Does IPv6 Support Multihomed Sites

It is often stated that multihoming of sites is an IPv6 problem. Multihoming is not a protocol problem. In the case of IPv6, the challenges are due to a set of prefix allocation policies enforced by the RIRs.

Multihoming is widely used by enterprises for the following reasons:

• Connect sites of a network with global reach: Organizations with multinational infrastructures will connect to multiple service providers in different countries.

• Backup for the link to the SP: An enterprise can have several links into the same provider that protect each other in the event of a failure.

• Backup SP: An enterprise can connect to several SPs in order to protect against SP failure.

Multihoming is a problem for IP in general and not for IPv6 alone. IPv4 faces the same issues with multihoming as IPv6. Current multihoming techniques impact the size of the Internet routing table. In February 2008, there were more than 250,000 entries in the IPv4 backbone BGP routing table.14 The root cause of the problem is a lack of a good framework for prefix aggregation. IPv6 routing is based on the same protocols as IPv4, so all multihoming mechanisms available in IPv4 can be used in IPv6. The size of the IPv6 prefixes—which, within the Internet routing tables, is driven through prefix allocation policies—facilitates better address management and good aggregation.

Figure 2-4 is a summary of the IPv6 prefix allocation policies. The address space is managed by IANA, which allocates prefixes to the RIRs, which in turn allocate prefixes to ISPs on the provider dependent track or directly to organizations (enterprises, educational institutions, and so forth) on the Provider Independent track.

Top Down Profiling
Figure 2-4 IPv6 Address Allocation Policies


A 2006 analysis of the IPv6 global routing tables, "Have We Reached 1000 Prefixes Yet? A Snapshot of the Global IPv6 Routing Table," presents the effectiveness of the policy approach at that stage in the deployment of IPv6.15 Geoff Huston's well-respected BGP Update site tracks and analyzes historic IPv4 and IPv6 BGP routing information, a valuable resource for up-to-date information.

These policies enforced by Registries preempt the use of multihoming as done in IPv4. In the absence of a multihoming mechanism that would work in the context of IPv6, enterprises are faced with significant operational challenges when integrating IPv6. Whenever an enterprise is dissatisfied with its provider and wants to switch to another one, it would have to renumber its network; and this is an expensive proposition. The provider-dependent allocation policies are not acceptable to enterprises.

To avoid a slowdown in IPv6 adoption due to these concerns, new policies were adopted by the RIRs and they provision for Provider Independent (PI) address space,16 which could be acquired directly from the RIR. These policies will help keep the IPv6 deployment momentum, but they do not solve the real problems of backbone routing table growth and organizations multihomed to several service providers. With a significantly larger address space, IPv6 can make the routing table problem considerably worse than it is in IPv4. The importance of this topic in the networking community mind is reflected in the support provided by IETF to research in this area. The list of suggestions and initiatives to solve the multihoming challenges was reported at the 53rd RIPE meeting and are

• CIDR boundary: The community decides on the longer prefix boundary that can be handled on the Internet.

• Metro/regional: IP address space is assigned to regions instead of organizations.

• Community codes: Prefixes are tagged with a BGP community attribute.

• Published list of IPv6 blocks: A list of prefixes approved for multihoming will be published, and filters will be opened for them.



• Policy: RIRs would implement policies that offer provider-independent address space. As of early 2008, all RIRs adopted a PI address space policy with the exception of RIPE ( 2005_1_orig.html,,,

• IETF Multi6 WG: This is the IETF working group that works on IPv6 multihoming solutions (

• IETF Shim6 WG: A shim layer that enables the decoupling between the IP address could be used by the application and used by transport (http:/ /

• Global, Site, End-system (GSE): Protocols that separate the user identifier from its locator.

• Maximum prefix: Each origin AS can advertise a limited number of prefixes.

Answer: The IPv6 protocol itself provides the same level of support for multihoming as IPv4 supports. Perceived challenges are just a reflection of address allocation policies implemented to enforce aggregation of prefixes in the Internet backbone routing table. IPv6 can leverage the same multihoming techniques as IPv4, and alternative mechanisms are being investigated in IETF.

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