Routing Considerations With IPv6


Routing Configurations with IPv6
Like IPv4 classless interdomain routing (CIDR), IPv6 uses longest prefix match routing. IPv6 uses modified versions of most of the common routing protocols to handle longer IPv6 addresses and different header structures.
Larger address spaces make room for large address allocations to ISPs and organizations. An ISP aggregates all of the prefixes of its customers into a single prefix and announces the single prefix to the IPv6 Internet. The increased address space is sufficient to allow organizations to define a single prefix for their entire network.
But how does this affect router performance? A brief review of how a router functions in a network helps illustrate how IPv6 affects routing. Conceptually, a router has three functional areas:
The control plane handles the interaction of the router with the other network elements, providing the information needed to make decisions and control the overall router operation. This plane runs processes such as routing protocols and network management. These functions are generally complex.
The data plane handles packet forwarding from one physical or logical interface to another. It involves different switching mechanisms such as process switching and Cisco Express Forwarding (CEF) on Cisco IOS software routers.
Enhanced services include advanced features applied when forwarding data, such as packet filtering, quality of service (QoS), encryption, translation, and accounting.
IPv6 presents each of these functions with specific new challenges.

IPv6 Control Plane
Enabling IPv6 on a router starts its control plane operating processes specifically for IPv6. Protocol characteristics shape the performance of these processes and the amount of resources necessary to operate them:
IPv6 address size - Address size affects the information-processing functions of a router. Systems using a 64-bit CPU, bus, or memory structure can pass both the IPv4 source and destination address in a single processing cycle. For IPv6, the source and destination addresses require two cycles each-four cycles to process source and destination address information. As a result, routers relying exclusively on software processing are likely to perform slower than when in an IPv4 environment.
Multiple IPv6 node addresses - Because IPv6 nodes can use several IPv6 unicast addresses, memory consumption of the Neighbor Discovery cache may be affected.
IPv6 routing protocols - IPv6 routing protocols are similar to their IPv4 counterparts, but since an IPv6 prefix is four times larger than an IPv4 prefix, routing updates have to carry more information.
Routing table Size -Increased IPv6 address space leads to larger networks and a much larger Internet. This implies larger routing tables and higher memory requirements to support them.

IPv6 Data Plane
The data plane forwards IP packets based on the decisions made by the control plane. The forwarding engine parses the relevant IP packet information and does a lookup to match the parsed information against the forwarding policies defined by the control plane. IPv6 affects the performance of parsing and lookup functions:
Parsing IPv6 extension headers - Applications, including mobile IPv6, often use IPv6 address information in extension headers, thus increasing their size. These additional fields require additional processing. For example, a router using ACLs to filter Layer 4 information needs to apply the ACLs to packets with extension headers as well as those without. If the length of the extension header exceeds the fixed length of the hardware register of the router, hardware switching fails, and packets may be punted to software switching or dropped. This severely affects the forwarding performance of the router.
IPv6 address lookup - IPv6 performs a lookup on packets entering the router to find the correct output interface. In IPv4, the forwarding decision process parses a 32-bit destination address. In IPv6, the forwarding decision could conceivably require parsing a 128-bit address. Most routers today perform lookups using an application-specific integrated circuit (ASIC) with a fixed configuration that performs the functions for which it was originally designed - IPv4. Again, this could result in punting packets into slower software processing, or dropping them all together.

RIPNg Routing Protocol
IPv6 routes use the same protocols and techniques as IPv4. Although the addresses are longer, the protocols used in routing IPv6 are simply logical extensions of the protocols used in IPv4.
RFC 2080 defines Routing Information Protocol next generation (RIPng) as a simple routing protocol based on RIP. RIPng is no more or less powerful than RIP, however, it provides a simple way to bring up an IPv6 network without having to build a new routing protocol.
RIPng is a distance vector routing protocol with a limit of 15 hops that uses split horizon and poison reverse updates to prevent routing loops. Its simplicity comes from the fact that it does not require any global knowledge of the network. Only neighboring routers exchange local messages.

RIPng includes the following features:
Based on IPv4 RIP version 2 (RIPv2) and is similar to RIPv2
Uses IPv6 for transport
Includes the IPv6 prefix and next-hop IPv6 address
Uses the multicast group FF02::9 as the destination address for RIP updates (this is similar to the broadcast function performed by RIP in IPv4)
Sends updates on UDP port 521
Is supported by Cisco IOS Release 12.2(2)T and later
In dual-stacked deployments, both RIP and RIPng are required.

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