IPv6CCNA 200-301

EUI-64 Address Calculated Incorrectly — Wrong IPv6 Address

Presenting Symptom

A host configured with IPv6 stateless address autoconfiguration (SLAAC) using EUI-64 cannot reach the default gateway or other hosts on the same subnet.

Network Context

Small branch office with a Cisco Catalyst 2960 switch and a Cisco ISR 4321 router running IOS XE 16.9. The router is configured as the IPv6 default gateway with SLAAC enabled. A single Windows 10 host is connected to the switch and obtains an IPv6 address via SLAAC. The host's EUI-64 derived address does not match the prefix advertised by the router, causing connectivity issues.

Diagnostic Steps

Verify IPv6 address on the host

ipconfig /all (Windows) or show ipv6 interface brief (Cisco device)

IPv6 Address. . . . . . . . . . : 2001:db8:acad:1:0200:27ff:fe00:1234(Preferred)

Note the IPv6 address. The EUI-64 portion should be derived from the MAC address. If the address looks incorrect (e.g., wrong prefix or interface ID), proceed to check the router's advertised prefix.

Check the router's IPv6 interface configuration and advertised prefix

show ipv6 interface gigabitethernet 0/0/0

IPv6 is enabled, link-local address is FE80::1
Global unicast address(es):
  2001:DB8:ACAD:1::1, subnet is 2001:DB8:ACAD:1::/64
Hosts use stateless autoconfig for addresses.

Verify the prefix being advertised. The prefix should be /64. If the prefix length is not /64, EUI-64 will not work correctly. Also check that the 'ipv6 nd prefix' command includes the correct prefix.

Examine the router's IPv6 neighbor cache for the host

show ipv6 neighbors

IPv6 Address                      Age Link-layer Addr State Interface
2001:DB8:ACAD:1:0200:27FF:FE00:1234  0  0050.7966.1234  REACH Gi0/0/0

The neighbor cache shows the host's IPv6 address and its MAC address. Compare the EUI-64 derived interface ID (last 64 bits) with the expected value from the MAC. The EUI-64 format inserts 'FF:FE' in the middle of the MAC and flips the U/L bit. If the address in the neighbor cache does not match the host's actual address, the host may have calculated it incorrectly.

Check the host's MAC address and manually compute expected EUI-64

On host: getmac /v (Windows) or ifconfig (Linux/Mac)

Physical Address: 00-50-79-66-12-34

Take the MAC address (e.g., 0050.7966.1234). Split into two halves: 005079 and 661234. Insert 'FF:FE' in the middle: 005079FFFE661234. Flip the seventh bit (U/L bit) of the first byte: 00 -> 02. Result: 0250.79FF.FE66.1234. The host's IPv6 address should be 2001:DB8:ACAD:1:0250:79FF:FE66:1234. If the host shows a different interface ID, the EUI-64 calculation is wrong.

Root Cause

The host's operating system (e.g., Windows 10) is using a privacy extension (RFC 4941) that generates temporary random interface identifiers instead of the EUI-64 based address. This results in a different IPv6 address than expected from the MAC address, causing confusion during troubleshooting. Alternatively, the host may have a manually configured IPv6 address that does not follow EUI-64.

Resolution

To disable privacy extensions on Windows 10 and force EUI-64 addressing: 1. Open an elevated Command Prompt. 2. Run: netsh interface ipv6 set privacy state=disabled 3. Run: netsh interface ipv6 set global randomizeidentifiers=disabled 4. Restart the network interface: netsh interface ipv6 reset Alternatively, on a Cisco device, ensure the interface is configured with 'ipv6 nd prefix default no-advertise' and then advertise a specific prefix with 'ipv6 nd prefix 2001:DB8:ACAD:1::/64 86400 43200'.

Verification

After disabling privacy extensions, run 'ipconfig /all' on the host. The IPv6 address should now show the EUI-64 derived address (e.g., 2001:db8:acad:1:0250:79ff:fe66:1234). On the router, run 'show ipv6 neighbors' and confirm the address matches the expected EUI-64 format.

Prevention

1. Standardize on a single method for IPv6 address assignment (SLAAC, DHCPv6, or static) across the network. 2. If using SLAAC, ensure all hosts have privacy extensions disabled if EUI-64 is required for troubleshooting. 3. Document the expected EUI-64 addresses for critical devices to simplify verification.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a multiple-choice question asking why a host's IPv6 address does not match the expected EUI-64 format. The exam tests understanding of EUI-64 calculation, the effect of privacy extensions, and the ability to identify incorrect address assignment. Key fact: EUI-64 inserts 'FF:FE' and flips the U/L bit.

Exam Tips

Memorize the EUI-64 process: split MAC, insert FFFE, flip the seventh bit of the first byte.

Know that Windows privacy extensions (random identifiers) override EUI-64 by default.

Be able to identify a non-EUI-64 address by the absence of 'FF:FE' in the interface ID.

Commands Used in This Scenario

show ipv6 neighbors

Displays the IPv6 neighbor discovery cache, showing the mapping of IPv6 addresses to MAC addresses on directly connected links, used to verify neighbor reachability and troubleshoot IPv6 connectivity.

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