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Difficulty: MediumIPv4 Static Routing

A network administrator is configuring static routes on router R1 to provide redundancy for destination network 172.20.50.0/24172.20.50.0/24. The primary path to this network is currently learned via OSPF with an administrative distance of 110110. The administrator wants to add a backup static route via next-hop IPv4 address 192.168.12.2192.168.12.2 and evaluate static route forwarding behavior on broadcast networks. Which two statements regarding static route configuration and forwarding logic are correct? (Select two.)

  1. To function as a backup (floating) static route for the OSPF path, the static route must be configured with an administrative distance value greater than 110.Answer
  2. B
    Configuring the backup static route with an administrative distance of 90 ensures it remains unused until the primary OSPF route goes down.
  3. Configuring an IPv4 static route specifying only an exit interface on a broadcast Ethernet link causes the router to issue ARP requests for destination host IPs, relying on proxy ARP from the next-hop router.Answer
  4. D
    A static route with a /24 prefix length and an administrative distance of 10 will take forwarding precedence over an OSPF route with a /26 prefix length.

Answer

The two correct statements are that a floating static route backing up OSPF must have an administrative distance greater than 110, and that specifying only an exit interface on a broadcast Ethernet link requires proxy ARP on the adjacent router to resolve destination IP addresses.
The statement requiring an administrative distance greater than 110 for a backup route is correct because OSPF has a default AD of 110, so a floating static route must have a higher AD (e.g., 120) to remain inactive until OSPF fails. The statement regarding exit-interface configuration on Ethernet links is also correct because broadcast media require next-hop MAC address resolution, causing the router to send ARP queries for destination IPs which require proxy ARP from the neighboring router.

Step-by-Step Solution

1
Analyze the floating static route administrative distance requirement.
OSPF has a default administrative distance (AD) of 110. For a static route to serve as a backup (floating static route), its AD must be set higher than 110 (e.g., 120). Setting an AD lower than 110 makes it the preferred active route immediately.
Lower administrative distance values indicate higher route trustworthiness in the Cisco IOS routing table.
2
Evaluate exit-interface behavior on Ethernet broadcast networks.
When a static route points only to an Ethernet exit interface (without a next-hop IP address), the router considers the destination network directly connected and sends an ARP request for the destination IP address of every traversing packet, relying on the next-hop device to answer via proxy ARP.
Ethernet is a multiaccess media type, so the router cannot automatically determine the next-hop Layer 2 MAC address without an explicit next-hop IP or proxy ARP response.
3
Verify forwarding precedence rules (Longest Prefix Match vs. AD).
Longest prefix match always takes precedence over administrative distance. A /26/26 route is more specific than a /24/24 route and will always be chosen for packets matching that subnet, regardless of AD.
Administrative distance is only used to select between routes with identical prefix lengths from different sources when populating the routing table.

Key Concept

IPv4 Static Route Administrative Distance and Next-Hop Resolution Mechanics
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Topics

IPv4 Static Routing

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