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Difficulty: MediumIPv6 Address Types and EUI-64

An interface on a Cisco IOS router is dynamically generating its IPv6 address using the EUI-64 standard. Output from the command `show ipv6 interface GigabitEthernet0/0` reveals that the link-local address assigned to the interface is `fe80::1a2b:3cff:fe4d:5e6f`. Based on this address, what is the burned-in MAC address of the Ethernet interface?

  1. A
    1a2b.3c4d.5e6f
  2. 182b.3c4d.5e6fAnswer
  3. C
    102b.3c4d.5e6f
  4. D
    182b.3cff.fe4d

Answer

The MAC address of the interface is 182b.3c4d.5e6f.
To reverse the EUI-64 generation process and recover the original 48-bit MAC address from the interface ID `1a2b:3cff:fe4d:5e6f`: first, remove the inserted `FFFE` bytes from the midpoint to get `1a2b:3c4d:5e6f`. Next, convert the first byte (`1A` hex) to binary (`0001 1010`) and flip the 7th bit (the Universal/Local bit), resulting in `0001 1000` (`18` hex). Assembling the modified octets gives the hardware MAC address `182b.3c4d.5e6f`.

Step-by-Step Solution

1
Extract the 64-bit Interface Identifier from the IPv6 address.
The Interface ID portion of `fe80::1a2b:3cff:fe4d:5e6f` is `1a2b:3cff:fe4d:5e6f`.
In IPv6 link-local addresses, the last 64 bits represent the Interface Identifier.
2
Remove the inserted 16-bit hex sequence `ff:fe` (`FFFE`) from the middle of the Interface ID.
Removing `ff:fe` from `1a:2b:3c:ff:fe:4d:5e:6f` yields 6 bytes: `1a:2b:3c:4d:5e:6f`.
The IEEE EUI-64 process inserts `FFFE` into the middle of a 48-bit MAC address to expand it to 64 bits.
3
Invert the 7th bit (Universal/Local bit) of the first byte.
The first byte `1A` in binary is `0001 1010`. Inverting the 7th bit (2nd bit from the left) changes it to `0001 1000`, which is `18` in hexadecimal.
EUI-64 inverts the Universal/Local bit (7th bit of the first byte) when creating an interface identifier from a MAC address.
4
Format the resulting 48 bits into standard Cisco MAC address notation.
Combining `18`, `2b`, `3c`, `4d`, `5e`, and `6f` produces `182b.3c4d.5e6f`.
Cisco CLI format uses three sets of four hexadecimal digits separated by dots.

Key Concept

Reversing the EUI-64 interface identifier process to determine the burned-in MAC address
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Topics

IPv6 Address Types and EUI-64

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