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Difficulty: MediumRouting Concepts and Dynamic Routing Protocols

An organization uses Open Shortest Path First (OSPF) as its primary interior gateway routing protocol to maintain connectivity to a remote branch office. A network administrator needs to implement a backup path using a static route over a secondary leased line. The backup path must remain inactive in the routing table under normal operations and automatically take over only if the OSPF path fails. Which configuration strategy satisfies this requirement?

  1. Configure the static route with an administrative distance higher than 110, such as 120.Answer
  2. B
    Assign the static route a lower metric value than the OSPF link cost while keeping its administrative distance at default.
  3. C
    Enable Port Address Translation (PAT Overload) on the secondary interface to prioritize backup route failover.
  4. D
    Change host default gateway IP addresses to point directly to the secondary router interface.

Answer

The static route should be configured with an administrative distance higher than OSPF's default value of 110 (such as 120) to create a floating static route.
Routers select paths based on Administrative Distance (AD) first when multiple sources provide routes to the same target subnet. Since OSPF routes have a default AD of 110 and standard static routes have a default AD of 1, configuring a static route with an AD higher than 110 (such as 120) ensures it remains inactive as a 'floating static route' until the OSPF route is withdrawn.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary dynamic routing protocol.
OSPF has a default Administrative Distance of 110110.
Routers use Administrative Distance to evaluate trustworthiness between different route sources before comparing metrics.
2
Determine the default Administrative Distance of a static route.
A standard static route has a default AD of 11, which would normally override OSPF (110110).
To prevent the static route from overriding OSPF during normal operations, its AD must be manually adjusted.
3
Configure a floating static route.
Assign an AD greater than 110110 (e.g., 120120) to the static route.
The router will prefer the OSPF route (AD 110110) and place it in the routing table. If the OSPF route disappears, the static route (AD 120120) becomes the best available path and is inserted into the routing table.

Key Concept

Floating Static Route and Administrative Distance Hierarchy
Estimated Time:1m 0s
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Topics

Routing Concepts and Dynamic Routing Protocols

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