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Difficulty: MediumFloating Static Routes

An enterprise router receives a primary path for the prefix 192.168.50.0/24192.168.50.0/24 via OSPF. An engineer attempts to configure a floating static backup route using the command `ip route 192.168.50.0 255.255.255.0 10.0.0.2 105`. However, after entering the command, the routing table immediately replaces the OSPF path with the static route. What is the cause of this behavior?

  1. The static route was assigned an administrative distance of 105, which is lower than the default OSPF administrative distance of 110.Answer
  2. B
    The static route was assigned an administrative distance of 105, which is higher than the default OSPF administrative distance of 90.
  3. C
    The numerical value 105 at the end of the static route command sets the routing metric cost rather than the administrative distance.
  4. D
    The router requires an administrative distance of 255 to designate a static route as a floating backup route.

Answer

The static route was assigned an administrative distance of 105, which is lower than the default OSPF administrative distance of 110.
Cisco IOS selects routes based on Administrative Distance (AD), where lower AD values indicate higher trustworthiness. Default OSPF has an AD of 110. Because the engineer configured the static route with an AD of 105, it is lower than OSPF's AD of 110. Consequently, the router prefers the static route and installs it into the routing table immediately, preventing it from functioning as a backup route. A proper floating static route for OSPF must be configured with an AD greater than 110 (such as 120).

Step-by-Step Solution

1
Identify the default Administrative Distance (AD) for the primary routing protocol (OSPF).
OSPF has a default AD of 110.
Cisco IOS uses AD values to determine route trustworthiness when multiple routing sources offer paths to the exact same destination.
2
Evaluate the AD assigned to the static route in the command `ip route 192.168.50.0 255.255.255.0 10.0.0.2 105`.
The trailing value 105 configures the static route AD to 105.
The optional numeric argument at the end of a static route statement specifies its administrative distance.
3
Compare the static route AD against the primary OSPF route AD.
Because 105<110105 < 110, the static route has a lower (more preferred) AD than OSPF.
Lower AD values take precedence in routing table installation. To act as a floating static backup, the static route AD must be configured with a value strictly greater than the primary protocol AD (e.g., AD > 110).

Key Concept

Floating Static Route Administrative Distance Requirements
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Topics

Floating Static Routes

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