A routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a prior knowledge only of networks attached to it directly. A routing protocol shares this information first among immediate neighbors, and then throughout the network. This way, routers gain knowledge of the topology of the network. For a discussion of the concepts behind routing protocols, see: Routing.
The term routing protocol may refer specifically to one operating at layer three of the OSI model, which similarly disseminates topology information between routers.
Many routing protocols used in the public Internet are defined in documents called RFCs.[1][2][3][4]
Although there are many types of routing protocols, two major classes are in widespread use in the Internet: link-state routing protocols, such as OSPF and IS-IS; and path vector or distance vector protocols, such as BGP, RIP and EIGRP.
The specific characteristics of routing protocols include
- the manner in which they either prevent routing loops from forming or break them up if they do
- the manner in which they select preferred routes, using information about hop costs
- the time they take to converge
- how well they scale up
- many other factors
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Routed versus routing protocols
Confusion often arises between routing protocols and routed protocols. While routing protocols help the router decide which paths to send traffic along, routed protocols are responsible for the actual transfer of traffic between devices running L3 protocols such as IP.[5] Specifically, a routed protocol is any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from one host to another based on the addressing scheme, without knowing the entire path from source to destination. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. Almost all layer 3 protocols, and those that are layered over them, are routable, with IP being an example. Layer 2 protocols such as Ethernet are necessarily non-routable protocols, since they contain only a link-layer address, which is insufficient for routing: some higher-level protocols based directly on these without the addition of a network layer address, such as NetBIOS, are also non-routable.
In some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP which runs over IP; care is taken in the implementation of such systems not to create a circular dependency between the routing and routed protocols. That a routing protocol runs over particular transport mechanism does not mean that the routing protocol is of layer (N+1) if the transport mechanism is of layer (N). Routing protocols, according to the OSI Routing framework, are layer management protocols for the network layer, regardless of their transport mechanism:
- IS-IS runs over the data link layer
- OSPF, IGRP, and EIGRP run directly over IP; OSPF and EIGRP have their own reliable transmission mechanism while IGRP assumed an unreliable transport
- RIP runs over UDP
- BGP runs over TCP
Examples
Interior routing protocols
Interior Gateway Protocols (IGPs) exchange routing information within a single routing domain. A given autonomous system [6] can contain multiple routing domains, or a set of routing domains can be coordinated without being an Internet-participating autonomous system. Common examples include:fh
- IGRP (Interior Gateway Routing Protocol)
- EIGRP (Enhanced Interior Gateway Routing Protocol)
- OSPF (Open Shortest Path First)
- RIP (Routing Information Protocol)
- IS-IS (Intermediate System to Intermediate System)
Note that IGRP, a Cisco proprietary routing protocol, is no longer supported. EIGRP accepts IGRP configuration commands, but the internals of IGRP and EIGRP are completely different.
References
- ^ INTERNET PROTOCOL, RFC 791, J Postel, September 1981.
- ^ BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS, RFC 922, Jeffrey Mogul, October 1984
- ^ Towards Requirements for IP Routers, RFC 1716, P. Almquist, November 1994
- ^ Requirements for IP Version 4 Routers, RFC 1812, F. Baker,June 1995
- ^ Routing and Routed Protocols
- ^ Guidelines for creation, selection, and registration of an Autonomous System (AS), RFC 1930, J. Hawkison & T. Bates,March1996
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