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Difficulty: Very hardIPv4 Static Routing

Router R1 connects an enterprise edge network to two service providers, ISP-A and ISP-B. The primary default path is dynamically learned from ISP-A via eBGP, which assigns an Administrative Distance (AD) of 2020. A network engineer needs to configure a floating static default route toward ISP-B through next-hop IP 198.51.100.1198.51.100.1 as a redundant backup path that will remain inactive in the routing table until the primary eBGP path fails. Which Cisco IOS command correctly achieves this objective?

  1. ip route 0.0.0.0 0.0.0.0 198.51.100.1 25Answer
  2. B
    ip route 0.0.0.0 0.0.0.0 198.51.100.1 15
  3. C
    ip route 0.0.0.0 0.0.0.0 198.51.100.1 1
  4. D
    ip route 0.0.0.0 255.255.255.255 198.51.100.1 25

Answer

The command 'ip route 0.0.0.0 0.0.0.0 198.51.100.1 25' correctly specifies a default route with an Administrative Distance higher than the primary eBGP AD of 20, ensuring it remains inactive until link failure.
A floating static route acts as a backup path and must remain inactive in the Routing Information Base (RIB) until the primary route fails. Because the primary route is dynamically learned via eBGP with an Administrative Distance (AD) of 20, the static backup route must be explicitly configured with an AD value strictly greater than 20. The option specifying 'ip route 0.0.0.0 0.0.0.0 198.51.100.1 25' correctly applies an AD of 25 to the default network 0.0.0.0/0, ensuring it functions strictly as a backup.

Step-by-Step Solution

1
Identify the primary route source and its administrative distance.
Primary default route is learned via eBGP, which has an Administrative Distance of 2020.
Floating static routes must be assigned an Administrative Distance higher than the primary protocol's AD.
2
Determine the correct IPv4 default static route prefix and mask syntax.
Default route destination is represented as '0.0.0.0 0.0.0.0'.
A quad-zero prefix and mask match any destination network not explicitly matched by more specific routes.
3
Select the appropriate Administrative Distance for the backup floating route.
Specify an Administrative Distance greater than 2020 (e.g., 2525).
Setting the AD to 2525 keeps the static route hidden from the Routing Information Base (RIB) while the primary eBGP route (AD 2020) is active.

Key Concept

Floating Static Route Administrative Distance Selection
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Topics

IPv4 Static Routing

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