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Difficulty: HardFloating Static Routes

In an enterprise WAN edge topology, router HQ-GW receives a path to subnet 10.80.0.0/1410.80.0.0/14 via a redistributed routing process resulting in an EIGRP External route (default Administrative Distance of 170170). An administrator needs to configure a floating static backup route for destination subnet 10.80.0.0/1410.80.0.0/14 using next-hop IP 192.168.200.1192.168.200.1 over an auxiliary link.

What minimum Administrative Distance integer value must be appended to the Cisco IOS command so that the static route remains dormant until the primary EIGRP External route fails?

Answer:ip route 10.80.0.0 255.240.0.0 192.168.200.1 【171】

Answer

171
Cisco IOS routes are populated into the routing table based on Administrative Distance (AD), where lower values indicate higher trustworthiness. EIGRP External routes possess a default AD of 170. For a static route to act as a floating backup (remaining inactive until the primary path fails), its AD must be set higher than the primary path's AD. The minimum integer value greater than 170 is 171.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary routing source.
EIGRP External routes have a standard Cisco IOS default AD of 170.
Routing decisions in Cisco IOS select the path with the lowest Administrative Distance.
2
Determine the required Administrative Distance condition for a floating static route.
The floating static route's AD must be strictly greater than 170.
If the static route's AD were less than or equal to 170, it would either override the primary path or cause equal-cost multi-pathing/route instability rather than serving strictly as a backup.
3
Calculate the minimum integer AD required for the static route command.
170 + 1 = 171.
171 is the lowest whole integer value greater than the primary path's AD of 170.

Key Concept

Floating Static Route Administrative Distance Selection
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Topics

Floating Static Routes

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