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50-398: 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. RIP implements

100-525: 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 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

150-407: A distance-vector routing protocol 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

200-536: A network is based on link cost, and is implemented through link-state routing protocols . The term distance vector refers to 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

250-517: A request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their routing table . The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update

300-412: A router advertises updated information from its routing table. This process continues until 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

350-488: A router requests and processes neighbouring routing tables, and keeps its routing table and hop count for reachable networks up to date, but does not needlessly send its own routing table into the network. Silent mode is commonly implemented to hosts. RIPv1 uses classful routing. The periodic routing updates do not carry subnet information, lacking support for variable length subnet masks (VLSM). This limitation makes it impossible to have different-sized subnets inside of

400-448: A significant increase in convergence times. More recently, 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

450-467: A single composite metric for each route, from a formula of five variables: bandwidth , delay , reliability , load , and MTU ; though on Cisco routers, by default, only bandwidth and delay are used in this calculation. Distance-vector routing protocol Distance-vector routing protocols use the Bellman–Ford algorithm to calculate the best route. Another way of calculating the best route across

500-422: A small random time variable to the update time, to avoid routing tables synchronizing across a LAN. It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice. Sally Floyd and Van Jacobson showed in 1994 that, without slight randomization of the update timer, the timers synchronized over time. RIPv1 can be configured into silent mode, so that

550-448: A valid direction in its table. Red indicates invalid entries in 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

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600-565: Is an obsolete network addressing architecture used in the Internet from 1981 until the introduction of Classless Inter-Domain Routing (CIDR) in 1993. The method divides the IP address space for Internet Protocol version 4 (IPv4) into five address classes based on the leading four address bits. Classes A, B, and C provide unicast addresses for networks of three different network sizes. Class D

650-612: Is assigned the reserved port number 520. Based on the Bellman–Ford algorithm and the Ford–Fulkerson algorithm , distance-vector routing protocols started to be implemented from 1969 onwards in data networks such as the ARPANET and CYCLADES . The predecessor of RIP was the Gateway Information Protocol (GWINFO) which was developed by Xerox in the mid-1970s to route its experimental network. As part of

700-412: Is for multicast networking and the class E address range is reserved for future or experimental purposes. Since its discontinuation, remnants of classful network concepts have remained in practice only in limited scope in the default configuration parameters of some network software and hardware components, most notably in the default configuration of subnet masks . In the original address definition,

750-514: Is not suitable for large networks as it limits 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

800-481: 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 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

850-412: The ARPANET (network number 10), and before the wide proliferation of local area networks (LANs). As a consequence of this architecture, the address space supported only a low number (254) of independent networks. Before the introduction of address classes, the only address blocks available were these large blocks which later became known as Class A networks. As a result, some organizations involved in

900-428: The Bellman–Ford algorithm . In these protocols, each router does not possess information about 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,

950-422: The Internet . It exchanges information between routers through 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

1000-701: The Xerox Network Systems (XNS) protocol suite GWINFO transformed into the XNS Routing Information Protocol. This XNS RIP in turn became the basis for early routing protocols, such as Novell 's IPX RIP, AppleTalk 's Routing Table Maintenance Protocol (RTMP), and the IP RIP. The 1982 Berkeley Software Distribution of the UNIX operating system implemented RIP in the routed daemon . The 4.2BSD release proved popular and became

1050-403: The split horizon with poison reverse technique to reduce 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

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1100-426: The split horizon , route poisoning , and holddown mechanisms to prevent incorrect routing information from being propagated. In RIPv1 routers broadcast updates with their routing table every 30 seconds. In the early deployments, routing tables were small enough that the traffic was not significant. As networks grew in size, however, it became evident there could be a massive traffic burst every 30 seconds, even if

1150-466: The 3 high-order bits set to 1, 1, and 0, and designating the next 21 bits to number the networks, leaving each network with 256 local addresses. The leading bit sequence 111 designated an at-the-time unspecified addressing mode (" escape to extended addressing mode "), which was later subdivided as Class D ( 1110 ) for multicast addressing, while leaving as reserved for future use the 1111 block designated as Class E. This architecture change extended

1200-480: The Internet Protocol. It divided the address space into primarily three address formats, henceforth called address classes , and left a fourth range reserved to be defined later. The first class, designated as Class A , contained all addresses in which the most significant bit is zero. The network number for this class is given by the next 7 bits, therefore accommodating 128 networks in total, including

1250-494: The address 224.0.0.9 , as opposed to RIPv1 which uses broadcast . Unicast addressing is still allowed for special applications. ( MD5 ) authentication for RIP was introduced in 1997. Route tags were also added in RIP version 2. This functionality allows a distinction between routes learned from the RIP protocol and routes learned from other protocols. RIPng (RIP next generation) is an extension of RIPv2 for support of IPv6 ,

1300-625: The address itself; any network device would inspect the first few bits of the IP address to determine the class of the address and thus its netmask. The blocks numerically at the start and end of classes A, B and C were originally reserved for special addressing or future features, i.e., 0.0.0.0 / 8 and 127.0.0.0 / 8 are reserved in former class A; 128.0.0.0 / 16 and 191.255.0.0 / 16 were reserved in former class B but are now available for assignment; 192.0.0.0 / 24 and 223.255.255.0 / 24 are reserved in former class C. While

