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Difficulty: Very hardFloating Static Routes

An enterprise router receives an internal EIGRP route for the summary prefix 172.16.0.0/16 (Administrative Distance 90) and learns a specific path for subnet 172.16.10.0/24 via OSPF (Administrative Distance 110). A network engineer attempts to configure a backup floating static route for destination 172.16.10.0/24 using the command `ip route 172.16.10.0 255.255.255.0 192.168.1.2 100`. Under normal operating conditions while all links remain fully operational, which routing behavior will occur for traffic sent to host 172.16.10.50?

  1. The router routes traffic using the static route via next-hop 192.168.1.2 because its Administrative Distance of 100 is lower than the OSPF route's Administrative Distance of 110.Answer
  2. B
    The router routes traffic using the EIGRP route because EIGRP has a lower Administrative Distance (90) than the configured static route (100).
  3. C
    The router load-balances traffic equally between the OSPF path and the static path because both match the /24 destination prefix.
  4. D
    The static route remains inactive in the RIB and only enters the routing table if the OSPF neighbour adjacency fails.

Answer

Traffic to host 172.16.10.50 will be forwarded using the static route via next-hop 192.168.1.2 because its configured Administrative Distance of 100 is lower than OSPF's Administrative Distance of 110.
For identical prefix lengths (/24), the router compares Administrative Distance (AD) values to determine which route enters the routing table. OSPF has a default AD of 110. Since the configured static route has an AD of 100, which is lower than 110, Cisco IOS selects the static route as the active primary path. Consequently, traffic destined for 172.16.10.50 is immediately forwarded via next-hop 192.168.1.2.

Step-by-Step Solution

1
Evaluate the prefix lengths available for the destination address 172.16.10.50.
Two prefix lengths match: /16 (EIGRP summary) and /24 (OSPF and static). The longest prefix match rule prioritizes the /24 subnet over the /16 summary.
Cisco routers always select the most specific route (longest subnet mask) first during packet forwarding lookup.
2
Compare the Administrative Distance (AD) values of competing routes for the 172.16.10.0/24 destination.
The OSPF route has a default AD of 110, while the configured static route has an AD of 100.
When multiple sources offer a route to the exact same prefix, Cisco IOS selects the route with the lowest Administrative Distance to install into the Routing Information Base (RIB).
3
Determine the operational status of the floating static route.
Because AD 100 < AD 110, the static route is active in the routing table immediately, preempting the primary OSPF route rather than acting as a floating backup.
A floating static route must have an AD set strictly higher than the primary route (e.g., AD 115 or higher) to remain dormant until the primary route fails.

Key Concept

Floating Static Route Administrative Distance Selection
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Topics

Floating Static Routes

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