Explaining Routing Administrative Distance (AD) and Metric

A network may implement multiple routing protocols, allowing routers within the network to acquire route information from various sources. In situations where several paths are available, routers must determine the most optimal route. To facilitate this selection process, routers utilize an administrative distance value, where a lower number indicates a more favorable route. For instance, if a route is obtained from both RIP and EIGRP, a Cisco router will prefer the EIGRP route and record it in the routing table. This preference arises from the fact that EIGRP has a default administrative distance of 90, whereas RIP has a higher administrative distance of 120.

To view the administrative distance of all routes on your router, you can execute the command: show ip route.

Router_A#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets
D 10.0.0.0 [90/30720] via 192.168.0.2, 00:00:09, FastEthernet0/0
C 192.168.0.0/24 is directly connected, FastEthernet0/0
Router_A#

In the output above, the router possesses only one entry in its routing table, which has been acquired through a dynamic routing protocol, specifically the EIGRP route.

The following table lists the administrative distance default values:

When a router acquires two distinct paths to the same network through the same routing protocol, it must determine which route is superior to be included in the routing table. The metric serves as the criterion for evaluating the quality of each route, with a lower value indicating a more favorable option. Each routing protocol employs its specific metric; for instance, RIP utilizes hop counts as its metric, whereas OSPF relies on cost.

The following example explains why RIP calculates its metric and why it chooses one path over another.

All routers have been configured with RIP. Router 1 has two routes to access the subnet 10.0.0.0/24. One route traverses Router 2, while the alternative route goes through Router 3 and subsequently Router 4. Since RIP relies on hop count as its metric, the route via Router 2 will be selected to reach the 10.0.0.0/24 subnet, as it is only one router away. In contrast, the other route will have a higher metric of 2, given that the subnet is two routers away.

The subsequent table outlines the parameters utilized by various routing protocols to determine their metrics.

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