1350-500: The addressing capacity of the Internet but did not prevent IP address exhaustion . The problem was that many sites needed larger address blocks than a Class C network provided, and therefore they received a Class B block, which was in most cases much larger than required. Due to the rapid growth of the Internet, the pool of unassigned Class B addresses (2 , or about 16,000) was rapidly being depleted. Starting in 1993, classful networking

1400-404: The basis for subsequent UNIX versions, which implemented RIP in the routed or gated daemon. Ultimately, RIP had been extensively deployed before the standard, written by Charles Hedrick, was passed as RIPv1 in 1988. The routing metric used by RIP counts the number of routers that need to be passed to reach a destination IP network. The hop count 0 denotes a network that is directly connected to

1450-400: The current time (or iteration) in 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

1500-447: The distance-vector protocols that have been described as 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

1550-544: The early development of the Internet received very large address space allocations (16,777,216 IP addresses each). Expansion of the network had to ensure compatibility with the existing address space and the IPv4 packet structure, and avoid the renumbering of the existing networks. The solution was to expand the definition of the network number field to include more bits, allowing more networks to be designated, each potentially having fewer hosts. Since all existing network numbers at

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1600-448: The hop count limit of 15 remained. RIPv2 has facilities to fully interoperate with the earlier specification if all Must Be Zero protocol fields in the RIPv1 messages are properly specified. In addition, a compatibility switch feature allows fine-grained interoperability adjustments. In an effort to avoid unnecessary load on hosts that do not participate in routing, RIPv2 multicasts the entire routing table to all adjacent routers at

1650-436: 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 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

1700-416: The information they receive from other routers and cannot determine if 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

1750-413: The metric between the routers be "number of jumps". Now suppose that A 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

1800-401: The most significant eight bits of the 32-bit IPv4 address was the network number field which specified the particular network a host was attached to. The remaining 24 bits specified the local address, also called rest field (the rest of the address), which uniquely identified a host connected to that network. This format was sufficient at a time when only a few large networks existed, such as

1850-574: The next generation Internet Protocol. The main differences between RIPv2 and RIPng are: RIPng sends updates on UDP port 521 using the multicast group ff02::9 . RIP messages use the User Datagram Protocol on port 520 and all RIP messages exchanged between routers are encapsulated in a UDP datagram. RIP defined two types of messages: The routing information protocol uses the following timers as part of its operation: Cisco 's proprietary Interior Gateway Routing Protocol (IGRP)

1900-443: The path from C to A is through itself (B), it updates its table with 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

1950-430: The reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that

2000-493: The router. 16 hops denote a network that is unreachable, according to the RIP hop limit. There are three standardized versions of the Routing Information Protocol: RIPv1 and RIPv2 for IPv4 , and RIPng for IPv6 . The original specification of RIP was published in 1988. When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation broadcasts to 255.255.255.255

2050-413: The routers had been initialized at random times. In most networking environments, RIP is not the preferred choice of routing protocol , as its time to converge and scalability are poor compared to EIGRP , OSPF , or IS-IS . However, it is easy to configure, because RIP does not require any parameters, unlike other protocols. RIP uses the User Datagram Protocol (UDP) as its transport protocol, and

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2100-507: The same network class . In other words, all subnets in a network class must have the same size. There is also no support for router authentication, making RIP vulnerable to various attacks. Due to the deficiencies of the original RIP specification, RIP version 2 (RIPv2) was developed in 1993, published in 1994, and declared Internet Standard 56 in 1998. It included the ability to carry subnet information, thus supporting Classless Inter-Domain Routing (CIDR). To maintain backward compatibility,

2150-571: The same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets. RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds. The RIPv1 protocol adds

2200-445: The shortest path across a network purely on the basis of 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

2250-454: The shortest-path from itself (A) to D, via C. For instance: A receives 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. Classful address A classful network

2300-409: The subtraction of 2 adjusts for the use of the all-bits-zero host value to represent the network address and the all-bits-one host value for use as a broadcast address. Thus, for a Class C address with 8 bits available in the host field, the maximum number of hosts is 254. Today, IP addresses are associated with a subnet mask . This was not required in a classful network because the mask was implied by

2350-402: The time were smaller than 64, they had only used the 6 least-significant bits of the network number field. Thus it was possible to use the most-significant bits of an address to introduce a set of address classes while preserving the existing network numbers in the first of these classes. The new addressing architecture was introduced by RFC   791 in 1981 as a part of the specification of

2400-402: The zero network, and including the IP networks already allocated. A Class B network was a network in which all addresses had the two most-significant bits set to 1 and 0 respectively. For these networks, the network address was given by the next 14 bits of the address, thus leaving 16 bits for numbering host on the network for a total of 65 536 addresses per network. Class C was defined with

2450-558: Was a somewhat more capable protocol than RIP. It belongs to the same basic family of distance-vector routing protocols . Cisco has ceased support and distribution of IGRP in their router software. It was replaced by the Enhanced Interior Gateway Routing Protocol (EIGRP) which is a completely new design. While EIGRP still uses a distance-vector model, it relates to IGRP only in using the same composite routing metric. Both IGRP and EIGRP calculated

2500-425: Was replaced by Classless Inter-Domain Routing (CIDR), in an attempt to solve this problem. Under classful network addressing, the 32-bit IPv4 address space was partitioned into five classes (A-E) as shown in the following tables. In the following bit-wise representation, The number of addresses usable for addressing specific hosts in each network is always 2 - 2 , where N is the number of rest field bits, and

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