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Difficulty: HardIPv6 Addressing and Prefixes

A network engineer executes the following configuration commands on a Cisco router interface with a MAC address of 54e1.ad12.345654\text{e}1.\text{ad}12.3456:

text
interface GigabitEthernet0/0/0
ipv6 address 2001:db8:abc:10::/64 eui-64
ipv6 address fe80::1 link-local

Which two statements accurately describe the resulting IPv6 addressing and prefix properties for this interface? (Select two.)

  1. The dynamically generated Global Unicast Address on the interface is 2001:db8:abc:10:56e1:adff:fe12:3456/642001:\text{db8}:\text{abc}:10:56\text{e}1:\text{adff}:\text{fe}12:3456/64.Answer
  2. The explicitly configured link-local address fe80::1\text{fe80}::1 replaces the auto-generated EUI-64 link-local address while staying within the reserved fe80::/10\text{fe80}::/10 scope.Answer
  3. C
    The automatically generated Global Unicast Address on the interface is 2001:db8:abc:10:54e1:adff:fe12:3456/642001:\text{db8}:\text{abc}:10:54\text{e}1:\text{adff}:\text{fe}12:3456/64 because EUI-64 preserves the original MAC address byte values.
  4. D
    The interface prefix length of /64/64 allows for 264−22^{64} - 2 usable host addresses after deducting reserved subnet network and broadcast identifiers.

Answer

The correct statements are that the dynamically generated Global Unicast Address is 2001:db8:abc:10:56e1:adff:fe12:3456/64, and the explicitly configured link-local address fe80::1 replaces the auto-generated EUI-64 link-local address within the fe80::/10 scope.
The EUI-64 process takes the 48-bit MAC address 54e1.ad12.3456, inserts FFFE in the middle to make it 64 bits, and flips the 7th bit of the first byte from 54 hex to 56 hex, yielding the interface ID 56e1:adff:fe12:3456. Combining this with the configured prefix 2001:db8:abc:10::/64 produces 2001:db8:abc:10:56e1:adff:fe12:3456/64. Additionally, manual link-local configuration overrides the default EUI-64 link-local address and uses the fe80::/10 scope.

Step-by-Step Solution

1
Calculate the EUI-64 Interface Identifier from the MAC address
Interface ID is 56e1:adff:fe12:3456
Split MAC address 54e1.ad12.3456 into two 24-bit halves (54-E1-AD and 12-34-56). Insert FFFE in the center (54E1:ADFF:FE12:3456). Invert the 7th bit of the first byte: 54 in hex is 01010100 in binary; flipping bit 7 yields 01010110 binary, which is 56 in hex.
2
Combine prefix with EUI-64 Interface Identifier
Global Unicast Address is 2001:db8:abc:10:56e1:adff:fe12:3456/64
The prefix specified is 2001:db8:abc:10::/64. Appending the 64-bit interface identifier derived in Step 1 forms the full address.
3
Evaluate link-local address configuration and reserved scope
fe80::1 is assigned as link-local within fe80::/10
Explicit link-local configuration replaces the automatically derived EUI-64 link-local address. The prefix fe80::/10 covers fe80:: through febf::, so fe80::1 is valid.

Key Concept

EUI-64 Interface ID Generation and IPv6 Address Scopes
Estimated Time:2m 0s
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Topics

IPv6 Addressing and Prefixes

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