Which statement about GLBP is true?
A. Unused bandwidth is never utilized
B. They communicate under TCP port 3222 for both source and destination.
C. They become active if any of the existing forwarding switches fail.
D. They become restricted if any of the existing forwarding switches fail.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp_fhrp/configuration/15-mt/fhp-15-mt-book/Configuring-GLBP.html#GUID-1A23B304-3BDF-45C4-AFFD-99B2AF5A858E
The advantage of GLBP is that it additionally provides load balancing over multiple devices (gateways) using a single virtual IP address and multiple virtual MAC addresses.
GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 224.0.0.102, UDP port 3222 (source and destination).
With the introduction of the GLBP SSO functionality, GLBP is stateful switchover (SSO) aware. GLBP can detect when a device is failing over to the secondary router processor (RP) and continue in its current group state.