Route redistribution

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In a router, route redistribution allows a network that uses one routing protocol to route traffic dynamically based on information learned from another routing protocol.

Contents

Routing

On the Internet, routers forward data packets between computer networks. [1] [2] In order for a router to determine where to forward each packet, it may use:

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. 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 or if networking devices from multiple vendors are used. [8] In such networks, it is often useful to redistribute information from one routing protocol into another. [9]

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 merge 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 loops.

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 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 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).

Related Research Articles

Interior Gateway Routing Protocol (IGRP) is a distance vector interior gateway protocol (IGP) developed by Cisco. It is used by routers to exchange routing data within an autonomous system.

<span class="mw-page-title-main">Router (computing)</span> Device that forwards data packets between computer networks

A router is a computer and networking device that forwards data packets between computer networks, including internetworks such as the global Internet.

<span class="mw-page-title-main">Border Gateway Protocol</span> Protocol for communicating routing information on the Internet

Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator.

Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single autonomous system (AS).

The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols 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 source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. In 2013, Cisco permitted other vendors to freely implement a limited version of EIGRP with some of its associated features such as High Availability (HA), while withholding other EIGRP features such as EIGRP stub, needed for DMVPN and large-scale campus deployment. Information needed for implementation was published with informational status as RFC 7868 in 2016, which did not advance to Internet Standards Track level, and allowed Cisco to retain control of the EIGRP 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. Some distance-vector protocols also take into account network latency and other factors that influence traffic on a given route. To determine the best route across a network, routers using a distance-vector protocol exchange information with one another, usually routing tables plus hop counts for destination networks and possibly other traffic information. Distance-vector routing protocols also require that a router inform its neighbours of network topology changes periodically.

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 Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS).

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Dial on Demand Routing (DDR) is a routing technique where a network connection to a remote site is established only when needed. In other words, if the router tries to send out data and the connection is off, then the router will automatically establish a connection, send the information, and close the connection when no more data needs to be sent. DDR is advantageous for companies that must pay per minute for a WAN setup, where a connection is always established. Constant connections can become needlessly expensive if the company does not require a constant internet connection.

The Virtual Router Redundancy Protocol (VRRP) is a computer networking protocol that provides for automatic assignment of available Internet Protocol (IP) routers to participating hosts. This increases the availability and reliability of routing paths via automatic default gateway selections on an IP subnetwork.

Dynamic routing, also called adaptive routing, is a process where a router can forward data via a different route for a given destination based on the current conditions of the communication circuits within a system. The term is most commonly associated with data networking to describe the capability of a network to 'route around' damage, such as loss of a node or a connection between nodes, as long as other path choices are available. Dynamic routing allows as many routes as possible to remain valid in response to the change.

Administrative distance (AD) or route preference is a number of arbitrary unit assigned to dynamic routes, static routes and directly-connected routes. The value is used in routers to rank routes from most preferred to least preferred. When multiple paths to the same destination are available in its routing table, the router uses the route with the lowest administrative distance.

Dynamic Multipoint Virtual Private Network (DMVPN) is a dynamic tunneling form of a virtual private network (VPN) supported on Cisco IOS-based routers, and Huawei AR G3 routers, and on Unix-like operating systems.

Protocol-dependent modules (PDMs) are used by the routing protocol EIGRP to make decisions about adding routes learned from other sources; for example other routers or routing protocols to the routing table. In fact EIGRP has the capability for routing several different protocols including IPv4 and IPv6 using protocol-dependent modules (PDMs). The PDM is also capable of carrying information from the routing table to the topology table. EIGRP offers support for various routed protocols, and has added support for Service Routing (SAF) PDMs. The only other routing protocol that comes with support for multiple network layer protocols is Intermediate System-to-Intermediate System (IS-IS).

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 packets are forwarded through the networks of the internet from router to router until they reach their destination computer. Routing algorithms determine the specific choice of route. 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. The ability of routing protocols to dynamically adjust to changing conditions such as disabled connections and components and route data around obstructions is what gives the Internet its fault tolerance and high availability.

In computer networking, CDP spoofing is a technique employed to compromise the operation of network devices that use Cisco Discovery Protocol (CDP) for discovering neighboring devices. CDP spoofing is a network security threat that can be mitigated by taking precautionary measures.

Static routing describes a process by which routing is configured with fixed values which 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 those protocols. Routes require at least two attributes; the destination and the gateway, but may contain additional attributes such as a metric. Some implementations treat the network address and subnet mask as separate values, however in practice both of the values have to be considered for any given routing decision to determine the longest prefix match. Static routes together with connected routes and routes from configuration protocols such as DHCP or Router Advertisements provide the routes which are then redistributed using dynamic routing protocols. While static routes are entered into the system and remain there until removed or changed manually, dynamic routing protocols create and delete routes dynamically at runtime without intervention. Thus the term static here refers to the nature of remaining unchanged by the system itself. The most prominent example of a static route is a default route which is often used on devices with a statically configured IP address to provide the device with access to the rest of the network or the internet by default. In contrast to a so called connected route which is automatically generated upon address assignment based on the used subnet mask, a static route must be manually configured. Due to this the configuration may fail if there is no route to the provided gateway at the time of configuration, other than the connected route which will always succeed as it does not require a gateway. The gateway of a static route need not be an address, but can also specify an interface in most implementations.

<span class="mw-page-title-main">Packet Tracer</span> Network simulation software

Packet Tracer is a cross-platform visual simulation tool designed by Cisco Systems that allows users to create network topologies and imitate modern computer networks. The software allows users to simulate the configuration of Cisco routers and switches using a simulated command line interface. Packet Tracer makes use of a drag and drop user interface, allowing users to add and remove simulated network devices as they see fit. The software is mainly focused towards Cisco Networking Academy students as an educational tool for helping them learn fundamental CCNA concepts. Previously students enrolled in a CCNA Academy program could freely download and use the tool free of charge for educational use.

IP routing is the application of routing methodologies to IP networks. This involves not only protocols and technologies but includes the policies of the worldwide organization and configuration of Internet infrastructure. In each IP network node, IP routing involves the determination of a suitable path for a network packet from a source to its destination in an IP network. The process uses static configuration rules or dynamically obtained from routing protocols to select specific packet forwarding methods to direct traffic to the next available intermediate network node one hop closer to the desired final destination, a total path potentially spanning multiple computer networks.

References

  1. ↑ Leeds City College, CCNA – Routing Protocols and Concepts, http://www.leedscitycollege.ac.uk/courses/index.php/3260/ccna-routing-protocols-and-concepts/(Nov. 16, 2010). Archived 2012-02-02 at the Wayback Machine
  2. ↑ Diane Teare, Implementing Cisco IP Routing (ROUTE): Foundation Learning Guide 22 (Cisco Press, Mar. 2013)
  3. ↑ Teare, Implementing Cisco IP Routing (ROUTE) at 22–25
  4. ↑ 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).
  5. ↑ Teare, Implementing Cisco IP Routing (ROUTE) at 28–29
  6. ↑ Id. at 26–28.
  7. ↑ Id. at ch. 5.
  8. ↑ Cisco Systems, Inc., Redistributing Routing Protocols, http://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8606-redist.html (last updated Mar. 22, 2012).
  9. ↑ Teare, Implementing Cisco IP Routing (ROUTE) at 330.
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