Cisco Systems CCNA 2 Defining a maximum count, Eliminating routing loops through split horizon

7.1.3 Defining a maximum count

The previous section described a situation in which slow convergence created the impression that a fictitious path to a network existed, which leads to a routing loop. Routing loops have a packet that circles a network, uses up bandwidth, and never reaches its destination. Distance vector algorithms are designed to prevent these loops by defining a maximum hop count. This value is known as a routing metric. A metric is the criteria used by a router to determine the best path to a destination network. Metrics vary for different protocols. Some protocols such as RIP use only the metric of hop counts. Other routing protocols may use bandwidth, delay, and other factors. If the only metric used by a routing protocol is hop count then a router makes its path determination decisions based on the lowest number of routers that a packet will have to pass through to reach its destination.

The maximum hop count value defines how many routers a packet can pass through before the destination network is unreachable. Each time a packet passes through a router the distance number is increased. When the default or defined maximum is reached the network is considered unreachable and the looping ceases. A non-technical example is a timed test. If someone takes a timed test they have a predefined amount of time to complete it. When the maximum amount of time has been reached, the test ends even if some questions have not been answered.

7.1.4 Eliminating routing loops through split horizon

Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that it sent. The following example explains how this problem occurs:

1.Router A passes an update to Router B and Router D that indicates that Network 1 is down. Router C transmits an update to Router B that indicates that Network 1 is available at a distance of four, by way of Router D. This does not violate split-horizon rules.

2.Router B incorrectly concludes that Router C still has a valid path to Network 1 at a less favorable metric. Router B sends an update to Router A to inform Router A of the new route to Network 1.

3.Router A now determines that it can send to Network 1 by way of Router B, Router B determines that it can send to Network 1 by way of Router C, and Router C determines that it can send to Network 1 by way of Router D. Any packet that is introduced into this environment will loop between routers.

4.Split-horizon attempts to avoid this situation. As shown in Figure [1], if a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split-horizon reduces incorrect routing information and reduces routing overhead. [2]

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