Answers

Figure 1: EUI-64 IPv6 Addressing Topology

interface GigabitEthernet0/1
 mac-address 0200.1234.5678
 ipv6 address 2000::/64 eui-64
!
interface GigabitEthernet0/2
 mac-address ac13.7294.6256
 ipv6 address 2001:BAE:274F:BAED::/64 eui-64

Example 5: R1 Config

interface GigabitEthernet0/1
 mac-address c081.abed.c724
 ipv6 address 2000::/64 eui-64
!
interface GigabitEthernet0/2
 mac-address 7824.fbce.a103
 ipv6 address 2001:BA82:B0AE:CAE::/64 eui-64

Example 6: R2 Config

interface GigabitEthernet0/1
 mac-address 0014.abc2.8abe
 ipv6 address 2000::/64 eui-64
!
interface GigabitEthernet0/2
 mac-address ac13.feda.beca
 ipv6 address 2001:BA81:8326:2753::/64 eui-64

Example 7: R3 Config

interface GigabitEthernet0/1
 mac-address aced.abec.fdac
 ipv6 address 2000::/64 eui-64
!
interface GigabitEthernet0/2
 mac-address 9876.1234.7654
 ipv6 address 2001:BEDE:AB81:9173::/64 eui-64

Example 8: R4 Config

Commentary

The EUI-64 process uses as input the MAC address of the interface. Cisco devices use a MAC address from a couple of sources: either the MAC address assigned to the interface by the manufacturer, or the address configured with the mac-address command. This lab uses a configured MAC address so that the address value is more obvious. Note that non-Ethernet interfaces like serial interfaces do not have MAC addresses, so when building an IPv6 address for a serial interface, IOS borrows the MAC address from lowest numbered Ethernet interface on the device.

The EUI-64 process takes the MAC address (12 hex digits), inserts FFFE in the middle, and inverts the 7^th 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; this 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 2 re-lists the IPv6 addresses you should see by the end of the lab, along with the prefix value. You should have configured the prefix in the ipv6 address prefix/prefix-length eui-64 command, not the entire IPv6 address from the table. With the eui-64 parameter, the router completes the last part of the IPv6 address.

Table 2 – Final IPv6 Address Assignments

Table 3 then shows some help in seeing the MAC address needed to make the router calculate the desired IPv6 address. The table lists the unabbreviated last 16 hex digits of the IPv6 address from table 1. 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 7^th bit. The right-most column lists that MAC address for each case, which you should see in the various mac-address address commands in the configuration examples.

Table 3 – Prefixes and MAC Addresses

For this lab, there were 8 different IPv6 addresses given, for each of these the prefix must be extracted. For the eight different ipv6 address ipv6-prefix/64 eui-64 interface subommands, they use the prefixes as listed in Table 3.

Additionally, once you calculate the MAC address that will result in the desired IPv6 address, you need to configure the mac-address address interface subcommand. For example, on R1’s G0/1 interface, the goal is to use EUI-64 to set the address to 2000::12FF:FE34:5678. The last 16 hex digits, unabbreviated, is 0000:12FF:FE34:5678. Remove the FFFE and you have 0000:1234:5678. Insert the 7^th bit and you have 0200:1234:5678. As a result, R2’s G0/1 interface lists the mac-address 0200.1234.5678 command.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

1. From the CLI of each router, verify the address calculated by the device when it uses EUI-64; this can be done with the show ipv6 interface or show ipv6 interface brief commands.