If a routing table contains a static route, a RIP route, and an EIGRP route to the same destination network, which route will the Cisco router use by default to forward packets?

Introduction / Context:
When multiple routes to the same destination exist in a Cisco router, the device must decide which one to install in the routing table. This decision is made first using administrative distance, which represents how trustworthy a source of routing information is. Static routes, dynamic distance vector protocols such as RIP, and advanced protocols such as EIGRP all have different default administrative distances. Understanding this hierarchy is crucial for designing deterministic routing behaviour and for troubleshooting unexpected paths in a network.

Given Data / Assumptions:

• The router has three routes for the same destination prefix.
• One route is a static route.
• One route is learned from RIP.
• One route is learned from EIGRP.
• No special administrative distance changes have been configured manually.

Concept / Approach:
Cisco uses administrative distance to choose between routes learned from different sources. The lower the administrative distance, the more preferred the route. Default values are well known: connected routes are 0, static routes are 1, Enhanced Interior Gateway Routing Protocol internal routes are 90, Open Shortest Path First routes are 110, and Routing Information Protocol routes are 120. Therefore, if the router has a static route and dynamic routes to the same destination and no distances have been changed, the static route is chosen because it has the lowest administrative distance value of 1.

Step-by-Step Solution:
1. List the administrative distances for each route source: - Static route: distance 1. - EIGRP internal route: distance 90. - RIP route: distance 120. 2. The router compares the administrative distance values for the three competing routes. 3. The smallest value represents the most preferred route. In this case, 1 is smaller than 90 and 120. 4. Therefore, the static route is installed in the routing table and used to forward traffic to the destination network by default.

Verification / Alternative check:
You can confirm this behaviour by configuring a static route alongside RIP and EIGRP on a lab router and then using show ip route. The route for the destination will be marked with the code S for static. If you change the administrative distance of the static route using the ip route command with an added distance parameter that is greater than 90, the router will then prefer the EIGRP route. This experiment demonstrates that administrative distance is the primary factor when choosing between different routing sources.

Why Other Options Are Wrong:
Any available route because all are equal: They are not equal, because each routing source has a different default administrative distance value.
RIP route because distance vector protocols are preferred: There is no general preference for distance vector over other protocols. RIP also has a higher distance value than EIGRP and static routes.
EIGRP route because it has the most advanced metric: While EIGRP uses an advanced composite metric, the router only considers the metric after it has chosen the protocol based on administrative distance. Static routes with distance 1 override both dynamic protocols by default.
None, the router will drop the traffic: A route is present, so the router will forward packets according to the best available route rather than dropping them.

Common Pitfalls:
Many learners initially assume that the most sophisticated protocol, such as EIGRP or OSPF, will always be chosen. Others mistakenly believe that path metric alone decides between routing protocols. In reality, administrative distance is applied first across protocols, and then metrics and other tie breakers are applied within the selected protocol. To avoid confusion, remember the key default values and understand that static routes are generally preferred over dynamic routes unless you intentionally alter their administrative distance.

Final Answer:
Static route because it has the lowest administrative distance