What is the administrative distance of internal EIGRP routes by default?
Correct Answer: A
Detailed Explanation: Administrative distance (AD) is a value used by routers to select the best path when there are multiple routes to the same destination learned from different routing protocols. It is essentially a measure of trustworthiness: the lower the AD, the more reliable the route is considered. Each routing protocol is assigned a default AD value. EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco proprietary routing protocol that uses a composite metric (based on bandwidth, delay, reliability, load, and MTU) to determine the best route. For internal EIGRP routes—the routes learned from routers within the same autonomous system—the default administrative distance is 90. This value is lower than many other routing protocols, such as OSPF (which has an AD of 110) and RIP (which has an AD of 120), meaning that internal EIGRP routes are generally preferred over these other protocols when multiple routes exist. Understanding the concept of administrative distance is critical for network troubleshooting and design. When a router has learned the same destination network via different routing protocols, it uses the administrative distance to decide which route to install in the routing table. For example, if a router learns about a route to a particular network through both EIGRP and OSPF, it will choose the EIGRP route because its AD of 90 is lower than OSPF’s 110. This mechanism ensures that the most reliable and preferred routes are selected, thereby improving network stability and performance. It is also important to note that EIGRP has a different administrative distance for external routes (routes learned from outside the EIGRP autonomous system), which by default is higher (typically 170). This higher AD for external routes reflects the lower level of trust compared to internal routes. Network professionals preparing for the CCNA exam should be comfortable with these default values as they frequently appear in troubleshooting scenarios and design questions. Adjustments to the administrative distance can be made on Cisco routers if a network designer wishes to influence the route selection process further. However, the default values (90 for internal EIGRP, 170 for external EIGRP, 110 for OSPF, and 120 for RIP) are the ones most commonly encountered and are foundational to understanding how routers make decisions about route selection. In summary, internal EIGRP routes have a default administrative distance of 90, making them a preferred choice over routes from protocols with higher AD values. This preference helps maintain optimal routing decisions in a multi-protocol environment.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.