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Difficulty: MediumIPv4 Static Routing

An engineer is configuring a backup link on router R2 to reach the internal network 10.50.0.0/1610.50.0.0/16. The primary path is currently learned dynamically via OSPF, which has a default administrative distance of 110. Which Cisco IOS command correctly configures a floating static route that remains inactive unless the primary OSPF route fails?

  1. ip route 10.50.0.0 255.255.0.0 172.16.12.2 120Answer
  2. B
    ip route 10.50.0.0 255.255.0.0 172.16.12.2 90
  3. C
    ip route 10.50.0.0 255.255.0.0 172.16.12.2 110
  4. D
    ip route 10.50.0.0 0.0.255.255 172.16.12.2 120

Answer

The command 'ip route 10.50.0.0 255.255.0.0 172.16.12.2 120' correctly configures the floating static route.
A floating static route acts as a secondary path that is installed into the routing table only when the primary path fails. To achieve this behavior, the static route must be configured with an Administrative Distance (AD) higher than that of the primary routing protocol. Since OSPF has a default AD of 110, setting the static route AD to 120 ensures it stays in reserve.

Step-by-Step Solution

1
Identify the primary route's Administrative Distance (AD)
OSPF has a default AD of 110.
Floating static routes must have an AD strictly greater than the primary route's AD to remain hidden in the routing table until the primary path drops.
2
Verify Cisco IOS IPv4 static routing syntax
The correct format is 'ip route <destination-network> <subnet-mask> <next-hop-ip> [distance]'.
Standard subnet masks (dotted-decimal) are mandatory in Cisco IOS static routing syntax.
3
Select the option with an AD greater than 110 and proper subnet mask syntax
The command specifying an AD of 120 with mask 255.255.0.0 meets both requirements.
An AD of 120 ensures the static route is kept in backup state while OSPF is active.

Key Concept

Floating Static Route Administrative Distance Selection
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Topics

IPv4 Static Routing

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