Many routing protocols can be used to automate the process of path discovery, with each of these protocols providing different benefits and having different costs. Here are some of the more common of these protocols.
>> Border Gateway Protocol (BGP): Version 4 is the exterior routing protocol of choice on the Internet and is used as an interior routing protocol in some environments. BGP routers exchange routes to organizational networks (as identified by autonomous system numbers), with these routes being tweaked according to the monetary cost of the connected link, the bandwidth available through the advertising organization's link and other details. In practical terms, this means that BGP is a contract vector-routing protocol, where any number of routes may be available for a destination, but the chosen path will most often be determined by the terms of the contracts that govern the available paths.
>> ICMP Router Discovery Protocol (IRDP): ICMP is best known for providing the kind of diagnostic and alert messages that are used by programs such as ping and traceroute. ICMP also provides a series of router-information messages that can be used to discover and advertise the presence of a default router on a network. In this model, routers can advertise themselves as offering default routes with a specific preference weight (operator assigned), and workstations can pick the best router based on the preference value. This means that managers do not have to configure workstations with specific routes, nor do they have to assign subnet-specific routes to DHCP assignment pools. Instead, they can simply enable IRDP on the workstations and walk away. However, IRDP is useful only for choosing default routes and cannot be used to assign routes with any higher granularity, so this restricts its functionality.
>> Intermediate System to Intermediate System (IS-IS): OSPF is a derivative of IS-IS, which was the first fully blown link-state routing protocol. IS-IS was intended for use with the OSI protocols but was extendible to other protocol families. Because of this flexibility and its earlier presence, it is still found on some older large-scale networks, but its younger cousins, OSPF and NLSP, are much more common.
>> NetWare Link-Services Protocol (NLSP): NLSP is the IPX-centric version of IS-IS. NLSP provides a link-state replacement for the IPX-specific version of RIP and the NetWare SAP (Service Advertisement Protocol), such that IPX routes and services have to be transmitted over WAN links only whenever those resources change rather than being advertised every 60 seconds.
>> Open Shortest Path First (OSPF): OSPF was originally designed as an IP-specific link-state protocol similar to IS-IS, though it has diverged from that design dramatically over the years.
>> Routing Information Protocol (RIP): In theory, RIP supports multiple protocols, including IP and IPX, though in practice these are covered by two separate protocols that have slightly different mechanics. For example, the IP implementation of RIP broadcasts all known routes every 30 seconds, while the IPX version broadcasts routes every 60 seconds.
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