Routing Table Maintenance Protocols

Routing tables are held by routers to define paths to destinations in an internetwork. They are normally created and maintained by routing table maintenance protocols.

Routing table maintenance protocols that maintain routing tables for IP networks are normally referred to as IP routing protocols. This term is used throughout this section.

Each routing table maintenance protocol uses a different algorithm to determine when new routes are available and what the best routes are that should be added to the routing table. These algorithms compare routes on the basis of some measurement of distance or cost associated with a route. 

Routes consist of three elements: a destination, the identity of the next hop in the path to the destination and the distance or cost of the complete route to the destination. The distance or cost of the route is referred to as a metric . 

In some protocols the metric is purely the number of hops to the destination; in others, it is a measure of the number of seconds to reach the destination. Modern protocols allow the cost of each network link to be set during network design. In this case the network link cost is normally an indication of the speed of the network, although it could be used to indicate other types of cost such as the lease cost of a telecommunications circuit. 

Each protocol suite (such as TCP/IP, DECnet, AppleTalk) has its own routing table maintenance protocol. Some, such as TCP/IP, offer alternates. 

The routing table maintenance protocols for each suite are:

TCP/IP

· Routing Information Protocol (RIP)

· Open Shortest Path First (OSPF)

· HELLO

· Exterior Gateway Protocol (EGP)

· Border Gateway Protocol (BGP)

· Static Routes

NetWare

· Routing Information Protocol (RIP)

Xerox Network Service (XNS)

· Routing Information Protocol (RIP)

AppleTalk

· Routing Table Maintenance Protocol (RTMP)

DECnet

· DECnet phase IV

Banyan VIrtual NEtworking System Protocol (VINES)

· VINES Routing Update Protocol (VINES RTP)

TCP/IP, NetWare and XNS all have routing information protocols. All derive from the XNS routing information protocol, but all are different .