RIP Packet Format
The RIPv2 specification (described in RFC 1723) enables more information to be included in RIP packets and provides a simple authentication mechanism. Figure 3-1 shows the IP RIP 2 packet format:
The field definitions for a RIP packet are as follows:
Up to 25 occurrences of the Address Family Indicator, Address, and Metric fields are permitted in a single IP RIP packet. (That is, up to 25 routing table entries can be listed in a single RIP packet.)
If the address family indicator specifies an authenticated message, only 24 routing table entries can be specified.
If the router has a default network path, RIP advertises a route that links the router to the pseudo network 0.0.0.0. The network 0.0.0.0 does not exist; RIP treats 0.0.0.0 as a network to implement the default routing feature. The Cisco IOS software will advertise the default network if a default was learned by RIP, or if the router has a gateway of last resort and RIP is configured with a default metric.
RIP sends updates to the interfaces in the specified networks. If an interfaces network is not specified, it will not be advertised in any RIP update.
Ciscos implementation of RIP Version 2 supports plain text and MD5 authentication, route summarization, classless interdomain routing (CIDR), and variable-length subnet masks (VLSMs).
RIP Configuration Task List
To configure RIP, complete the tasks in the following sequence. You must enable RIP. The remaining tasks are optional.
Link State Protocols
Link state routing protocols require routers to periodically send routing updates to all other routers in the internetwork. However, each router sends only that portion of the routing table that describes the state of its own links. Link state routing protocols are fast to converge their routing updates across the network in comparison to distance vector protocols.
Their fast convergence makes link state protocols less prone to routing loops than distance vector protocols. However, they also require more CPU power and system memory. One of the primary reasons additional CPU power and memory are needed is that link state protocols are based on the distributed map concept, which means every router has a copy of the network map that is regularly updated.
Link state protocols are based on link state algorithms, which are also called shortest path first (SPF) algorithms or Dijkstra algorithms.
A simple way to think about how link state technology operates is to picture the network as a large jigsaw puzzle; the number of pieces in your puzzle is dependent upon the size of your network. Each piece of the puzzle holds only one router or one LAN. Each router draws itself on that jigsaw piece, including little arrows to other routers and LANs. Those pieces are then replicated and sent throughout the network from router to router, until each router has a complete and accurate copy of each and every piece of the puzzle. Each router then assembles these pieces by using the shortest path first (SPF) algorithm. The SPF algorithm determines how the various pieces of the puzzle fit together.
The Link-State Database
As mentioned, the principle of link-state routing is that all the routers within a network maintain an identical copy of the network topology. From this map, each router will perform a series of calculations that will determine the best routes. This network topology is contained within a database, where each record represents the links to a particular node in the network.
Each record contains several pieces of information: an interface identifier, a link number, and metric information regarding the state of the link. With that information, each router can quickly compute the shortest path from itself to all other routers.