When a Citrix ADC high availability (HA) pair failover occurs, by what method does the Citrix ADC communicate to the network switches and routers that IP-to-MAC address bindings have changed?

The correct answer is D. Gratuitous ARPs (GARPs) to update the network devices. Gratuitous ARPs (GARPs) are the mechanism Citrix ADC uses during HA failover because a GARP is an unsolicited ARP broadcast that announces "this IP address now maps to this MAC address" - exactly what's needed when the secondary node takes over and network devices must update…

High Availability

Question

Options

AReverse ARP (RARP) to update the network devices
BProxy ARP to update the network devices
CMAC-based forwarding (MBF) to update the routers
DGratuitous ARPs (GARPs) to update the network devices

How the community answered

(27 responses)

B
4% (1)
C
4% (1)
D
93% (25)

Explanation

Gratuitous ARPs (GARPs) are the mechanism Citrix ADC uses during HA failover because a GARP is an unsolicited ARP broadcast that announces "this IP address now maps to this MAC address" - exactly what's needed when the secondary node takes over and network devices must update their ARP caches to forward traffic to the new active node.

Why the distractors are wrong:

A (RARP): Reverse ARP resolves a MAC address to an IP address (the opposite direction) - it's used for diskless workstations finding their own IP, not for updating network mappings after failover.
B (Proxy ARP): Proxy ARP is when a router answers ARP requests on behalf of a host in another subnet - it's a routing aid, not a failover notification mechanism.
C (MBF): MAC-Based Forwarding is a Citrix ADC feature for handling asymmetric routing by forwarding return traffic based on MAC address, not a method for notifying external switches of address changes.

Memory tip: Think of "Gratuitous" as "unprompted" - nobody asked, but the new active node volunteers the announcement. During HA failover, the secondary shouts to the whole network: "Hey everyone, update your tables - I'm in charge now!" That unsolicited broadcast is the defining characteristic of a GARP.

Topics

#Gratuitous ARP#HA Failover#MAC Advertisement#Network Convergence

Community Discussion

Anjali D.Nov 15, 2025

Good question to nail down before the exam. The correct answer is D, Gratuitous ARPs. When an HA failover happens, the new primary ADC sends out GARPs on the network to announce that the virtual IP addresses are now reachable via a different MAC address. This forces the switches and routers to update their ARP caches immediately, so traffic stops going to the old primary and starts reaching the new one without waiting for normal ARP cache timeouts to expire. The distractors are worth understanding too. RARP is used to find an IP address from a known MAC, which is the opposite of what we need here. Proxy ARP is about one device answering ARP requests on behalf of another, and MBF is a Citrix ADC forwarding mode, not a mechanism for notifying the network of address changes. None of those fit the failover notification scenario the way GARPs do. Does anyone in the group have a lab setup where they have actually captured the GARP traffic during a failover? Seeing the actual packets helps this concept stick. Would love to hear if anyone ran a Wireshark trace on this.

Nina C.Nov 16, 2025

One thing worth adding for exam purposes is that the GARP goes out as a broadcast so every host on the segment updates its ARP cache, not just switches and routers, which matters if a question asks specifically what devices get notified during failover.

Mei-Ling H.Nov 22, 2025

The word "communicate" in the stem is the clue, because you need a mechanism the ADC sends out on its own after taking over, with no external request triggering it. RARP is a legacy protocol for a host to discover its own IP, so option A is doing a completely different job. Proxy ARP answers ARP requests on behalf of another device, which is reactive, not something the new primary fires off proactively. So that leaves D, the gratuitous ARP, which the new primary broadcasts unsolicited to tell every switch and router "this virtual IP now lives at my MAC address, update your tables." Quick clarifying question for anyone who knows the ADC internals well: when the new primary sends those GARPs after failover, does it send them out of all interfaces simultaneously, or only the interface that owns the virtual IP at that moment? I ask because I want to understand if there is a timing window where some switches could still be forwarding to the old primary's MAC while others have already updated.

Nina C.Nov 26, 2025

This one actually showed up on my practice test last week and I almost second-guessed myself into picking B because proxy ARP sounds like it involves updating other devices, but then I remembered that gratuitous ARP is specifically the unsolicited broadcast a device sends out to say "hey, update your tables, this IP now maps to MY MAC," which is exactly what the new primary ADC needs to do after a failover so traffic stops going to the old one. The answer is D.

Kemal J.Nov 30, 2025

Yep, D is right. After HA failover the new primary blasts out Gratuitous ARPs so every switch and router in the path flushes the old MAC binding and rewires it to the new primary's interface, no manual intervention needed.

Mei-Ling H.Nov 30, 2025

Good point to remember for the exam, though watch the wording: the GARP updates Layer 3 ARP caches on hosts and routers (IP-to-MAC mapping), while the switch MAC address table gets refreshed as a side effect of seeing the source MAC arrive on the new port, which is just normal MAC learning, not something unique to GARPs.

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