Each router running the RIPng protocol keeps the information about all known networks in the internal data structure called RIPng route table. It is essentially the RIPng-specific incarnation of the distance vector with few additional items that are necessary for RIPng operation and cooperation with other routing protocols. For every network, the routing table contains the following data:

• destination prefix
• metric
• next hop
• route tag
• route change flag
• route expiration timer
• garbage collection timer

The first three items together are known as a route and carry information about the reachability of a network from the viewpoint of the particular router. The destination prefix consists of an IPv6 network address and prefix length and identifies the network that the route table entry is about. The metric is the distance to that network, or rather the current estimate of the distance. For directly connected networks, the metric is set to the cost of that network. Finally, next hop contains the IP address of the router that is next on the (provisional) optimum path to the destination network. For directly connected networks, the next hop is irrelevant, usually empty.

The route tag is not of an immediate interest for RIPng; the standard only requires that the route tag be preserved and redistributed with the route. It is typically used for tagging routes that were imported from other routing protocols, for example BGP.

The route change flag and both timers are internal parameters that control the route processing. Their role in RIPng operation will be explained in the next subsection.

The routers running RIPng exchange entire route tables or their portions between each other. Such an exchange takes place in the following three cases:

1. About every 30 seconds, each router sends regular updates to all neighbours.
2. Whenever a router detects a change in network topology (e.g., a link going down), it sends to all neighbours a triggered update containing only the routes that are affected by the topology change.
3. Routers may also send their routing table or part of it after receiving an explicit Request message from another router or host.

The routes are exchanged in the form of a data structure known as route table entry (RTE).

The current version of the RIPng protocol [RFC2080] defines two types of messages: Request and Response. Both types have the same format as shown in Figure 6-3 and every message contains one or more RTEs.

Figure 6-3 RIPng Message