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High Availability for CompTIA Network+ N10-009

High availability (HA) ensures network services remain accessible despite component failures. CompTIA Network+ N10-009 tests HA concepts including redundancy, failover, FHRP (HSRP/VRRP), NIC teaming, and availability calculations. These concepts appear in both implementation and operations domains — understanding how HA is designed and measured is essential for the exam.

8 min

3 sections · 7 exam key points

1 practice questions

Availability and Redundancy Concepts

Availability is measured as a percentage of time a system is operational. 99.9% ('three nines') = ~8.7 hours downtime per year. 99.99% ('four nines') = ~52 minutes. 99.999% ('five nines') = ~5 minutes. Each additional '9' requires significantly more redundancy and cost. SLA (Service Level Agreement) specifies the availability commitment between a service provider and customer.

MTTR (Mean Time To Repair): average time to restore a failed component. MTBF (Mean Time Between Failures): average time between failures. Higher MTBF = more reliable hardware. Lower MTTR = faster recovery (spare parts, procedures, monitoring). Availability = MTBF / (MTBF + MTTR). Both metrics inform HA design.

Redundancy eliminates single points of failure (SPOF): dual power supplies, redundant links (EtherChannel, dual ISP), redundant switches, server clustering, and RAID storage. Hot spare: a standby component that takes over immediately (no manual intervention). Warm spare: takes over quickly after minimal configuration. Cold spare: requires manual installation and configuration.

First Hop Redundancy Protocols

FHRP (First Hop Redundancy Protocol) provides router redundancy for hosts that have a single default gateway configured. Without FHRP, if the default gateway router fails, all hosts on that subnet lose internet and inter-VLAN connectivity.

HSRP (Hot Standby Router Protocol): Cisco proprietary. Two or more routers share a virtual IP and virtual MAC. One is Active (forwards traffic), others are Standby (waiting). If Active fails, Standby takes over. Hosts configure the virtual IP as their default gateway. Uses UDP port 1985. VRRP (Virtual Router Redundancy Protocol): IETF standard (RFC 5798) — vendor-neutral equivalent of HSRP. One Master router, one or more Backup routers. GLBP (Gateway Load Balancing Protocol): Cisco proprietary, allows multiple routers to actively forward traffic simultaneously (load balancing, not just failover).

NIC Teaming and Link Redundancy

NIC teaming (bonding): combining multiple physical NICs on a server into a single logical interface. Provides bandwidth aggregation and/or failover. Active-passive: one NIC active, other on standby. Active-active: both NICs carry traffic simultaneously. Requires switch support for LACP (802.3ad) when using active-active.

Dual-homed servers: servers connected to two different switches for switch-level redundancy. If one switch fails, the server remains connected via the other switch. Clustering: multiple servers operate as a single logical service. Active-active clusters process requests across all nodes; active-passive clusters have standby nodes that activate on failure.

FHRP Protocol Comparison

Protocol	Standard	Active Role	Load Balancing
HSRP	Cisco proprietary	Active (1 at a time)	No (failover only)
VRRP	IEEE/IETF RFC 5798	Master (1 at a time)	No (failover only)
GLBP	Cisco proprietary	AVG + multiple AVFs	Yes (all routers active)

Key exam facts — Network+

99.9% = ~8.7 hrs/yr downtime; 99.99% = ~52 min/yr; 99.999% = ~5 min/yr
MTBF = time between failures; MTTR = time to repair; Availability = MTBF/(MTBF+MTTR)
HSRP: Cisco, Active/Standby; VRRP: IEEE standard, Master/Backup
FHRP provides virtual IP as default gateway — survives router failure
Hot spare: instant failover; warm: quick recovery; cold: manual intervention
NIC teaming: multiple NICs combined for bandwidth or redundancy
GLBP = Cisco FHRP with load balancing across multiple active routers

Common exam traps

VRRP and HSRP are the same protocol

HSRP is Cisco proprietary (Active/Standby terminology, UDP 1985); VRRP is the IETF open standard (Master/Backup terminology, uses multicast). They serve the same purpose but are not interoperable

Practice questions — High Availability

These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.

Q1.A network team needs to provide default gateway redundancy for hosts in a subnet. The environment uses multiple vendors' routers. Which FHRP should be used?

A.HSRP

B.VRRP

C.GLBP

D.STP

Explanation: VRRP (Virtual Router Redundancy Protocol) is the IEEE/IETF open standard for router redundancy (RFC 5798) and is supported by multiple vendors. HSRP and GLBP are Cisco proprietary. STP prevents Layer 2 loops but does not provide router redundancy.

Frequently asked questions — High Availability

What is the difference between high availability and fault tolerance?

High availability minimizes downtime through redundancy and fast failover — brief interruptions may occur during failover. Fault tolerance provides continuous operation with zero interruption even during component failure — requires duplicate, synchronized systems that switch instantly. Fault tolerance is more expensive and complex. Storage systems using RAID and mirrored databases are fault-tolerant; clustered servers with FHRP are highly available.

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