How does a Cisco router choose the best path when multiple routes to a destination exist? Learn how administrative distance and metric work together to determine route selection—essential for Cisco Certified Field Technician (CCT) exam success.
Question
A router has multiple paths to a destination network. What determines which path is chosen?
A. Interface bandwidth
B. Routing table priority
C. Next-hop IP address
D. Administrative distance and metric
E. Source MAC address
Answer
D. Administrative distance and metric
Explanation
The administrative distance and metric determine the best path when multiple routes exist.
When a Cisco router has multiple paths to a destination network, it uses administrative distance and metric to determine which path to choose.
Administrative Distance (AD)
AD is a value that rates the trustworthiness of a route source (such as a routing protocol or static route). When multiple routing protocols offer routes to the same destination, the router selects the route with the lowest administrative distance as the most preferred and installs it in the routing table. For example, a directly connected route (AD 0) is preferred over a static route (AD 1), which is preferred over OSPF (AD 110), and so on.
Metric
If multiple routes to the same destination are learned via the same routing protocol (and thus have the same administrative distance), the router then compares the metric—a protocol-specific value representing the cost or desirability of a path (e.g., hop count for RIP, bandwidth and delay for EIGRP, cost for OSPF). The route with the lowest metric is chosen. If multiple routes have the same metric, the router may install multiple equal-cost paths and load balance between them.
Selection Process
- Longest Prefix Match: The router first checks for the most specific route (longest prefix).
- Administrative Distance: If multiple routes with the same prefix exist from different sources, the route with the lowest AD is selected.
- Metric: If multiple routes with the same prefix and AD exist, the route with the lowest metric is chosen.
Routers use administrative distance to select the most trustworthy route source, and metric to choose the best path among routes from the same protocol
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