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Difficulty: MediumIPv6 Addressing and Deployment

A network administrator is deploying IPv6 across a new corporate branch office. To optimize IP allocation, the administrator subnets the organization's assigned prefix into /68/68 subnets and configures one of these /68/68 subnets on the gateway router's local interface. Although Router Advertisement (RA) messages are actively transmitted across the LAN, host workstations configured to use Stateless Address Autoconfiguration (SLAAC) fail to acquire global unicast addresses. Which of the following best explains why SLAAC is failing to automatically configure host addresses on this subnet?

  1. SLAAC mandates a prefix length of exactly /64 on the interface to generate 64-bit host interface identifiers.Answer
  2. B
    SLAAC requires a DHCP relay agent configured on the gateway router whenever subnet masks longer than /48 are used.
  3. C
    SLAAC cannot generate addresses until an AAAA DNS record is published on the network for each client device.
  4. D
    SLAAC is restricted to configuring link-local FE80::/10 addresses and cannot assign global unicast prefixes.

Answer

SLAAC mandates a prefix length of exactly /64 on the interface to generate 64-bit host interface identifiers.
Stateless Address Autoconfiguration (SLAAC), defined in RFC 4862, requires a /64 subnet prefix length. Since IPv6 host interface identifiers (generated via EUI-64 or privacy extensions) are 64 bits long, the host expects a 64-bit prefix from ICMPv6 Router Advertisement messages to complete the 128-bit IPv6 address structure. If a subnet prefix longer than /64 (such as /68) is configured on the gateway router, hosts will refrain from using SLAAC to autoconfigure global unicast addresses.

Step-by-Step Solution

1
Analyze the SLAAC specification standard (RFC 4862) regarding prefix requirements.
SLAAC combines a /64 network prefix received from Router Advertisements with a 64-bit interface identifier (EUI-64 or opaque random string) to form a 128-bit IPv6 address.
Because IPv6 host interface IDs are defined as 64 bits in standard SLAAC autoconfiguration, the network prefix must be exactly /64.
2
Evaluate the network configuration described in the scenario.
The router interface was configured with a /68 prefix length.
When a router advertises a prefix length other than /64 in ICMPv6 Router Advertisement Prefix Information Options, client SLAAC implementations ignore the prefix for autoconfiguration.
3
Select the option that correctly identifies the /64 prefix requirement constraint.
The requirement for an exact /64 prefix length explains the autoconfiguration failure.
Correcting the interface subnetting to /64 will allow client workstations to automatically build their global unicast IPv6 addresses.

Key Concept

SLAAC Prefix Length Requirement (/64)
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Topics

IPv6 Addressing and Deployment

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