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Difficulty: MediumFloating Static Routes

An edge router receives the primary route to remote destination network 10.10.0.0/1610.10.0.0/16 via external BGP (eBGP) from ISP-1 using default settings. The network administrator attempts to configure a floating static backup route toward ISP-2 using the command `ip route 10.10.0.0 255.255.0.0 192.168.1.2 15`. Which behavior will be observed on the router regarding this destination?

  1. The static route immediately replaces the eBGP route as the primary active route in the routing table because its Administrative Distance of 15 is lower than the default eBGP Administrative Distance of 20.Answer
  2. B
    The static route functions correctly as an inactive backup route and enters the routing table only when the eBGP path drops.
  3. C
    The router installs both routes into the routing table and load balances traffic equally across ISP-1 and ISP-2.
  4. D
    Cisco IOS rejects the command because static route Administrative Distance values cannot be configured lower than 20.

Answer

The static route immediately replaces the eBGP route as the primary active route in the routing table because its Administrative Distance of 15 is lower than the default eBGP Administrative Distance of 20.
In Cisco IOS, lower Administrative Distance values take precedence. The default Administrative Distance for external BGP (eBGP) is 20. Because the static route was configured with an explicit Administrative Distance of 15, the router considers it more reliable than the eBGP route. As a result, the static route becomes the primary active path in the routing table rather than remaining an inactive backup.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary routing protocol.
The primary route is learned via external BGP (eBGP), which has a default Cisco IOS Administrative Distance of 2020.
Administrative Distance indicates the trustworthiness of a routing source.
2
Analyze the configured Administrative Distance of the static route.
The trailing argument in `ip route 10.10.0.0 255.255.0.0 192.168.1.2 15` specifies an Administrative Distance of 1515.
The optional metric value at the end of the `ip route` syntax overrides the default static route AD of 1.
3
Compare the AD values to determine routing table selection.
Since 15<2015 < 20, the static route is considered more believable than the eBGP route.
Routers select the route with the lowest Administrative Distance when multiple sources advertise the exact same prefix length.

Key Concept

Floating Static Route Administrative Distance Precedence
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Topics

Floating Static Routes

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