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12. Renumbering Considerations

IPv6 networks may need to be renumbered for various reasons, such as changing Internet Service Providers (ISPs), corporate mergers, or network reorganizations. Neighbor Discovery includes features to facilitate smooth renumbering operations. This section discusses considerations for renumbering IPv6 networks.

12.1. Renumbering Overview​

Network renumbering involves changing the IPv6 address prefixes used by hosts on a network. The primary mechanisms for renumbering in IPv6 are:

  1. Router Advertisements with New Prefixes: Routers advertise new prefixes with appropriate lifetimes while gradually phasing out old prefixes.
  2. DHCPv6: Dynamic Host Configuration Protocol for IPv6 can also manage address assignment during renumbering.
  3. Stateless Address Autoconfiguration (SLAAC): Hosts automatically generate new addresses based on new prefixes advertised by routers.

12.2. Router Advertisement-Based Renumbering​

Neighbor Discovery's Router Advertisement mechanism is designed to support graceful renumbering:

12.2.1. Introducing New Prefixes​

When introducing a new prefix:

  1. Initial Advertisement: Routers begin advertising the new prefix with:

    • Valid Lifetime: Set to a reasonable duration (e.g., 2 hours or more)
    • Preferred Lifetime: Initially set equal to Valid Lifetime
    • A (Autonomous) flag: Set to indicate hosts should use SLAAC

  2. Coexistence Period: Both old and new prefixes are advertised simultaneously, allowing:

    • New connections to use new addresses
    • Existing connections to continue using old addresses
    • Gradual migration without service interruption

12.2.2. Deprecating Old Prefixes​

To phase out an old prefix:

  1. Set Preferred Lifetime to Zero: This causes hosts to:

    • Stop using the old prefix for new connections
    • Continue using existing connections with old addresses
    • Not generate new addresses from the deprecated prefix

  2. Reduce Valid Lifetime: Gradually decrease the Valid Lifetime to allow existing connections to complete.

  3. Final Removal: Once Valid Lifetime expires or is set to zero, hosts remove the addresses and prefixes from their configuration.

12.2.3. Timeline Recommendations​

A typical renumbering timeline might be:

  • Week 0: Begin advertising new prefix alongside old prefix (both preferred)
  • Week 1-2: Deprecate old prefix (Preferred Lifetime = 0, Valid Lifetime = 1 week)
  • Week 2-3: Continue reducing Valid Lifetime of old prefix
  • Week 3+: Stop advertising old prefix entirely

The exact timeline depends on network requirements and the nature of applications and connections.

12.3. Implementation Considerations​

12.3.1. Host Behavior​

Hosts SHOULD (SHOULD):

  • Monitor Router Advertisements: Continuously process Router Advertisements to learn about new prefixes and lifetime changes.
  • Respect Lifetimes: Honor Preferred and Valid Lifetimes when selecting source addresses for new connections.
  • Deprecate Gracefully: When a prefix becomes deprecated (Preferred Lifetime = 0), avoid using it for new connections but maintain existing connections.
  • Remove Expired Addresses: Delete addresses when their Valid Lifetime expires.

12.3.2. Router Behavior​

Routers SHOULD (SHOULD):

  • Coordinate Renumbering: All routers on a link should advertise consistent prefix information.
  • Use Conservative Timers: Allow sufficient time for renumbering to complete without disrupting services.
  • Monitor Progress: Network administrators should monitor renumbering progress to ensure smooth transition.

12.3.3. Application Considerations​

Applications should be designed to:

  • Handle Address Changes: Be prepared for addresses to change during the lifetime of long-running applications.
  • Prefer Temporary Addresses: For privacy-sensitive applications, use temporary addresses [RFC4941] that change periodically.
  • Implement Happy Eyeballs: Support both old and new addresses during transition periods [RFC8305].

12.4. DHCPv6 and Renumbering​

When using DHCPv6 for address assignment:

  • Lease Times: DHCPv6 servers can control renumbering by adjusting lease times for old and new addresses.
  • RECONFIGURE Messages: Servers can use RECONFIGURE messages to instruct clients to renew their configurations and obtain new addresses.
  • Coordination with RA: DHCPv6-based renumbering should be coordinated with Router Advertisements to ensure consistency.

12.5. DNS Considerations​

During renumbering, DNS records must be updated:

  1. Add New Records: Create AAAA records for new addresses.
  2. Reduce TTL: Lower the Time-To-Live (TTL) for DNS records before renumbering to minimize caching issues.
  3. Maintain Old Records: Keep old AAAA records available during the transition period.
  4. Remove Old Records: After renumbering is complete and old addresses are no longer in use, remove obsolete DNS records.

Dynamic DNS (DDNS) can automate this process for hosts that support it.

12.6. Operational Recommendations​

Network operators should:

  1. Plan Carefully: Develop a detailed renumbering plan with timelines and rollback procedures.
  2. Communicate: Inform users and stakeholders about the renumbering schedule.
  3. Monitor: Use network monitoring tools to track renumbering progress and identify issues.
  4. Test: If possible, test renumbering procedures in a lab environment before deploying to production.
  5. Document: Maintain documentation of prefix assignments and renumbering history.

12.7. Multi-Homing and Renumbering​

Multi-homed networks (connected to multiple ISPs) face additional challenges:

  • Multiple Prefixes: Hosts may have addresses from multiple prefixes simultaneously.
  • Source Address Selection: Proper source address selection (RFC 6724) is critical for choosing the right address for outbound connections.
  • Provider Failover: When one provider's prefix becomes unavailable, hosts should seamlessly transition to using addresses from other prefixes.

12.8. Enterprise Considerations​

For enterprise networks:

  • Internal Numbering: Internal address assignments may need to change less frequently than edge prefixes.
  • ULA Usage: Unique Local Addresses (ULA, RFC 4193) can provide stable internal addressing independent of ISP changes.
  • NAT66: While generally discouraged, some enterprises may use NAT66 to isolate internal renumbering from external changes, though this has significant drawbacks.

12.9. Automation and Tools​

To facilitate renumbering:

  • Configuration Management: Use automated configuration management tools to update router configurations.
  • Monitoring Systems: Implement monitoring to detect renumbering-related issues.
  • Scripts and APIs: Develop scripts or use APIs to automate DNS updates and other renumbering tasks.

12.10. Summary​

Neighbor Discovery's Router Advertisement mechanism provides robust support for IPv6 network renumbering. By carefully managing prefix lifetimes and coordinating changes across routers, DHCPv6 servers, and DNS, network operators can achieve smooth transitions with minimal service disruption. Proper planning, communication, and monitoring are essential for successful renumbering operations.

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