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Interior Gateway Protocols: Distance Vector
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The first distance vector protocol was the Routing Information Protocol, (RIP) The discussion that follows on RIP fully explains how a distance vector routing protocol works We will review the generic distance vector algorithm, followed by discussion of RIP version 1 as the first protocol of this type
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The distance vector algorithm (sometimes referred to as the Bellman-Ford algorithm for the people who invented it), requires each machine involved in the routing process to keep track of its "distance" from all other possible destinations What happens is that a router is programmed to construct a table of all possible destinations (in terms of network numbers) and measure the distance to each location The only information a router has at boot time is what networks are directly attached to it So how can it find out about all other possible destinations Each router is programmed so that it announces all the destinations it knows about, along with the distance to each Once a router starts hearing all the advertisements from other routers, it can start to list all the possible destinations and calculate the shortest path to these destinations the one kept in the routing table So a distance vector protocol will, on a regular basis, send out an update that contains all the information contained in the machine's routing table These updates are sent only to neighboring routers on directly connected segments One of the key differences between distance vector and link state routing is that link state protocols send information to routers on remote segments Once these updates get to a router, the router will then use a collection of algorithms and timers to decide what entries should be put into the routing table of the machine that is receiving the updates Another key aspect of distance vector protocols is that any given machine knows only the next hop in the sequence to deliver the packet to its ultimate destination In link state protocols, each machine has a complete map of the network it is in (or, more accurately, the routing area it is in but we will come to that later) To finish this introduction, let's take a high-level overview of the code behind the distance vector algorithm: 1Each router has a unique identification on the internetwork 2Every router will identify a metric to be associated with each link directly connected to itself 3All routers will start advertising directly connected links with a metric of zero 4Every router transmits to neighbors the complete information regarding all destinations and their metric when the router first boots, then periodically, and whenever a router becomes aware of a change 5Each router determines which directly connected neighbor offers the lowest metric to each l i
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RIP: The Routing Information Protocol
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The first Interior Gateway Protocol (IGP) was the Routing Information Protocol, or "RIP" as it became known RIP was designed for an environment that had only a relatively small number of machines, and these machines were connected with links that had identical characteristics As the first IGP, RIP gained widespread use and was distributed free with BSD Unix as the routed daemon process As today's heterogeneous networks grow and become more diverse, RIP has been improved upon with more modern, more full-featured distance vector protocols RIP is still in widespread use and is the only routing protocol that Unix machines universally understandOkay, before we get into the workings of RIP, let's refresh our memories regarding what a routing table looks like To display a Cisco router's routing table, do the following: Codes:C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default Gateway of last resort is not set 160400 255255255224 is subnetted, 2 subnets C 1604132 is directly connected, Ethernet0 R 1604164 [120/1] via 1604134, 00:00:12, Ethernet0 In the first column, the router tells us how it learned about the routing table entries The legend at the top of the display explains what these abbreviations mean For example, C means that the network is directly connected to the router on the specified interface S stands for static route and means that the route to this destination network is hard-coded into the router's configuration R means that the router learned of this destination network via a RIP update The rest of the entry in the router's routing table states which interface to use for the specified network, and which router to send the packet to next if the destination network is on another part of the internetwork In this case, the RIP entry is telling us to use the router with address 1604134 in order to get to the 1604164subnet, and that this router is connected to the Ethernet 0 interface Let's look at the rest of this entry in more detail The [120/1] entry in the RIP line first gives the value of the administrative distance, and then the metric The administrative distance is a value assigned to routing protocols RIP has a value of 120, OSPF of 110, IGRP of 100, EIGRP of 90, static of 1, and directly connected of 0
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This administrative distance is used if two routing protocols advertise the same route to the same router In this case, the router will prefer the route supplied by the routing protocol with the lowest value for administrative distance The logic behind this is that, because Cisco routers give you the
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routing protocols are not comparable; RIP only goes up to a metric of 16, while IGRP goes to more than 16 million, so this other measure is used to show preference for the routing protocols in use EIGRP is most preferred, IGRP next, then OSPF and finally RIP The 00:00:12 value is a counter that indicates the amount of time that has passed since the router received an advertisement for this route We will now discuss how RIP handles its responsibilities as a routing protocol in more detail The Basics of How RIP Works Once configured on an internetwork, the operation of RIP and its interaction with the routing table is fairly straightforward By default, every 30 seconds each RIPenabled device sends out a RIP update message, comprising routing information from the machine's routing table This message includes the following: Destination address of host or network The IP address of the gateway sending the update A metric that indicates the distance (in terms of hops) to the destination It is worth noting that interfaces on a Unix machine or a router can be defined as passive If that is done, the interface in question does not send out any RIP updates; it merely listens for updates from other machines Once a routing device receives an update, it processes the new information, which it compares with that in the existing routing table If the routing update includes a new destination network, it is added to the routing table If the router receives a route with a smaller metric to an existing destination, it replaces the existing route If an entry in the update message has the same destination network and gateway but a different metric, it will use the new metric to update the routing table This covers how routers handle a static network using RIP; some additions to the protocol are in place to handle changes in topology, such as a downed link If the preceding were the only logic coded into a router, it would not recover from a downed link because it remembers only the best route to any given destination If the gateway, or link to that gateway, should fail, the routing table might never reflect the change That's because, so far, the logic defined depended upon a gateway notifying its neighbors of whether its metrics had changed If the gateway could no longer communicate, it could not notify its neighbors of a change To handle such situations, RIP employs timers: It sends out messages (every 30 seconds by default), and the protocol assumes that if a gateway or specific route is not heard from within 180 seconds, it no longer is available Once it determines that a route is unavailable, the router sends a special message that notifies its neighbors of the unavailable route After 270 seconds, if nothing is heard from the gateways or route, this information is flushed from the router's routing table These timers are known as the update, invalid, and flush timers, respectively
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To understand fully how a router deals with link or other failures on the internetwork, we need to explore other issues, starting with the count to infinity problem With a router that uses RIP as its
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explore other issues, starting with the count to infinity problem With a router that uses RIP as its routing protocol, "infinity" turns out to be 16 Clearly, some further explanation is needed here RIP's interpretation of infinity as the number 16 relates to the Time To Live (TTL) field in the IP layer header Each time a packet travels through a router, its TTL field (with initial value of 15) is decreased by one When the TTL value reaches 0, the packet is discarded and no longer exists in the internetwork This feature is there to stop a packet caught in a routing loop from being switched back and forth forever between routers Obviously we want the TTL value to be high enough to allow us to send correctly routed packets through whatever network size we want to implement, but small enough so that packets are not kept in a routing loop for too long In a RIP-based network, this value is fixed at 15 With IGRP, we can set the value to whatever we like Let's see how this situation of a circulating packet can occur, and what features in RIP have been implemented to minimize its occurrence Referring to Fig 4-2, consider the situation wherein a PC on network 3 needs to send a message to a PC on network 1
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