Cisco: IPv6 basics and configuration how-to

Today, I had to deal with IPv6 configuration for my CCIE preparation and I said to write a small tutorial about IPv6 for it’s basic stuff. First of all IPv6 is not more complicated that IPv4, it just look like it is. Second, and this is my personal opinion, I think that as long as IPv4 will be on the market and things like NAT can overcome the problem of  IP addresses being exhausted, IPv6 will not be implemented on a large scale. Let’s be honest, I’m more OK in a discussion saying that I had a problem with the IP address than to say 2001:128:1F:633:207:85FF:FE80:71B8 (IPv6).

But what we like and what we have to know as network engineer, that’s a different story. For this reason I said that some information about IPv6 and how to configure it and test a point-to-point connection on a Cisco device, would not hurt anyone.

Some essential points about IPv6 (if you want more there is a lot of information on Internet):
-use of 128 bits compared with 32bits )IPv4
-IPv4 are represented in hexadecimal rather than decimal
-use colon-separated field of 16bits rather than 8 bits decimal points separator
The IPv6 can be write down in different formats. Let’s take:
this can look like:
2001:1:0:0:A1:CC0:1AB:397A – leading zero in each 16bits group can be eliminated
2001:0001::00A1:0CC0:01AB:397A – two or more 16bits “0000” groups can be write like :: (double colons) one time in the IPv6 address
2001:1::A1:CC0:1AB:397A-the simplest form to write the IPv6 address
A small, but very important hint. Take care of the following common mistake when you are writing IPv6 in short format. Let’s say that we have 2001:0000:0000:0000:00A1:0000:0000:397A and you want to write it in short format with the rule that 2 or more “0000” groups can be write like :: , and you write 2001::A1::397A which is wrong, because after this nobody or no device will know where it was 2  groups of zero and where 3. Taking the wrong format, you want to write the long format back and you can write it as: 2001:0000:0000:00A1:0000:0000:0000:397A, and obvious you can see the mistake.
There are more types of IPv6 address which you can find in the table below:
IPv6 addresses type

There are 3 ways in which you can assign an IPv6 to a Cisco device:
-manual =  just like in the IPv4
-stateful autoconfiguration = you use a DHCP server that will assign and IPv6 address to the interface
-stateless autoconfiguration = the interface ID in IPv6 is configured locally by the host to be globally unique using the EUI-64 procedure

In short terms the EUI-64 procedure is used to autoconfigure the last 64bits of the 128bits IPv6 address, which is the interface ID portion. EUI-64 method take the MAC address of the interface (48 bits) which is unique and derives the interface ID (64bits). You’ll ask probably how come 48bits become 64bits. EUI-64 insert in the middle of the MAC address a 16bits portion equal to FFFE and set the universal/local bit (7th bit) to indicate global scope. More clearly, let’s say that the interface has the MAC address 00:07:88:80:71:b9 and IPv6 apply the EUI-64 method this will become 0207:88FF:FE80:71b9 (last 64bits = interface ID).

After this short information (trust me it is short compared to detailed books) I hope that the things look a little bit more clear to you regarding IPv6 and you understood that this is not an IT network monster of any kind. Let me show you a small example of how you can configure the Cisco interfaces for IPv6 connectivity. Routing and more advanced stuff in a future post maybe.

For this tutorial I will use the same topology like in the previous post, please have a look to it here, but for the IPv6 configuration will only focus on the LAN side, the connection between R1 and R2. There is already a working IPv4 link between this 2 devices. Please click below to see the example:

IPv6 Cisco interface config

If for some reasons the tutorial above is not available for you, please check this text file which consist of the configuration needed to enable IPv6 configuration between 2 Cisco devices.

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Calin is a network engineer, with more than 20 years of experience in designing, installing, troubleshooting, and maintaining large enterprise WAN and LAN networks.

4 thoughts on “Cisco: IPv6 basics and configuration how-to”

  1. "let’s say that the interface has the MAC address 00:07:88:80:71:b9 and IPv6 apply the EUI-64 method this will become 0207:88FF:FE80:71b9 (last 64bits = interface ID)."

    unless I'm HIGHLY mistaken, the second address is incorrect. if should read: 0007:88FF:FE80:71b, or just ::7:88FF:FE80:71b

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