In a router, route redistribution allows a network that uses one

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select paths between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...

to route

traffic Traffic is the movement of vehicles and pedestrians along land routes. Traffic laws govern and regulate traffic, while rules of the road include traffic laws and informal rules that may have developed over time to facilitate the orderly an ...

dynamically based on information learned from another routing protocol.

Routing

On the Internet, routers forward

data packet In telecommunications and computer networking, a network packet is a formatted unit of data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the '' payload''. Control inform ...

s between

computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

s. In order for a router to determine where to forward each packet, it may use: *

default Default may refer to: Law * Default (law), the failure to do something required by law ** Default (finance), failure to satisfy the terms of a loan obligation or failure to pay back a loan ** Default judgment, a binding judgment in favor of eit ...

static route Static routing describes a process by which routing is configured with fixed values that do not change at runtime unless manually edited. Static routes are used with and without dynamic Routing protocols and usually share the same routing table as ...

s that are manually configured, * on-demand routing, * dynamic routing protocols, or *

Policy-based routing In computer networking, policy-based routing (PBR) is a technique used to make routing decisions based on policies set by the network administrator. When a router (computing), router receives a packet it normally decides where to forward it based ...

. In a small network such as a home or home office, a default route is often used to send all traffic to the user's

Internet service provider An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, no ...

. Likewise, medium-sized networks such as branch offices or small Internet service providers may use default routes for traffic intended for the public Internet. But in medium-sized and large networks, routers use dynamic routing protocols to determine the best paths to various network destinations. Sometimes, a network may use more than one dynamic routing protocol, for example, if two different companies

merge Merge, merging, or merger may refer to: Concepts * Merge (traffic), the reduction of the number of lanes on a road * Merge (linguistics), a basic syntactic operation in generative syntax in the Minimalist Program * Merger (politics), the comb ...

or if networking devices from multiple vendors are used. In such networks, it is often useful to redistribute information from one routing protocol into another.Teare, ''Implementing Cisco IP Routing (ROUTE)'' at 330.

Where used

Route redistribution may be used to allow different networks belonging to the same company to communicate with each other. It may also be used when two companies

and their networks use different routing protocols. And it may be used in conjunction with VRFs or MP-BGP.

Route redistribution pitfalls

If not implemented carefully, route redistribution may cause

routing loop A routing loop is a common problem with various types of networks, particularly computer networks. They are formed when an error occurs in the operation of the routing algorithm, and as a result, in a group of nodes, the path to a particular desti ...

s. Each routing protocol contains loop-avoidance mechanisms, but the information needed for loops to be avoided is lost when routes are redistributed. For example, a

distance-vector routing protocol A distance-vector routing protocol in data networks determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of routers a packet has to pass; one router counts as one hop ...

may prevent loops by the "split-horizon" rule; that is to say, if a router learns a route from a particular interface it will not re-advertise the route out the same interface. In other words if A learns from B that the path to C is through B then it will not tell B to route packets destined for C through A. Likewise, a

link-state routing protocol Link-state routing protocols are one of the two main classes of routing protocols used in packet switching networks for computer communications, the others being distance-vector routing protocols. Examples of link-state routing protocols include ...

may keep a database containing the state of different links in the network, representing a "map" (so to speak) of the network. But the portion of the network whose routes are learned by redistribution are not "mapped" in the same way. The "where did I learn this route?" information is lost in the redistribution process. The chance of loops is increased if there are more than one router that performs redistribution in the same network, and if redistribution occurs in both directions (for example, both from EIGRP into OSPF and from OSPF into EIGRP).

References

External links

*David Davis, ''Cisco administration 101: Routing redistribution'', http://www.techrepublic.com/article/cisco-administration-101-routing-redistribution (Dec. 8, 2005). *Cisco Systems, Inc., ''Configuring a Gateway of Last Resort Using IP Commands'', http://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default.html (last updated Aug. 10, 2005). {{DEFAULTSORT:Route redistribution Routing algorithms