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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

A core router receives a packet destined for 192.168.10.45192.168.10.45. The router's Routing Information Base (RIB) contains four distinct candidate routes for reaching this destination network:

- A static route to 192.168.10.0/24192.168.10.0/24 configured with an administrative distance of 130130.
- An OSPF route to 192.168.10.0/24192.168.10.0/24 with an administrative distance of 110110 and metric of 5050.
- An EIGRP internal route to 192.168.10.0/24192.168.10.0/24 with an administrative distance of 9090 and metric of 2560025600.
- A RIPv2 route to 192.168.10.0/24192.168.10.0/24 with an administrative distance of 120120 and metric of 22.

Which route will the router install into its active forwarding table to handle traffic destined for 192.168.10.45192.168.10.45?

  1. The EIGRP internal route because it possesses the lowest administrative distance value among all candidate routes.Answer
  2. B
    The RIPv2 route because its hop-count metric of 2 represents the lowest numerical metric among the dynamic routing protocols.
  3. C
    The static route because administrator-configured static routes always take precedence over dynamic protocols regardless of AD values.
  4. D
    The OSPF route because link-state protocols are prioritised over distance-vector protocols at Layer 3.

Answer

The router will select the EIGRP internal route because it has the lowest administrative distance (9090) among all routes advertising the exact same prefix mask length (192.168.10.0/24192.168.10.0/24).
When a router receives multiple routing updates for the exact same destination network prefix (192.168.10.0/24192.168.10.0/24), it uses Administrative Distance (AD) to evaluate which protocol source is most trustworthy. EIGRP internal routes have a default AD of 9090, which is lower (and thus preferred) over OSPF (110110), RIPv2 (120120), and the explicitly configured static route (130130).

Step-by-Step Solution

1
Evaluate Longest Prefix Match (LPM)
All candidate routes advertise the exact same network prefix mask (192.168.10.0/24192.168.10.0/24), so prefix length comparison results in a tie.
Routers check prefix length specificity first before comparing routing source trustworthiness.
2
Compare Administrative Distance (AD) across competing protocols
EIGRP internal AD (9090) < OSPF AD (110110) < RIPv2 AD (120120) < Custom Static AD (130130).
Administrative Distance defines the trustworthiness of a route source; lower AD numbers indicate higher priority.
3
Select winning route for Forwarding Information Base (FIB)
The EIGRP route is selected and installed into the active routing table.
Metrics are only compared between routes learned from the exact same routing protocol instance, not across different protocols.

Key Concept

Administrative Distance Precedence in Route Selection
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Topics

Routing Concepts and Dynamic Routing Protocols

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