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.
25-556: 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 . IGRP is a proprietary protocol . IGRP was created in part to overcome the limitations of RIP (maximum hop count of only 15, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth , delay , load , and reliability ; to compare two routes these metrics are combined into
50-415: A Transmission Control Protocol (TCP) session. Routers with BGP implementation determine the shortest path across a network based on a range of factors other than hops. BGP can also be configured by administrators so that certain routes are preferred or avoided. BGP is used by internet service providers (ISPs) and telecommunication companies. Among the distance-vector protocols that have been described as
75-478: A classful routing protocol. Because the protocol has no field for a subnet mask , the router assumes that all subnetwork addresses within the same Class A, Class B, or Class C network have the same subnet mask as the subnet mask configured for the interfaces in question. This contrasts with classless routing protocols that can use variable length subnet masks . Classful protocols have become less popular as they are wasteful of IP address space . In order to address
100-462: A telecommunications network . It specifies the latency for a bit of data to travel across the network from one communication endpoint to another. It is typically measured in multiples or fractions of a second. Delay may differ slightly, depending on the location of the specific pair of communicating endpoints. Engineers usually report both the maximum and average delay, and they divide the delay into several parts: A certain minimum level of delay
125-421: A DV from B that tells A there is a path via B to D, with a distance (or cost) of 7. Since the current "shortest-path" to B is 3, then A knows it has a path to D that costs 7+3=10. This path to D of length 10 (via B) is shorter than the existing "shortest-path" to D of length 28 (via C), so it becomes the new "shortest-path" to D. Network latency Network delay is a design and performance characteristic of
150-424: A hybrid, because it uses routing methods associated with link-state routing protocols , is the proprietary Enhanced Interior Gateway Routing Protocol (EIGRP). It was developed by Cisco in the 1980s and was designed to offer better convergence and cause less network traffic between routers than the link-state routing protocol Open Shortest Path First (OSPF). Another example of a distance-vector routing protocol
175-421: A number of loop-free distance vector protocols have been developed — notable examples are EIGRP , DSDV and Babel . These avoid loop formation in all cases, but suffer from increased complexity, and their deployment has been slowed down by the success of link state routing protocols such as OSPF . In this network we have 4 routers A, B, C and D: [REDACTED] We mark the current time (or iteration) in
200-463: A single metric, using a formula which can be adjusted through the use of pre-set constants. By default, the IGRP composite metric is a sum of the segment delays and the lowest segment bandwidth. The maximum configurable hop count of IGRP-routed packets is 255 (default 100), and routing updates are broadcast every 90 seconds (by default). IGRP uses protocol number 9 for communication. IGRP is considered
225-444: Is Babel . The Bellman–Ford algorithm does not prevent routing loops from happening and suffers from the count to infinity problem . The core of the count-to-infinity problem is that if A tells B that it has a path somewhere, there is no way for B to know if the path has B as a part of it. To see the problem, imagine a subnet connected like A–B–C–D–E–F, and let the metric between the routers be "number of jumps". Now suppose that A
250-491: Is completely unsupported. In the new Cisco CCNA curriculum (version 4), IGRP is mentioned only briefly, as an "obsolete protocol". Distance vector Distance-vector routing protocols use the Bellman–Ford algorithm to calculate the best route. Another way of calculating the best route across a network is based on link cost, and is implemented through link-state routing protocols . The term distance vector refers to
275-404: Is sent to all its neighbours that are configured to use the same distance-vector routing protocol. Once a router has this information it is able to amend its own routing table to reflect the changes and then inform its neighbours of the changes. This process has been described as ‘routing by rumour’ because routers are relying on the information they receive from other routers and cannot determine if
SECTION 10
#1732802399424300-411: Is taken offline. In the vector-update-process B notices that the route to A, which was distance 1, is down – B does not receive the vector update from A. The problem is, B also gets an update from C, and C is still not aware of the fact that A is down – so it tells B that A is only two jumps from C (C to B to A). Since B doesn't know that the path from C to A is through itself (B), it updates its table with
325-430: The algorithm with T, and begin (at time 0, or T=0) by creating distance matrices for each router to its immediate neighbours. As we build the routing tables below, the shortest path is highlighted in green, and a new shortest path is highlighted in yellow. Grey columns indicate nodes that are not neighbors of the current node, and are therefore not considered as a valid direction in its table. Red indicates invalid entries in
