The active virtual gateway assigns forwarders
GLBP elects an active virtual gateway (AVG) for a group. The AVG answers ARP for the group’s virtual IP and can return different virtual MAC addresses for different hosts. An active virtual forwarder (AVF) handles traffic sent to the virtual MAC it owns.
With round-robin load sharing, the AVG cycles through available virtual forwarder MAC addresses in its ARP replies. Hosts still use the same default gateway IP. Forwarding is shared through their MAC mappings, so successive packets from one host normally use the same forwarder while its ARP entry is valid.
Gateway election and forwarding failover are separate
A router can act as both AVG and AVF, and a group can have multiple AVFs. If an AVF fails, a secondary forwarder can take responsibility for its virtual MAC so existing hosts can continue using that mapping. If the AVG fails, another group member takes over its ARP-answering role.
Within one HSRP or VRRP group, a single active router normally forwards traffic sent to the group’s virtual MAC. Within one GLBP group, multiple active forwarders can carry traffic for the same virtual gateway IP.
Try it in a lab
- On Cisco equipment supporting GLBP, put two routers and two hosts on one LAN, configure one virtual IP and enable round-robin load sharing. Confirm the elected AVG and both active virtual forwarder MAC addresses before testing.
- Send consecutive new ARP requests for the virtual IP from the two hosts. Confirm that each host uses the same gateway IP and can receive a different virtual MAC when both forwarders are available.
- In the lab, take down a forwarder that is separate from the AVG. Check which surviving device assumes its virtual MAC and verify that an affected host can resume forwarding through the gateway.