352-001 Question #193: Real Exam Question with Answer & Explanation

The correct answer is B: automatic protection switching. Automatic protection switching (APS) restores connectivity at the physical or data link layer within 50ms, making the failover completely transparent to Layer 3 protocols which never detect the outage.

Question

A network designer wants to improve a company network design due to multiple network crashes. Which technology would allow for the restore of a network connection without informing the Layer 3 protocol?

Options

ABidirectional Forwarding Detection
Bautomatic protection switching
CUniDirectional Link Detection
DEthernet OAM

Explanation

Automatic protection switching (APS) restores connectivity at the physical or data link layer within 50ms, making the failover completely transparent to Layer 3 protocols which never detect the outage.

Common mistakes.

A. BFD actively detects forwarding plane failures and then notifies Layer 3 routing protocols to trigger reconvergence - it informs Layer 3 of the failure rather than hiding it.
C. UDLD detects unidirectional Ethernet link failures and disables the affected port, but it does not restore connectivity or switch to an alternate path transparently.
D. Ethernet OAM (IEEE 802.3ah/802.1ag) provides link monitoring and fault notification but does not perform automatic protection switching or restore connectivity in a way that is transparent to Layer 3.

Concept tested. Automatic protection switching for Layer 3 transparent failover

Reference. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/interface/configuration/xe-3s/ir-xe-3s-book/ir-aps.html

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