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Difficulty: HardIPv4 Static Routing

A network administrator intends to configure a floating static route on Router R1 to serve as a backup to a primary route dynamically learned via OSPF (Administrative Distance 110) for destination network 172.16.50.0/24172.16.50.0/24. However, while the primary OSPF link remains healthy and operational, the administrator observes that the newly added static route immediately overrides the OSPF route in the IPv4 routing table. Which command did the administrator incorrectly execute on Router R1?

  1. ip route 172.16.50.0 255.255.255.0 192.168.1.2 90Answer
  2. B
    ip route 172.16.50.0 255.255.255.0 192.168.1.2 120
  3. C
    ip route 172.16.50.0 255.255.255.0 192.168.1.2 110
  4. D
    ip route 172.16.50.0 0.0.0.255 192.168.1.2 120

Answer

The command 'ip route 172.16.50.0 255.255.255.0 192.168.1.2 90' was incorrectly configured because assigning an Administrative Distance lower than 110 causes the static route to immediately preempt the primary OSPF path.
In Cisco IOS, static routes have a default Administrative Distance (AD) of 1. A floating static route acts as a backup path and must be configured with an AD higher than the primary route's protocol AD. Because OSPF has a default AD of 110, setting the static route's AD to 90 gives it higher priority (lower AD number) than OSPF. Consequently, the router immediately installs the static route into the routing table and ignores the active OSPF route.

Step-by-Step Solution

1
Analyze the requirement for a floating static route.
A floating static route must have an Administrative Distance (AD) higher than the primary routing protocol so that it remains inactive in the routing table until the primary route fails.
Cisco IOS routing logic selects paths based on the lowest Administrative Distance first.
2
Compare the AD of the primary route (OSPF) against the attempted static route.
OSPF has a default AD of 110. To act as a backup, the static route AD must be greater than 110 (such as 120).
If the static route is configured with an AD lower than 110 (such as 90), the router prefers the static route over the OSPF route.
3
Identify which misconfiguration caused the immediate routing table override.
The command specifying an AD of 90 is preferred over OSPF's AD of 110, causing immediate installation into the routing table.
Lower AD numbers indicate higher trustworthiness in Cisco IOS.

Key Concept

Floating Static Route Administrative Distance Selection
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Topics

IPv4 Static Routing

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