The addresses above the shim are called Upper Layer IDs (ULID). Above the shim are stable routable IPv6 addresses that allow applications to work as they have before and do not disrupt DNS information. The primary solution that exists today is to use a 'shim' that can be a new layer between the network layer and the transport layer. This early work is documented in RFC 3582, 'Goals for IPv6 Site-Multihoming Architectures.' Now the Site Multihoming by IPv6 Intermediation working group (shim6) is developing solutions to address sites that are multihomed. The IETF has performed much work on the subject of multihoming.

Therefore, alternatives must exist to allow the redundancy and failover between service providers without violating the address hierarchy rule. The customer addresses its web servers in one ISP's address space, and if that ISP fails, the web servers cannot be reached through the other ISP link. If one ISP link goes down, the other ISP does not readvertise the other ISP's address space. However, problems arise when sites have multiple address assignments from multiple ISPs. Customers must be allowed to be multihomed to the Internet. The redundancy and availability needs of customers must be addressed in some way. This is a requirement to reduce the latency that would result in back-hauling their Internet traffic to one central Internet attachment point. Multinational organizations want to have Internet connections on the different continents they operate, for example. Many larger organizations still have a desire to have redundant connections to the Internet. This address space is not likely to be routed by service providers, but it does give customers additional addresses should they need them. However, various registries (notably ARIN) have started to allow customers to obtain provider independent (PI) address space. Because of this fear, the addressing hierarchy has been enforced by the IANA, the IETF, the regional registries, and the ISPs. The address space would become fragmented, and the size of the Internet routing tables would expand out of control. If the rules of IPv6 addressing hierarchy were relaxed, many organizations could advertise their prefixes to the Internet. Therefore, if one ISP connection were to fail, the BGP routing tables would converge and the customer would maintain its Internet connectivity. The organization takes in routes from these providers (full routes, partial routes, or just the default route) and advertises its own address space from its own Autonomous System Number (ASN). This is part of an enterprise organization's disaster recovery and business continuity plan. Many large organizations that connect to today's IPv4 Internet enjoy the redundancy that comes from connecting to two or more ISPs. This would simplify device configurations and also reduce the need for BGP, which would reduce the number of protocols the routers needed to run. In turn, the customer could simply use a default route to point toward the ISP for routing traffic to all unknown prefixes. An ISP could simply use a static route to point to the address block that has been allocated to the customer. If Internet routers contain both IPv4 and IPv6, the problem gets worse.īecause IPv6 addresses are fully hierarchical, you probably do not need to use BGP, except in the default-free zone of the Internet backbone.

As the size of the routing table increases, so does the time of convergence. As the FIB gets larger, so does the lookup time, which affects the forwarding rate. The size of the Forwarding Information Base (FIB) and the Routing Information Base (RIB) increases with the number of routes. Both memory and processor capacity are factors in the maximum size of the IP routing table. With the increasing size of today's IPv4 Internet routing table, many devices struggle to handle the storage and the workload of processing the changes. Because IPv6 has the ability to have far more address blocks than IPv4, it would be impossible to have a large number of routes in the Internet backbone routers. IPv6 addresses are intended to be fully hierarchical to help reduce the size of the core Internet routing table. IPv6 addresses are allocated by service providers to end-user organizations.