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Difficulty: HardIPv4 Static Routing

A network engineer needs to configure a floating static route on a Cisco router to serve as a backup path for the destination network 172.28.32.0/21172.28.32.0/21. The primary path to this network is currently learned via OSPF, which operates with an Administrative Distance of 110. Which Cisco IOS command correctly configures the floating static route so that it only enters the routing table if the primary OSPF route fails?

  1. ip route 172.28.32.0 255.255.248.0 10.200.1.2 120Answer
  2. B
    ip route 172.28.32.0 255.255.248.0 10.200.1.2 90
  3. C
    ip route 172.28.32.0 255.255.240.0 10.200.1.2 120
  4. D
    ip route 172.28.32.0 255.255.248.0 10.200.1.2 110

Answer

ip route 172.28.32.0 255.255.248.0 10.200.1.2 120
The correct command specifies 'ip route 172.28.32.0 255.255.248.0 10.200.1.2 120'. A /21 subnet mask corresponds to 255.255.248.0. Since the primary route is learned via OSPF with an Administrative Distance (AD) of 110, a backup (floating) static route must be configured with an AD greater than 110 (such as 120). This keeps the route out of the routing table until the primary OSPF route is withdrawn.

Step-by-Step Solution

1
Determine the subnet mask representation for the prefix /21
A /21 network mask consists of 21 contiguous binary 1s: 11111111.11111111.11111000.00000000, which evaluates to 255.255.248.0 in dotted-decimal notation.
Cisco IOS IPv4 static route command syntax requires the netmask parameter in dotted-decimal format.
2
Identify the administrative distance (AD) of the primary dynamic routing protocol
OSPF has a default Administrative Distance of 110.
To create a floating static route, the static route must have an AD strictly greater than the primary route protocol's AD.
3
Select an Administrative Distance value for the floating static route
An AD of 120 is higher than 110, ensuring the route stays inactive in the routing table until OSPF fails.
Lower AD values are preferred by Cisco IOS. Setting the AD to 120 keeps the static route inactive (floating) while OSPF is active.
4
Construct the full Cisco IOS static route command
'ip route 172.28.32.0 255.255.248.0 10.200.1.2 120'
Matches the syntax: 'ip route <prefix> <mask> <next-hop-ip> [distance]'.

Key Concept

IPv4 Floating Static Route Configuration and Administrative Distance Selection
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Topics

IPv4 Static Routing

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