IP Services

The purpose of this document is to discuss the options that Cisco Gateway Load Balancing Protocol (GLBP)-one of Cisco First Hop routing Protocols family-offers in regard to load balancing across multiple gateways. For those unfamiliar with GLBP an introductory paper can be found in the reference section of this document.

GLBP differs from Cisco Hot Standby Redundancy Protocol (HSRP) and IETF RFC 3768 Virtual Router Redundancy Protocol (VRRP) in that it has the ability to load balance over multiple gateways. Like HSRP and VRRP an election occurs, but rather than a single active router winning the election GLBP elects an 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 routers that receive this MAC address assignment are known as Active Virtual Forwarders (AVF).

How network hosts are assigned to each GLBP router can be altered by using one of four load balancing options outlined below. Please note, these options are not able to be deployed concurrently. There is a negligible CPU difference in the operation of either of these options.

If no load-balance algorithm is specified then GLBP will operate in an identical fashion to HSRP, the AVG will only respond to Address Resolution Protocol (ARP) requests with its own Virtual Forwarder (VF) MAC address, and all traffic will be directed to the AVG with no occurrence of load balancing.

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.

For example, if there are two routers in a group and router A has double the forwarding capacity of router B, the weighting value of router A should be configured to be double the amount of router B.

Example of load balancing using weighted option:

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.

Host dependent load balancing will need to be used when using statefull Network Address Translation (NAT) because it requires each host to be returned the same virtual MAC address each time it sends an ARP request for the virtual IP address.

Host dependent load balancing is not recommended for situations where there are a small number of end hosts, for example less than 20, unless there is also a requirement that individual hosts must always use the same forwarder. The larger the number of host, the less likely it is to have an imbalance in distribution across forwarders.

This method uses an algorithm designed to equally distribute hosts among forwarders, this distribution changes only when the number for forwarders permanently changes.

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.

The GLBP load balancing discussed in this document has been available since the first release of GLBP, supported in the following versions:

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Host Dependent is required when an application requires traffic flows to be tracked (for example when using NAT). Round Robin is recommended as the default, it is suitable for all other requirements. Weighted can be used if there are disparities in the capabilities of gateways in the GLBP group. None provides no load balancing.