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Difficulty: MediumIPv6 Addressing and Prefixes

A network engineer configures a Cisco router interface with the command `ipv6 address 2001:db8:cafe:40::/64 eui-64`. The physical MAC address of the interface is `7081.05e1.a2b3`. Which two statements accurately describe the resulting IPv6 addressing state on this interface? (Select two.)

  1. The dynamically generated Global Unicast Address on the interface is 2001:db8:cafe:40:7281:5ff:fee1:a2b3.Answer
  2. The interface automatically derives a Link-Local address with the prefix fe80::/10 using the same modified EUI-64 interface identifier.Answer
  3. C
    The generated interface identifier is 7081:05ff:fee1:a2b3 because the 7th bit of the MAC address is maintained without inversion for global unicast traffic.
  4. D
    The available host count for this prefix is reduced by two to reserve the network address and broadcast address within the /64 subnet.

Answer

The dynamically generated Global Unicast Address is 2001:db8:cafe:40:7281:5ff:fee1:a2b3, and the interface automatically derives a Link-Local address with the prefix fe80::/10 using the same EUI-64 interface identifier.
The EUI-64 algorithm flips the 7th bit of the first MAC byte (0x70 to 0x72) and inserts FFFE into the middle, yielding the interface ID 7281:05ff:fee1:a2b3. Adding this to the prefix 2001:db8:cafe:40::/64 gives 2001:db8:cafe:40:7281:5ff:fee1:a2b3. Additionally, any IPv6-enabled interface automatically configures a Link-Local address under the fe80::/10 prefix using this same interface ID.

Step-by-Step Solution

1
Convert the MAC address to EUI-64 interface identifier form
MAC 7081.05e1.a2b3 splits into 70-81-05 and e1-a2-b3. Inverting the 7th bit of 0x70 (0011 0000 -> 0011 0010 = 0x72) yields 7281:05. Inserting FFFE between the halves yields 7281:05ff:fee1:a2b3.
EUI-64 requires flipping the U/L bit (7th bit) and placing FFFE in the middle of the 48-bit MAC address to form a 64-bit interface ID.
2
Combine the prefix with the EUI-64 interface identifier
Combining 2001:db8:cafe:40::/64 with 7281:05ff:fee1:a2b3 produces 2001:db8:cafe:40:7281:5ff:fee1:a2b3.
The prefix replaces the upper 64 bits and the EUI-64 identifier populates the lower 64 bits.
3
Evaluate link-local address generation
The router creates a link-local address within fe80::/10 using the interface ID 7281:5ff:fee1:a2b3.
IPv6 interfaces automatically assign a link-local address upon enabling IPv6 processing.

Key Concept

EUI-64 Interface Identifier Generation and IPv6 Address Scopes
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Topics

IPv6 Addressing and Prefixes

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