A Cisco IOS router (R1) is configured with three routes for the destination prefix 192.168… — Cisco CCNA

A Cisco IOS router (R1) is configured with three routes for the destination prefix $192.168.10.0/24$ : a primary static route (`ip route 192.168.10.0 255.255.255.0 10.0.12.2`), an active OSPFv2 dynamic route (Administrative Distance 110 via $10.0.14.2$ ), and a backup floating static route (`ip route 192.168.10.0 255.255.255.0 10.0.13.2 120`). Place the control plane and data plane events in the correct chronological sequence starting immediately after the physical interface connected to $10.0.12.2$ fails.

1The local physical interface associated with next-hop $10.0.12.2$ transitions to a down state.
2Cisco IOS removes the primary static route ( $192.168.10.0/24$ with AD 1) from the IPv4 Routing Information Base (RIB).
3The RIB evaluates remaining candidate routes for prefix $192.168.10.0/24$ and selects OSPF (AD 110) over the floating static route (AD 120).
4The Cisco Express Forwarding (CEF) Forwarding Information Base (FIB) updates the prefix entry $192.168.10.0/24$ to point to next-hop $10.0.14.2$ .
5Incoming IP packets destined for host $192.168.10.50$ are encapsulated and transmitted toward $10.0.14.2$ .

Cevap

The correct sequence of events is: Interface link failure → Primary static route RIB withdrawal → AD evaluation selecting OSPF over floating static → FIB table update → Forwarding of packets via the OSPF path.

When an active egress interface fails, Cisco IOS immediately removes the connected static route from the Routing Information Base (RIB). The router then re-evaluates all available candidate routes for the exact destination prefix (

192.168.10.0/24

). Because the prefix lengths are identical, the router compares Administrative Distance (AD). OSPF (AD 110) is preferred over the floating static route (AD 120). Once the RIB settles on the OSPF path, the control plane programs the Forwarding Information Base (FIB), and data plane traffic resumes forwarding out the OSPF next-hop.

Adım Adım Çözüm

Identify link status change

Interface going down invalidates directly associated static route next-hops.

Cisco IOS requires an active interface to keep static routes active in the RIB.

Withdraw primary static route from RIB

Primary static route with Administrative Distance 1 is purged.

Unreachable next-hop interfaces trigger route removal.

Compare Administrative Distance of remaining matching prefixes

OSPF route (AD 110) is selected instead of the floating static route (AD 120).

When prefix lengths match exactly (

192.168.10.0/24

), the route with the lowest Administrative Distance is installed.

Update Cisco Express Forwarding (CEF) FIB hardware tables

FIB updates pointer for

192.168.10.0/24

to point to

10.0.14.2

RIB modifications must sync to the FIB for hardware-accelerated forwarding.

Process data plane traffic

Packets targeting

192.168.10.50

switch path to

10.0.14.2

Subsequent ingress traffic matches the newly installed FIB lookup entry.

Anahtar Kavram

IPv4 Static Route RIB Lifecycle, Floating Static Administrative Distance Calibration, and CEF FIB Convergence

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

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