N10-009 Question 149
Single answerFirst Hop Redundancy Protocol (FHRP) / Virtual IP (VIP)A network administrator is configuring two Layer 3 switches to provide gateway redundancy for users on VLAN 20. Hosts in the VLAN are currently configured with a default gateway of 10.20.20.1. The administrator wants users to keep the same default gateway address even if one switch fails, with failover occurring automatically and without reconfiguring clients. Which solution best meets this requirement?
- A
Configure an FHRP so both switches share a virtual IP address of 10.20.20.1, and set hosts to use that virtual IP as their default gateway
- B
Assign 10.20.20.1 to Switch A and 10.20.20.2 to Switch B, then configure half of the hosts to use each switch as their default gateway
- C
Create a loopback interface with IP 10.20.20.1 on both switches so either device can answer ARP requests for the gateway
- D
Configure DHCP reservations so clients can quickly obtain a new default gateway if the primary switch fails
Show answer and explanation
Correct answer: A
Explanation
The best answer is to use an FHRP with a virtual IP address that matches the hosts' default gateway. In real environments, protocols such as HSRP, VRRP, and GLBP allow multiple routers or Layer 3 switches to present one logical gateway to end devices. This design is considered a best practice for default gateway resilience because endpoints do not need to know which physical device is currently forwarding traffic. If the active gateway fails, another device in the FHRP group takes over the VIP and continues forwarding. This aligns with standard vendor documentation for HSRP and VRRP, which describes a shared virtual router or gateway model for first-hop availability. For Network+ purposes, the key concept is that the VIP remains constant for clients, while the active forwarding device can change transparently during failover.
- A. Correct.
Correct. First Hop Redundancy Protocols such as HSRP, VRRP, and GLBP are designed to provide a resilient default gateway. They do this by presenting a virtual IP (VIP) and usually a virtual MAC address to hosts. Clients use the VIP, such as 10.20.20.1, as their default gateway, and if the active device fails, another participating device takes over automatically. This directly satisfies the requirement to keep the same gateway address and avoid client reconfiguration.
- B. Incorrect.
Incorrect. While splitting hosts across two physical default gateways can provide some limited resiliency, it does not preserve a single consistent gateway address for all users. If Switch A fails, hosts using 10.20.20.1 would lose gateway access unless manually or dynamically reconfigured. This approach also increases operational complexity and does not meet the stated requirement as cleanly as an FHRP with a VIP.
- C. Incorrect.
Incorrect. A loopback interface is not a substitute for an FHRP virtual gateway. Configuring the same IP address on both switches would create an IP conflict and can lead to unstable ARP behavior and duplicate IP issues. FHRPs exist specifically to coordinate ownership of a shared virtual gateway address safely and predictably.
- D. Incorrect.
Incorrect. DHCP can distribute default gateway information, but it is not intended to provide immediate first-hop failover for already leased clients. Clients typically retain their existing gateway until lease renewal or manual intervention. This does not provide the seamless, automatic failover expected from an FHRP-based VIP solution.