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Difficulty: HardFloating Static Routes

A network administrator is configuring a backup floating static route to reach an external corporate subnet 172.31.50.0/24. The primary path to this destination is learned dynamically via External BGP (eBGP). During testing while the primary eBGP connection is fully operational, the administrator observes that traffic toward 172.31.50.0/24 is being forwarded across the backup static path rather than the primary link. Which static route command entry in the router configuration caused this improper failover behavior?

  1. ip route 172.31.50.0 255.255.255.0 192.168.12.2 15Answer
  2. B
    ip route 172.31.50.0 255.255.255.0 192.168.12.2 25
  3. C
    ip route 172.31.50.0 255.255.255.0 192.168.12.2 90
  4. D
    ip route 172.31.50.0 255.255.255.0 192.168.12.2 110

Answer

The command 'ip route 172.31.50.0 255.255.255.0 192.168.12.2 15' sets the administrative distance lower than eBGP, causing it to preempt the primary route.
Cisco IOS assigns default Administrative Distance values to evaluate route trustworthiness. External BGP (eBGP) routes carry a default AD of 20. To create a floating static backup route for an eBGP path, the trailing administrative distance argument in the 'ip route' command must be set higher than 20. Specifying an administrative distance of 15 makes the static route more preferred than the eBGP route (15 < 20), causing the router to immediately install the static path into the routing table even when the eBGP link is active.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary routing protocol.
External BGP (eBGP) has a Cisco default Administrative Distance of 20.
Administrative Distance determines route trustworthiness when multiple sources advertise the exact same network prefix.
2
Evaluate the requirement for a floating static route.
A floating static route must have an AD strictly greater than the primary route's AD (greater than 20) so it stays inactive until the primary route drops out of the routing table.
The router installs the route with the lowest AD into the IP routing table.
3
Analyze the configured static route entries to identify why the backup route preempted the primary path.
Configuring an administrative distance of 15 gives the static route a lower AD than eBGP (15 < 20).
Because 15 is lower than 20, the router selects the static route over the operational eBGP path.

Key Concept

Floating Static Route Administrative Distance relative to eBGP
Estimated Time:1m 30s
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Topics

Floating Static Routes

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