TABLE OF CONTENTS Any time when communication takes place between any pair of nodes, especially when that communication involves nodes that reside on different network segments, a decision must be made about where each packet should go. This decision is often known as a packet routing decision, or a packet forwarding decision. The intermediate network devices, commonly known as routers, perform the routing functions that involve making the routing decision normally based on each packet's final destination.
The routing decision could be made based on manually configured routing information at each router, but such practice is obviously impractical for a complex network of middle to large scale. Routing protocols provide the necessary information that enable the routers to make correct routing decisions automatically. Since a packet's destination may be a unicast destination or a multicast destination (treating broadcast destination as a special case of multicast), routing protocols are designed for either unicast routing or multicast routing. We will focus on the routing protocols in this chapter.
In the IPv4 world, RIPv2 [RFC2453], the integrated IS-IS [RFC1195], and OSPFv2 [RFC2328] are commonly deployed unicast routing protocols in networks of small to middle scale such as enterprise environments, while BGP-4 [RFC4271] is the common routing protocol deployed among large organizations such as Internet Service Providers (ISPs). In general, since the routing concept is identical between IPv4 and IPv6, these routing protocols have been naturally extended to support IPv6. Even though the packet formats may have changed, the...
