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Difficulty: HardIPv4 Static Routing

A network administrator is evaluating the IPv4 static routing configuration on router R1. Router R1 needs to reach the remote subnet 10.80.16.0/2010.80.16.0/20. The primary routing path to this destination is currently provided by OSPF, which has an administrative distance of 110. The administrator must configure two static routes:

1. A static route using next-hop IPv4 address 172.16.30.2172.16.30.2 that immediately takes precedence over the OSPF route.
2. A floating static route using next-hop IPv4 address 172.16.40.2172.16.40.2 that serves as a backup only if the OSPF route becomes unavailable.

Which TWO static route commands must be configured on router R1 to achieve this objective? (Select TWO.)

  1. ip route 10.80.16.0 255.255.240.0 172.16.30.2Answer
  2. ip route 10.80.16.0 255.255.240.0 172.16.40.2 120Answer
  3. C
    ip route 10.80.16.0 255.255.240.0 172.16.40.2 90
  4. D
    ip route 10.80.16.0 255.255.240.0 172.16.30.2 110

Answer

The correct commands are 'ip route 10.80.16.0 255.255.240.0 172.16.30.2' for the primary route and 'ip route 10.80.16.0 255.255.240.0 172.16.40.2 120' for the floating static backup route.
To create a primary static route that overrides OSPF (AD 110), the standard command 'ip route 10.80.16.0 255.255.240.0 172.16.30.2' uses the default administrative distance of 1, which is lower than 110 and thus preferred. To configure a floating static backup route for the same subnet via next-hop 172.16.40.2, the command 'ip route 10.80.16.0 255.255.240.0 172.16.40.2 120' assigns an administrative distance of 120, which is higher than OSPF's 110, ensuring it only enters the routing table if the OSPF route drops.

Step-by-Step Solution

1
Convert prefix length /20 to dotted-decimal subnet mask format.
A /20 prefix corresponds to a subnet mask of 255.255.240.0.
Cisco IOS static route syntax requires the destination network mask in dotted-decimal format rather than CIDR notation.
2
Determine the required Administrative Distance (AD) for the primary static route.
Use the default static route AD of 1 (or any value lower than OSPF's AD of 110).
Lower administrative distance values are preferred in the Cisco routing table forwarding decision logic.
3
Determine the required Administrative Distance (AD) for the backup floating static route.
Configure an AD greater than 110 (such as 120) for the next-hop 172.16.40.2.
A floating static route must have a higher AD than the primary dynamic routing protocol (OSPF = 110) so it is installed in the routing table only when the primary path goes down.

Key Concept

Cisco IOS IPv4 static route syntax and floating static route administrative distance selection.
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Topics

IPv4 Static Routing

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