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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

An enterprise router receives two dynamic route updates for the destination network 172.16.40.0/24. The first update is received from OSPF with a metric cost of 45. The second update is received from RIPv2 with a hop count metric of 2. Default administrative distance values are configured across all protocols on the router. Which path will be installed into the active IP routing table?

  1. The OSPF route, because OSPF has a lower default administrative distance than RIPv2.Answer
  2. B
    The RIPv2 route, because a lower metric value takes precedence over administrative distance across different routing protocols.
  3. C
    The RIPv2 route, because RIPv2 processes network Layer 3 headers directly without requiring transport protocol encapsulation.
  4. D
    Both routes will be installed to perform load balancing between the OSPF and RIPv2 paths.

Answer

The OSPF route will be selected and installed into the routing table because OSPF has a lower default administrative distance (110) than RIPv2 (120).
When a router receives candidate routes to the exact same prefix length from two different dynamic routing protocols, it evaluates Administrative Distance (AD) first. By default, OSPF has an AD of 110 and RIPv2 has an AD of 120. The lower AD value indicates a more reliable route source, so the OSPF path is chosen for installation in the routing table.

Step-by-Step Solution

1
Identify the routing protocols providing updates for the target network
Two distinct protocols (OSPF and RIPv2) are offering paths to 172.16.40.0/24.
When multiple sources offer paths to the same destination network prefix, the router must decide which source is more trustworthy.
2
Compare the default Administrative Distance (AD) for both protocols
OSPF has a default AD of 110, while RIPv2 has a default AD of 120.
Administrative Distance ranks the reliability of routing protocols; lower numerical AD values indicate higher trustworthiness.
3
Determine which route is selected for routing table insertion
The OSPF route is selected due to its lower AD (110 vs 120).
Because AD resolves route selection between different protocols, protocol metric values (cost of 45 vs hop count of 2) are not compared against each other.

Key Concept

Administrative Distance in Multi-Protocol Routing Selection
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Topics

Routing Concepts and Dynamic Routing Protocols

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