IP Routing

Rate the Trustworthiness of a Routing Information Source

As I discussed in detail in Chapter 1, administrative weight is the trustworthiness of a routing information source. When a route is known via more than one source, the source with the lower administrative distance is installed in the routing table. The following subcommand:

distance weight [[ip-source-address ip-address-mask] [access-list-number]]

is used to specify administrative weight. Without the options, the command applies to all routes known via the routing protocol. Thus, in the following example, a distance of 10 is attached to all RIP-derived routes and a distance of 20 is attached to all EIGRP-derived routes:

hostname R
...
router rip
distance 10
...
router eigrp 109
distance 20

You can use the optional filters to attach an administrative weight to only the routes derived from routing sources that pass the filters. Thus, the distance command in the following code:

router rip
network 11.0.0.0
distance 160 11.1.1.0 0.0.0.255

attaches a distance of 160 to RIP routes derived from sources in the IP address range 11.1.1.0 through 11.1.1.255. The use of the optional filters in the distance command is discouraged; unless it is carefully planned, it can cause problems with routing loops.