350-432: The best route, routers regularly exchange information with neighbouring routers, usually their routing table , hop count for a destination network and possibly other traffic related information. Routers that implement distance-vector protocol rely purely on the information provided to them by other routers, and do not assess the network topology . Distance-vector protocols update the routing tables of routers and determine
375-405: The chance of forming loops and uses a maximum number of hops to counter the 'count to infinity' problem. These measures avoid the formation of routing loops in some, but not all, cases. The addition of a hold time (refusing route updates for a few minutes after a route retraction) avoids loop formation in virtually all cases, but causes a significant increase in convergence times. More recently,
400-502: The fact that the protocol manipulates vectors ( arrays ) of distances to other nodes in the network. The distance vector algorithm was the original ARPANET routing algorithm and was implemented more widely in local area networks with the Routing Information Protocol (RIP). Distance-vector routing protocols use the Bellman–Ford algorithm . In these protocols, each router does not possess information about
425-424: The full network topology . It advertises its distance value (DV) calculated to other routers and receives similar advertisements from other routers unless changes are done in the local network or by neighbours (routers). Using these routing advertisements each router populates its routing table. In the next advertisement cycle, a router advertises updated information from its routing table. This process continues until
450-435: The hops, that is numbers of routers that need to be passed to reach the destination network. RIP is an interior gateway protocol , so it can be used in local area networks (LANs) on interior or border routers. Routers with RIPv1 implementation exchange their routing tables with neighbouring routers by broadcasting a RIPv1 packet every 30 second into all connected networks. RIPv1 is not suitable for large networks as it limits
475-402: The information is actually valid and true. There are a number of features which can be used to help with instability and inaccurate routing information. The oldest routing protocol , and the oldest distance-vector protocol, is version 1 of the Routing Information Protocol (RIPv1). RIPv1 was formally standardised in 1988. It establishes the shortest path across a network purely on the basis of
500-492: The issues of address space and other factors, Cisco created EIGRP (Enhanced Interior Gateway Routing Protocol). EIGRP adds support for VLSM (variable length subnet mask) and adds the Diffusing Update Algorithm (DUAL) in order to improve routing and provide a loopless environment. EIGRP has completely replaced IGRP, making IGRP an obsolete routing protocol. In Cisco IOS versions 12.3 and greater, IGRP
525-433: The new value "B to A = 2 + 1". Later on, B forwards the update to C and due to the fact that A is reachable through B (From C's point of view), C decides to update its table to "C to A = 3 + 1". This slowly propagates through the network until it becomes infinity (in which case the algorithm corrects itself, due to the relaxation property of Bellman-Ford). RIP uses the split horizon with poison reverse technique to reduce
SECTION 20
#1732802399424550-586: The number of hops to 15. This hop limit was introduced to avoid routing loops, but also means that networks that are connected through more than 15 routers are unreachable. The distance-vector protocol designed for use in wide area networks (WANs) is the Border Gateway Protocol (BGP). BGP is an exterior gateway protocol and therefore implemented on border and exterior routers on the Internet . It exchanges information between routers through
575-460: The route on which a packet will be sent by the next hop which is the exit interface of the router and the IP address of the interface of the receiving router. Distance is a measure of the cost to reach a certain node. The least cost route between any two nodes is the route with minimum distance. Updates are performed periodically in a distance-vector protocol where all or part of a router's routing table
600-585: The routing tables of each router converge to stable values. Some of these protocols have the disadvantage of slow convergence. Examples of distance-vector routing protocols: Routers that use distance-vector protocol determine the distance between themselves and a destination. The best route for data through a data network is measured in terms of the numbers of routers (hops) a packet has to pass through to reach its destination network. Additionally, some distance-vector protocols take into account other traffic information, such as network latency . To establish
625-457: The table since they refer to distances from a node to itself, or via itself. For example: A receives a DV from C that tells A there is a path via C to D, with a distance (or cost) of 5. Since the current "shortest-path" to C is 23, then A knows it has a path to D that costs 23+5=28. As there are no other shorter paths that A knows about, it puts this as its current estimate for the shortest-path from itself (A) to D, via C. For instance: A receives
#423576