| IPv6 Network - Routing |
IPv6 Network - Routing
For IPv6 packets to reach remote destinations, they must generally pass through several IPv6 routers. This chapter explains how to set up IPv6 routing, both in a simple end-user environment and in an ISP or enterprise environment, where one or more routing protocols are deployed. Most residential gateways and small office/home office routers that are sold these days don’t support IPv6, so it’s common for a simple end-user network to use a regular computer as an IPv6 router. Routing/forwarding IPv6 packets is supported on Windows XP and all UNIX-like operating systems such as Linux, the BSD family, and MacOS, which sort of married into the BSD family as of version “X.” Figure 4-1 shows the layout of the example network we’ll be discussing in the rest of this chapter. A router sits between the host and the IPv6 Internet. The Ethernet link between the router and the host is depicted somewhat archaically as some length of coaxial Ethernet cable with terminators at both ends.
Routing IPv6
There is a difference between routing and forwarding a protocol. Routing is the process of maintaining a routing table, usually (but not necessarily) aided by routing protocols. With a routing table in place, any packets that come in that aren’t addressed to the router can be forwarded to their destination, or to another router closer to that destination. Not everyone cares to make the distinction between the two, so “routing” is sometimes used to describe both. In IPv6, there is more to being a router than just routing and forwarding. The host in Figure 4-1 not only depends on the router to forward packets to and from the Internet on its behalf; it also needs the router to provide an address prefix so it can autoconfigure an IPv6
address. So the router in Figure 4-1 must, like all well-behaved IPv6 routers, listen on the allrouters multicast address (ff02::2) for incoming router solicitation packets. When it receives one of those, it replies with a router advertisement (to the all-hosts group address, ff02::1) that contains the desired configuration information. Routers also send out router advertisements periodically to let hosts know that previously learned information is still valid. In IPv4, routers often supply configuration information to hosts by using the Dynamic Host Configuration Protocol (DHCP), but there is no requirement that DHCP runs on a router: the DHCP service can be provided by a regular server, if desired. Stateless autoconfiguration, on the other hand, is closely tied to routing functionality, and therefore supplying the information that clients need to autoconfigure themselves must be done on actual routers. There is also DHCP for IPv6 (DHCPv6), but it’s not widely supported yet, and it’s doubtful that it ever will as a mechanism for hosts to configure their IPv6 addresses. See Chapter 8 for more information on DHCPv6.
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