Failover

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Failover is the transfer of an active network service or role to a standby device after the active device fails.

Failover transfers an active network role to a standby router after the active router fails. In first-hop redundancy, both routers use the same virtual IP address (VIP) and virtual MAC address, so hosts keep using the same default gateway. section 19.1.3 section 19.4

After failover:

• The standby router becomes active and sends gratuitous ARP (GARP) replies to the broadcast MAC address. section 19.1.3
• Switches relearn the virtual MAC address on the port leading to the new active router. section 19.1.3
• End hosts retain their default gateway and ARP entries because the VIP and virtual MAC remain unchanged. section 19.4

R1 fails

When R1 fails, R2 takes over the active role and sends GARP messages. Switches update their MAC tables, while hosts continue using 10.0.0.1 as their gateway. section 19.1.3

Figure 19.4 An FHRP failover. (1) A hardware failure occurs on R1. (2) R2 takes over as the active router. (3) R2 sends GARP messages, sourced from the virtual MAC and destined for the broadcast MAC. (4) The switches learn the virtual MAC on the appropriate ports—those leading toward the new active router. (5) End hosts’ default gateways and ARP table entries for the VIP remain the same.

Trace the hardware failure, standby takeover, GARP broadcast, switch relearning, and unchanged host gateway. section 19.1.3

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Explains takeover, GARP signaling, switch relearning, and host transparency, but not configuration commands.

Examples

Uses an R1 hardware failure, R2 takeover, GARP broadcasts, switch-table updates, and gateway 10.0.0.1.

What it assumes

Introduces active and standby routers, VIPs, virtual MAC addresses, ARP, and FHRP relationships within the explanation.

What you can do after

Evaluate how FHRP failover preserves host gateway operation and updates switched forwarding paths.

How it connects

Connects FHRP with VIPs, virtual MAC addresses, ARP, GARP, switching, and preemption.

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Also explained in

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  100 SQL Server Mistakes and How to Avoid ThemFailover is the transfer of service from a failed or inactive primary system to a secondary system.
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  Amazon Web Services in Action, Third Edition: An in-depth guide to AWSFailover is the process of transferring service responsibility from a failed or unavailable primary component to an available replacement.
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  Microsoft Azure in ActionAutomatic failover is the automatic transfer of service operation to another region or component when the primary one becomes unavailable.
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  Infrastructure as Code, Patterns and Practices: With examples in Python and TerraformFailover is the transfer of service traffic or operation from a failing component or environment to a standby replacement.
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  PostgreSQL Mistakes and How to Avoid ThemFailover is the transition of service from a failed or unavailable primary system to a replica or alternate system.

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Related concepts

• active-passive configurationused by · An active-passive configuration is an architecture in which one environment serves traffic while a separate environment remains ready to take over after failure or for fallback.
• standby taskaddressed by · A standby task is an inactive Kafka Streams task that maintains a replicated state-store copy for faster failover when an active task is reassigned.
• uptimeused by · Uptime is the duration or proportion of a specified period during which a system or service is operational and available to provide its intended function.
• database serviceused by · A database service is a software or cloud service that provides managed storage, organization, querying, and retrieval of application data.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.