Chapter 4. IPv6 Routing Protocols

Numerous IPv4 routing protocols (RPs) are available for finding routes between networks, and almost every one of them has an IPv6 correspondent or extension: Routing Information Protocol next-generation (RIPng), Open Shortest Path First version 3 (OSPFv3), Intermediate System-to-Intermediate System (IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway Protocol (BGP). So far, IPv6 has brought few innovations to the IP routing paradigm. There are still interior gateway protocols (IGPs) and exterior gateway protocols (EGPs), distance vectorbased and link-state-based routing protocol algorithms, and so on.

The concept of the autonomous system, defined as a set of networks controlled by a common administrative entity, remains unchanged with the introduction of IPv6 RPs. The same autonomous system (and autonomous system number [ASN]) will route both IPv4 and IPv6. IPv6 IGPs, used to exchange routes within the autonomous system, are namely RIPng, OSPFv3, IS-IS for IPv6, and EIGRP for IPv6. Only BGP4 is available to exchange IPv6 routes between autonomous systems. Multiprotocol extensions provide support in BGP4 for IPv6 routing.

The requirements for IGPs and EGPs are quite different, in terms of routing table size, number of supported routers, convergence time, security, routing policy, and so forth. For that reason, they use different algorithms and mechanisms, which also affect the type of information they exchange and store. IGPs use distance vector and link state, whereas BGP uses the path vector RP algorithm. Table 4-1 presents the RP taxonomy, and highlights their IPv6 correspondent. For more details on how to choose the RP, refer to Top-Down Network Design, Second Edition, by Priscilla Oppenheimer.

Table 4-1. Taxonomy of Routing Protocols

Deployment Domain	Algorithm	RP	Scalability	Convergence Time	Metric	IPv6 Version
Interior gateway protocol (IGP)	Distance vector	RIP	15 hops	Slow	Hop count	RIPng
		EIGRP	1000s routers	Quick (via DUAL algorithm)	Bandwidth, delay, reliability, load	EIGRP for IPv6
	Link state	OSPF	1000s routers (100s/area)	Quick (via LSAs and HELLO)	Cost (function of bandwidth on Cisco routers)	OSPFv3
		IS-IS	1000s routers (100s/area)	Quick (via LSPs)	Configured host, delay, expense	IS-IS for IPv6
Exterior gateway protocol (EGP)	Distance vector	EGP			Integer <=255
	Path vector	BGP AF=IPv4	1000s routers	Slow (via UPDATE)	Function of path attributes and other configurable factors	BGP AF=IPv6

The rest of this section briefly reviews existing unicast RP technologies. The next section reviews each of the available IPv6 RPs and provides configuration examples. Then the last two sections cover the topic of multihoming and deployment aspects, respectively.