Understanding GLBP Router ARP Response in Cisco CCNA 200-125 Exam

A

GLBP (Gateway Load Balancing Protocol) is a Cisco proprietary protocol that allows multiple routers to share a virtual IP address and MAC address to represent a single virtual gateway. GLBP provides redundancy and load balancing for hosts on the LAN that use the virtual gateway as their default gateway.

When a host on the LAN sends an ARP request for the virtual gateway's MAC address, the GLBP routers in the virtual gateway group determine which router should reply to the ARP request. The router that replies to the ARP request is known as the Active Virtual Gateway (AVG). The AVG is responsible for forwarding traffic destined for the virtual gateway.

Now let's go through the options one by one to determine which one is correct.

A. The active virtual gateway will reply with one of four possible virtual MAC addresses. This statement is partially correct. The AVG will reply with a virtual MAC address, but there are more than four possible virtual MAC addresses. In fact, there are 1024 possible virtual MAC addresses that GLBP can use. The virtual MAC address is calculated based on the virtual IP address and the group number. Each GLBP group can have a different virtual MAC address.

B. All GLBP member routers will reply in round-robin fashion. This statement is not correct. GLBP uses an algorithm to determine which router should reply to an ARP request. The algorithm takes into account the priority of the router, its current load, and whether it is the AVG or not. The router that meets the criteria will be selected to reply to the ARP request. Round-robin is not used.

C. The active virtual gateway will reply with its own hardware MAC address. This statement is not correct. The AVG will reply with a virtual MAC address, not its own hardware MAC address.

D. The GLBP member routers will reply with one of four possible burned in hardware addresses. This statement is not correct. GLBP uses virtual MAC addresses, not burned-in hardware addresses.

Therefore, the correct answer is A. The active virtual gateway will reply with one of the 1024 possible virtual MAC addresses.