distance
  • Classful/classless capabilities of the protocols
    • These factors will be discussed in various sections of this module.
    Content 5.1 Operating a Network Using Multiple Routing Protocols 5.1.4 Using Seed Metrics Each routing protocol defines a metric for each route. The metric value determines the shortest or “best” path to an IP network. When a router redistributes routes from one routing domain to another, this information cannot be translated from one routing protocol to another. For example, a RIP hop cannot be dynamically recalculated to an OSPF cost by the router doing redistribution. Therefore, a seed metric artificially sets the distance, cost, and so on, to each external (redistributed) network from the redistribution point. Seed Metrics Example
    For example, if a boundary router receives a RIP route, the route has a hop count as a metric. To redistribute the route into OSPF, the router must translate the hop count into a cost metric that the OSPF router understands. This seed metric, also referred to as the default metric, is defined during redistribution configuration. When the seed metric for a redistributed route is established, the metric increases in increments normally within the autonomous system. Note
    The exception to this rule is OSPF E2 routes, which hold their initial metric regardless of how far they are propagated across an autonomous system. The default-metric command, used in routing process configuration mode, establishes the seed metric for all redistributed routes. Cisco routers also allow the seed metric to be specified as part of the redistribution command, either with the metric option or by using a route map. Whichever way it is done, the initial seed metric should be set to a value larger than the largest metric within the receiving autonomous system to help prevent suboptimal routing and routing loops. Figure lists protocol names with the default seed metrics for the various protocols.
    Content 5.1 Operating a Network Using Multiple Routing Protocols 5.1.5 Seed Metrics Example Figure illustrates a seed metric of 30 implemented by OSPF on the redistributed RIP routes. The link cost of the Ethernet link to router D is 100. Therefore, the cost for networks 1.0.0.0, 2.0.0.0, and 3.0.0.0 on router D is the seed metric (30) plus the link cost (100) = 130. Notice that the metrics of the three networks in the RIP cloud are irrelevant in the OSPF cloud, because the objective is to have each OSPF router forward traffic for the three networks to the border (redistributing) router. A metric of infinity tells the router that the route is unreachable, and therefore, it should not be advertised. When redistributing routes into RIP and EIGRP, you must specify a default metric. For OSPF, the redistributed routes have a default type 2 metric of 20, except for redistributed BGP routes, which have a default type 2 metric of 1. For IS-IS, the redistributed routes have a default metric of 0. But unlike RIP or EIGRP, IS-IS does not treat a seed metric of 0 as unreachable. Configuring a seed metric for redistribution into IS-IS is recommended. For BGP, the redistributed routes maintain the IGP routing metrics.
    Content 5.1 Operating a Network Using Multiple Routing Protocols 5.1.6 Defining Administrative Distance Most routing protocols have metric structures and algorithms that are not compatible with other protocols. It is critical for a network using multiple routing protocols to have seamless exchange of route information and the ability to select the best path across multiple protocols. Cisco routers use a value called administrative distance to select the best path when they learn two or more routes to the same destination from different routing protocols. Administrative distance rates the believability of a routing protocol. Cisco has assigned a default administrative distance value to each routing protocol supported on its routers. Each routing protocol is prioritized in the order of most believable to least believable. Some examples of prioritization are as follows: In Figure , the table lists the default administrative distance of the protocols that Cisco supports. The administrative distance is a value between 0 and 255. The lower the administrative distance value, the higher the believability of the protocol. Note
    IGRP is no longer supported as of Cisco IOS Software Release 12.3. For example, in Figure , if router A receives a route to network 10.0.0.0 from RIP and receives a route to the same network from OSPF, the router compares the administrative distance of RIP (120) with the administrative distance of OSPF (110). The router determines that OSPF is more reliable and adds the OSPF version of the route to the routing table.

    Prefix Lengths
    Varying prefix lengths of routes from different routing protocols can also affect routing decisions. The prefix length is the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet, regardless of routing protocols. For example, assume that a router has four routing processes running on it, and each process has received these routes: Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, you might assume that the first one is installed. However, since each of these routes has a different prefix length (subnet mask), they are considered different destinations. Therefore, they are all installed in the routing table. If a packet arrives on a router interface destined for 192.168.32.1, which route would the router choose? It depends on the prefix length, or the number of bits set in the subnet mask. Longer prefixes are always preferred over shorter ones when forwarding a packet. In this case, a packet destined to 192.168.32.1 is directed toward 10.1.1.1, because 192.168.32.1 falls within the 192.168.32.0/26 network (192.168.32.0 to 192.168.32.63). It also falls within the other two routes available, but the 192.168.32.0/26 has the longest prefix within the routing table (26 bits versus 24 or 19 bits). Likewise, if a packet destined for 192.168.32.100 arrives on one of the router’s interfaces, it is forwarded to 10.1.1.2, because 192.168.32.100 does not fall within 192.168.32.0/26 (192.168.32.0 through 192.168.32.63), but it does fall within the 192.168.32.0/24 destination (192.168.32.0 through 192.168.32.255). Again, it also falls into the range covered by 192.168.32.0/19, but 192.168.32.0/24 has a longer prefix length. Interactive Media Activity Drag and Drop: Administrative Distance Upon completion of this activity, the student will be able to identify the administrative distances for all entries in a routing table. Web Links What is Administrative Distance
    http://www.cisco.com/warp/public/
    105/admin_distance.html
    Content 5.1 Operating a Network Using Multiple Routing Protocols 5.1.7 Modifying Administrative Distances In some cases, a router selects a suboptimal path if it believes a routing protocol with a better administrative distance, even though it is actually a routing protocol with a worse route. Assigning an undesired routing protocol a larger administrative distance ensures that routers select routes from the desired routing protocol. You can use the distance command to change the default administrative distance for all protocols, except EIGRP and BGP. Figure describes the command parameters. For EIGRP, use the distance eigrp command. EIGRP assigns different