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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

An organization is configuring a core router that has multiple active routing protocols running simultaneously. The router receives two competing routes for the destination network 192.168.50.0/24. The first route is learned via OSPF with a metric of 45. The second route is learned via EIGRP (Internal) with a metric of 156800. Assuming default administrative distance values are used, which path will the router select for traffic destined to 192.168.50.0/24?

  1. The route learned via EIGRP, because internal EIGRP has a lower administrative distance than OSPF.Answer
  2. B
    The route learned via OSPF, because OSPF has a significantly lower route metric cost than EIGRP.
  3. C
    Both routes equally using Equal-Cost Multi-Path (ECMP) load balancing across both protocols.
  4. D
    Neither route, because competing routing protocols destination prefixes create an invalid loop condition.

Answer

The router will select the route learned via EIGRP because its default Administrative Distance (90) is lower than OSPF's default Administrative Distance (110).
When a router receives identical network destination prefixes from two different dynamic routing protocols, it compares their Administrative Distance (AD) values. Default AD for internal EIGRP is 90, and for OSPF it is 110. Because lower AD indicates higher trustworthiness, the router selects the EIGRP path regardless of raw metric values.

Step-by-Step Solution

1
Identify the destination prefixes and compare their subnet mask lengths.
Both protocols report the identical network prefix length (192.168.50.0/24), so longest prefix match does not eliminate either route.
Longest prefix match is always the first evaluation criteria when choosing between routes.
2
Compare the Administrative Distance (AD) of the competing dynamic routing protocols.
Internal EIGRP has a default AD of 90, while OSPF has a default AD of 110.
When different routing protocols advertise the exact same network prefix, the router uses Administrative Distance to determine source trustworthiness before comparing metrics.
3
Select the preferred route based on the lower Administrative Distance.
The EIGRP route (AD 90) is selected and placed into the IP routing table.
A lower Administrative Distance indicates a more trustworthy routing source.

Key Concept

Administrative Distance vs. Routing Metric in Route Selection
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Topics

Routing Concepts and Dynamic Routing Protocols

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