IPv6 Multi-Homing with Provider Independent Addressing

The primary attraction to using provider independent space is that an organization is not tied to a specific provider. An organization that is using provider independent space can change providers without having to go through and renumber its entire network when the provider address space changes.

Provider independent (PI) space also allows an organization to connect to multiple service providers with a single IPv6 address block. These multiple connections provide resiliency and redundancy in case a particular service provider network has issues.The PI model also allows an organization to deploy and manage a single block of address space without worrying about potential source address selection issues. If an organization was multihomed and used the provider assigned (PA) model, it would have to manage two address blocks at a minimum.

The end systems would get assigned multiple addresses (for instance, one from each service provider (SP) block), which could lead to higher address management overhead and potential source address selection issues (for example, If SP A is having transit issues, how does an end system know to use SP B addresses?).

The primary question when considering PI addressing is what to do when an organization has presence across regional registries. For example, Company A has sites and a headquarters in North America, and sites and data centers in Europe and Asia. Does Company A get PI space from ARIN, RIPE, and APNIC? Or do they just get PI space from ARIN? This question is not easily answered. The main focus from an SP perspective has to do with routing table growth. With the expansion of the address space to 128 bits, there is a corresponding potential for the routing tables to grow considerably. This potential for growth in the routing tables, and resulting service provider maintenance, will push SPs to look more closely at how they accept prefixes from their customers and how they advertise prefixes to their peers.

Current SP practices do not have regional PI block filtering in place (for example, an SP in Europe only accepting PI announcements from its customers). This means that an organization that gets a PI assignment from ARIN should be able to split up that block and make announcements in Europe and Asia. The other potential issue with how service providers handle provider independent address blocks has to do with prefix lengths that are accepted and further propagated. Each regional registry has its own policy on initial block size assignment. The minimum prefix length that will be assigned is a /48 prefix block for provider independent space. While it might be perfectly acceptable to your service provider to accept that announcement, the downstream service providers that peer to your service provider might not be willing to accept a /48 announcement. In this case, the other service providers are concerned about the size of the IPv6 routing table that their routers might have to carry.

These two issues highlight a critical issue when looking at developing an IPv6 addressing plan. It is the responsibility of the organization that is considering PI space to sit down with its SP to figure out what IPv6 prefix policies the SP has in place. Listed below are questions that should be part of the initial conversations with the SP:

  • Do you accept PI advertisements?
  • What is the longest prefix length you will accept from customers?
  • What is the longest prefix length you will accept from your peering partners?
  • What is the longest prefix length your peering partners will accept?

Most providers have settled on the /48 as the longest prefix that they will accept from customers or peering partners. This policy should be verified with the SP. The SP policy can be verified by looking at a route server or looking glass service to see how the SP is handling their current IPv6 prefix announcements. You can find route servers and looking glass servers at http://www.bgp4.as/looking-glasses.

Here are some examples for when it is recommended to use provider independent address space:

  • Your organization is connecting to multiple different providers
  • Agreements are in place with your service providers to accept your independent IPv6 prefix announcements

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