Routing protocol
Encyclopedia
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 node
s on a computer network
, the choice of the route being done by routing algorithms
. Each router has a priori 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.
Although there are many types of routing protocols, three major classes are in widespread use on IP
networks:
Many routing protocols are defined in documents called RFCs
.
The specific characteristics of routing protocols include
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:
. A given autonomous system
can contain multiple routing domains, or a set of routing domains can be coordinated without being an Internet-participating autonomous system. Common examples include:
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.
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
s on a computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
, the choice of the route being done by routing algorithms
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
. Each router has a priori 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
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
.
The term routing protocol may refer specifically to one operating at layer three of the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
, which similarly disseminates topology information between routers.
Although there are many types of routing protocols, three major classes are in widespread use on IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
networks:
- Interior gateway routingInterior gateway protocolAn interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
via link-state routing protocolLink-state routing protocolA link-state routing protocol is one of the two main classes of routing protocols used in packet switching networks for computer communications . Examples of link-state routing protocols include OSPF and IS-IS....
s, such as OSPFOpen Shortest Path FirstOpen Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
and IS-IS - Interior gateway routingInterior gateway protocolAn interior gateway protocol is a routing protocol that is used to exchange routing information within an autonomous system ....
via path vectorPath vector protocolA path vector protocol is a computer network routing protocol which maintains the path information that gets updated dynamically. Updates which have looped through the network and returned to the same node are easily detected and discarded...
or distance vectorDistance-vector routing protocolIn computer communication theory relating to packet-switched networks, a distance-vector routing protocol is one of the two major classes of routing protocols, the other major class being the link-state protocol...
protocols, such as RIPRouting Information ProtocolThe Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
, IGRPInterior Gateway Routing ProtocolInterior Gateway Routing Protocol is a distance vector interior routing protocol invented by Cisco. It is used by routers to exchange routing data within an autonomous system....
and EIGRPEnhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing Protocol - is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of... - Exterior gateway routingExterior Gateway ProtocolThe Exterior Gateway Protocol is a now obsolete routing protocol for the Internet originally specified in 1982 by Eric C. Rosen of Bolt, Beranek and Newman, and David L. Mills. It was first described in RFC 827 and formally specified in RFC 904...
. BGPBorder Gateway ProtocolThe Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
v4 is the routing protocol used by the public InternetInternetThe Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
.
Many routing protocols are defined in documents called RFCs
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
.
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
Routed versus routing protocols
A routed protocol can be routed by a router, i.e., it can be forwarded from one router to another. A routing protocol sends and receives packets containing routing information to and from other routers.In some cases, routing protocols can themselves run over routed protocols: for example, BGP runs over TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
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-ISIS-ISIntermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
runs over the data link layer - OSPFOpen Shortest Path FirstOpen Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
, IGRPInterior Gateway Routing ProtocolInterior Gateway Routing Protocol is a distance vector interior routing protocol invented by Cisco. It is used by routers to exchange routing data within an autonomous system....
, and EIGRPEnhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing Protocol - is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of...
run directly over IP; OSPF and EIGRP have their own reliable transmission mechanism while IGRP assumed an unreliable transport - RIPRouting Information ProtocolThe Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
runs over UDPUser Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring... - BGPBorder Gateway ProtocolThe Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
runs over TCPTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
Interior routing protocols
Interior Gateway Protocols (IGPs) exchange routing information within a single routing domainRouting domain
In computer networking, a routing domain is a collection of networked systems that operate common routing protocols and are under the control of a single administration...
. A given autonomous system
Autonomous system (Internet)
Within the Internet, an Autonomous System is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet....
can contain multiple routing domains, or a set of routing domains can be coordinated without being an Internet-participating autonomous system. Common examples include:
- IGRPInterior Gateway Routing ProtocolInterior Gateway Routing Protocol is a distance vector interior routing protocol invented by Cisco. It is used by routers to exchange routing data within an autonomous system....
(Interior Gateway Routing Protocol) - EIGRPEnhanced Interior Gateway Routing ProtocolEnhanced Interior Gateway Routing Protocol - is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of...
(Enhanced Interior Gateway Routing Protocol) - OSPFOpen Shortest Path FirstOpen Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
(Open Shortest Path First) - RIPRouting Information ProtocolThe Routing Information Protocol is a distance-vector routing protocol, which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops allowed for RIP is 15....
(Routing Information Protocol) - IS-ISIS-ISIntermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
(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.
Further reading
- Chapter "Routing Basics" in the CiscoCisco SystemsCisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
"Internetworking Technology Handbook"