Answers

Figure 1: Global IPv6 Addressing Topology

interface GigabitEthernet0/1
 mac-address 2c25.dbef.a3df
 ipv6 address 3000:23DA:FEDA:BCA1:9374:CBAE:EBDA:9182/64
!
interface GigabitEthernet0/2
 mac-address aa3d.feea.1353
 ipv6 address 2500:B:AE:83D:B91:A024:BCA2:8374/64

Example 1: R1 Config

interface GigabitEthernet0/1
 mac-address 9eab.28df.ab24
 ipv6 address 3000:23DA:FEDA:BCA1:BA71:FDEA:9817:BC71/64
!
interface GigabitEthernet0/2
 mac-address bc81.ba82.8362
 ipv6 address 2500:BA71:8263:BA71:BD87:1786:0:9283/64

Example 2: R2 Config

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 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 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 7^th bit.

Table 1 – Link Local Addresses to Use, Based on Configuring MAC Addresses

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.

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. Connect to the CLI of each router and verifying the address configured and calculated by the device match Table 1; this can be done with the show ipv6 interface or show ipv6 interface brief commands. Pay close attention to the link-local addresses in this lab.