfaces, then the MACs should be unique in the universe.

But it is possible to assign different MAC addresses to interfaces, and then make the mistake of duplicating MAC addresses – FYI, in case that’s what you’re asking.

7. stal February 4, 10:15

Hi Wendell,

How was it determined that the ipv6 address is a /64?

• CCENTSkills February 28, 15:11

Hey Stal,
Well, literally, Table 1 in the lab/question post mentions using /64 (in the caption to the table). More generally, the vast majority of IPv6 implementations use /64 prefix lengths.

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