GLBP stands for Gateway Load Balancing Protocol. In this article, I will explain where GLBP is used , where it shouldn’t be used with the topologies. GLBP is a Cisco preparatory protocol. In most networks, design requirements might be to use only standard based protocols. If that is the case, GLBP is not a standard based protocol and business cannot use it.
Unlike HSRP and VRRP, GLBP supports flow based load balancing. HSRP and VRRP can only supports active/standby redundancy or Vlan based load balancing. GLBP was invented to provide an active-active traffic forwarding the network traffic but there is almost no use case in today networks. In some cases, GLBP has create more problem than it should solve.
Figure - GLBP at the Enterprise Internet Edge
In the above picture, I depict classical Enterprise Internet edge network. Firewall, Layer 2 switch and Internet Gateways. Service Providers don’t use stateful devices such as Firewall at the Internet edge.
If in this network, GLBP is used, firewall would send an ARP for the default gateway and only one of the Internet Gateway routers would be used as a default gateway. If there would be two firewalls, another firewall could use the second Internet gateway routers as the default gateway and traffic from the firewall to the Internet could be load balanced.
Let’s say R2 is chosen as the default gateway, if that is the case, firewall sends the Internet traffic always to R2 only. This is called polarization. Situation can be worse. Imagine we have a BGP running on the Internet gateways and we implemented a Local Preference policy to choose R1 as default gateway.
In this case, all the traffic from the firewalls first would be sent to R2 , then R2 would forward the traffic over R1-R2 link to R1 and finally R1 would send the traffic to upstream provider for the Internet. GLBP creates polarization issue at the Internet Edge if there is a firewall. If active-active redundancy is required for the Layer 2 networks, most Enterprises rely on Multi Chassis Link Aggregation Group (Multi Chassis Ether channel in Cisco).
Cisco VSS and VPC technologies use Multi Chassis Link Aggregation concept. If scalability is concern, then Enterprises or Service Providers, for their layer 2 networks, rely on Layer 2 multipathing solution (Fabricpath is common).