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Difficulty: HardIPv4 Static Routing

An enterprise router R1 learns a primary path to the remote network 172.16.50.0/24172.16.50.0/24 dynamically through OSPF with an Administrative Distance of 110. A network administrator needs to configure a floating static backup route to 172.16.50.0/24172.16.50.0/24 via the next-hop IPv4 address 10.0.12.210.0.12.2. Which TWO statements accurately describe the requirements and operational behavior of this IPv4 static route configuration? (Select two.)

  1. The floating static route command must specify an administrative distance value greater than 110 (such as 120) so that it is installed in the routing table only if the primary OSPF route fails.Answer
  2. When configured using only the next-hop IPv4 address 10.0.12.210.0.12.2, the router must perform a recursive lookup in the routing table to resolve the outbound interface.Answer
  3. C
    Configuring the floating static route with an administrative distance of 90 ensures it operates as a standby route while OSPF remains operational.
  4. D
    If a static default route (0.0.0.0/00.0.0.0/0) with an administrative distance of 120 is also present, router R1 will prefer that default route over the OSPF route for destination 172.16.50.10172.16.50.10.

Answer

The floating static route must be configured with an administrative distance greater than 110 (such as 120) to remain inactive while OSPF is up, and specifying only a next-hop IP address requires the router to perform a recursive routing lookup to resolve the exit interface.
For a static route to function as a floating backup, its Administrative Distance must be configured higher than the primary route's protocol AD (110 for OSPF). Additionally, static routes configured with only an IP next-hop rely on recursive lookup logic to identify the outgoing interface.

Step-by-Step Solution

1
Analyze the Administrative Distance (AD) requirement for floating static routes.
OSPF has an AD of 110. Standard static routes have an AD of 1. To make a static route act as a floating backup, its AD must be manually set higher than the active protocol (e.g., AD > 110).
The Cisco IOS routing table installs the route with the lowest Administrative Distance when multiple sources offer routes to the exact same prefix.
2
Evaluate router forwarding lookup behavior for next-hop static routes.
Specifying only the next-hop IPv4 address (10.0.12.210.0.12.2) obligates the router to recursively search the routing table to match 10.0.12.210.0.12.2 against directly connected egress interfaces.
Without an explicitly named local exit interface, the control plane must resolve how to reach the next-hop address.
3
Evaluate Longest Prefix Match rules against Administrative Distance.
A default route (0.0.0.0/00.0.0.0/0) is less specific than a /24 route. Longest prefix matching occurs prior to AD comparison, so a /24 route always beats a /0 default route.
Prefix length is the primary criterion in IPv4 forwarding decisions.

Key Concept

Floating Static Route Administrative Distance and Recursive Next-Hop Lookup
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Topics

IPv4 Static Routing

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