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23.5 Routing Decision Criteria
Routing Decision Criteria Section 23: Implementing Dynamic Routing Multiple routes to a destination can exist. When a routing protocol algorithm updates the routing table, the primary objective of the algorithm is to determine the best route to include in the table. Each distance vector routing protocol uses a different routing metric to determine the best route. The algorithm generates a number called the metric value for each path through the network. With the exception of BGP, the smaller the metric, the better the path. Metrics can be calculated based on a single characteristic of a path. More complex metrics can be calculated by combining several path characteristics. The metrics that routing protocols most commonly use are as follows: Hop count: The number of routers that a packet passes through. Bandwidth: The data capacity of a link; for instance, normally, a 10Mb/s Ethernet link is preferable to a 64kb/s leased line. Delay: The length of time that is required to move a packet from the source to the destination. Load: The amount of activity on a network resource, such as a router or link. Reliability: Usually refers to the bit error rate of each network link. Cost: A configurable value that on Cisco routers is based by default on the bandwidth of the interface. The figure shows an example of multiple routes between two hosts and the way that different routing protocols compute metrics. OSPF can be configured to use bandwidth, delay, load, and reliability to calculate its metric but by default it uses only bandwidth and delay. Multiple routing protocols and static routes can be used at the same time. If there are several sources for routing information, an administrative distance value is used to rate the trustworthiness of each routing information source. By specifying administrative distance values, Cisco IOS Software can discriminate between sources of routing information. Administrative Distance Because multiple routing protocols can be active at the same time and may determine different "best" routes, the router needs a way to determine which routing protocol should be trusted the most. An administrative distance is an integer from 0 to 255. A routing protocol with a lower administrative distance is more trustworthy than one with a higher administrative distance. As shown in the figure, if router A receives a route to network 172.16.0.0, which is advertised by EIGRP and by OSPF at the same time, router A would use the administrative distance to determine that EIGRP is more trustworthy. Router A would then add the EIGRP route to the routing table because EIGRP is a more trusted routing source than OSPF. The administrative distance is an arbitrary value that Cisco IOS Software sets to handle cases when multiple routing protocols send information about the same routes. The table shows the default administrative distances for selected routing information sources. Routing Default Administrative Distances Route Source Default Distance Route Source Default Distance Connected network 0 Static route 1 EIGRP 90 OSPF 110 RIPv2 120 External EIGRP 170 Unknown or unbelievable 255 (will not be added to the routing table to pass traffic) If nondefault values are necessary, you can use Cisco IOS Software to configure administrative distance values on a perrouter, perprotocol, and perroute basis. Click the Play Button to watch a short video about metrics and administrative distance. Up Next: Understanding LinkState Routing Protocols