Cisco routers can also differentiate between IP routes based on the administrative distance. By adjusting the administrative distance, the administrator can implement a routing policy. This policy may be used during migration from one routing protocol to another or when multiple protocols exist in the network. Another use of the administrative distance is floating static routes, which are frequently used to supply a route when the routing protocol or link fails. Under these conditions, the static route is normally used with a DDR (dial-on-demand routing) circuit, and the administrator assigns a higher administrative distance to the static route than would be found with the dynamic protocols; once the dynamic routing protocols have exhausted all their routes, or the protocol has failed due to link failure, the highest administrative distance is the static route. Table 3.4 documents the administrative distances associated with various route sources. Note that by default a static route will supersede a dynamic routing protocol.

The administrative distance is set with the distance command. The highest value is 255, and it is placed on each interface.

The router will select routes based on their administrative distance before considering the routing metric. This is an important consideration in both design and troubleshooting as the router may not act as expected—in actuality, it is doing exactly what it was told. This issue is particularly common in route redistribution. Designers employ route redistribution when a routing protocol’s information must be propagated via another routing protocol. For example, the designer would use redistribution to transfer RIP routes into OSPF (Open Shortest Path First).

Selecting a Routing Protocol

One of the considerations novice network designers frequently forget is the selection of a routing protocol for IP. As a result, many networks begin with RIP version 1, and this installation remains in the network.

The following list presents some of the criteria for selecting a routing protocol:

•  Support for variable-length subnet masks (VLSM)

•  Network convergence time

•  Support for discontiguous subnets

•  Interoperability with existing hosts, servers, and routers

•  Scalability to support existing and future needs

•  Consideration for standards-based protocols

•  Interoperability with autonomous systems and redistribution

•  Usage of a small amount of bandwidth

•  Adaptability to changes in the network as implemented

Routing protocols also incorporate characteristics that may require additional consideration. For example, connections likely fit into one of the following three types:

•  Host-to-router

•  Router-to-router

•  Autonomous system-to-autonomous system

Host connections may obtain router information using a number of methods. These methods include:

•  A preconfigured gateway address on the host.

•  Use of the Proxy Address Resolution Protocol. Proxy ARP is also called the ARP hack, and it is enabled by default. It typically adds unnecessary broadcast traffic to the network. Proxy ARP routers will respond to ARPs for off-network resources and will make the original host believe that the remote host is local.

•  Use of the ICMP (Internet Control Message Protocol) Router Discovery Protocol (IRDP).

•  Use of the Gateway Discovery Protocol (GDP).

•  The previous items in concert with Cisco’s Hot Standby Router Protocol (HSRP).

•  RIP on the host, preferably in passive mode.

Router-to-router connections are typically called interior routes and use interior routing protocols such as RIP, OSPF, IGRP, or EIGRP. The routes will all be contained within one autonomous system. Connections between autonomous systems are referred to as exterior routes and use exterior routing protocols. The most common exterior gateway protocol is eBGP. Note the small e, denoting the exterior implementation of the protocol. eBGP, also called BGP, is aptly defined as the routing protocol of the Internet.

It is important to note that classless routing protocols, such as EIGRP, look for the longest, or most specific, match when evaluating a route. This is also true for classful routing protocols. However, the designer must bear in mind that the mask for these routes must remain consistent. The router will assume the natural mask or the interface’s mask.

Consider a router processing a packet destined for host 10.12.24.48. The following routes would be selected in order of appearance, as reflected in Table 3.5.