Gateway Load Balancing Protocol (GLBP)

GLBP is a Cisco proprietary protocol. This protocol selects a single Active Virtual Gateway (AVG). The job of the AVG is to assign virtual MAC addresses to each of the other GLBP routers and to assign each network host to one of the GLBP routers.

The following load balancing options are supported

None
Similar to HSRP


Weighted
This is the ability GLBP to place a weight on each device when calculating the amount of load sharing that will occur through MAC assignment. Each GLBP router in the group will advertise its weighting and assignment; the AVG will act based on that value.

There is no information that describes how this is achieved. 

See this for an example of how weights are assigned.

Host dependent
The MAC address of a host is used to determine which VF MAC address the host is directed towards. This ensures that a host will be guaranteed to use the same virtual MAC address as long as the number of VFs in the GLBP group is constant.
 
Round robin
With Round Robin each VF MAC address is used sequentially in ARP replies for the virtual IP address. Round Robin load balancing is suitable for any number of end hosts.

• Group numbers are restricted to the range from 0 to 1023.
• Uses mac address 0007.b400.0xxx. There seems to be no direct method of determining the mac address for each group. This must be a cisco proprietary equation?
• Each virtual forwarder in a group can be assigned the next mac address in sequence. As a result you can only have 1024 groups & 4 macs per group = 4096 mac's. Hence the last three octets are reserved of the mac address.
• The multicast address 224.0.0.102 is used to send hello messages
• Send traffic on UDP/3222
• Priority (not a metric so higher number wins)  is 0-255 with the default of value of 100.
• Authentication: No, clear text, MD5