What is GLBP? Explained with Examples

By | 14th November 2015

Understanding Gateway Load Balancing Protocol (GLBP)

Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary solution for redundancy and load balancing in an IP network.

GLBP allow automatic selection and simultaneous recovery from first hop router failures.

GLBP provides load balancing over multiple (router) gateways using a single virtual IP address and multiple virtual MAC addresses.

 Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets.

How GLBP Works.

GLBP works by making use of a single virtual IP address, which is configured as the default gateway on the hosts.

The different routers that assume the forwarding role use different virtual MAC addresses for the same virtual IP address which is used to forward packets.

Unlike HSRP and VRRP, GLBP does not use a single virtual MAC address for the entire group. Instead, the AVG assigns different virtual MAC addresses to each of the physical routers in the group.

There are two types of routers in a GLBP group use in redundancy and load balancing:

 Active Virtual Gateway(AVG):

Within a GLBP group, one virtual router (gateway) is elected as the Active Virtual Gateway(AVG), and its responsibility for the operation of the protocol. This AVG router has the highest priority value or IP address in the group,  it responds to all ARP requests for MAC addresses which it sends to the virtual router IP address.

Active Virtual Forwarder (AVF)

A router within a GLBP group is elected as Active Virtual Forwarder (AVF) This AVF is responsible for forwarding packets sent to the mac address returned by the AVG router.  Multiple active virtual forwarders can exist for each GLBP group.

So, when a client needs to send a packet to the known default gateway (AVG) with configured IP address, it requests for the MAC address by sending an ARP (address resolution protocol) request on the subnet.

The AVG will respond to these ARP requests with the virtual MAC address of each “active” virtual forwarders, based on a configured load sharing algorithm.


Types of GLBP load Balancing Mechanism.

There is two load-balancing mechanism that is used with GLBP. These including :

1. Round-robin: The default one. Each AVF, in turn, is included in address resolution replies for the virtual IP address.

2. Host-dependent: Based on the MAC address of a host where the same forwarder is always used for a particular host.

 – Weighted: Based on weight dependent share of a user between routers.

GLBP Load Balancing mechanism States.

There are different states for AVG and AVF in a GLBP group.

AVG is having six states. These including:

1- Disabled: means no Virtual IP address configured.

2- Initial: means the virtual IP address configured but virtual gateway configuration is incomplete.

3 – Listen: receiving hello messages and ready to “speak” state if AVG unavailable.

4 – Speak: means the Virtual gateway is attempting to become the AVG.

5 – Standby: ready to become the next AVG.

6 –  Active: means the current AVG and responsible for responding to ARP requests for the virtual IP address.

AVF is having four states. These including:

1- Disabled : means no Virtual MAC address assigned.

2 – Initial : The virtual MAC address is OK but virtual forwarder configuration is incomplete.

3 – Listen : Virtual forwarder is receiving hello and ready to “active” state if AVF unavailable.

4 – Active : current AVF and responsible for forwarding packets sent to the virtual forwarder MAC address.

Benefits of GLBP

* Allows full use of resources on all devices without the administrative burden of creating multiple groups

* Provides a single virtual IP address and multiple virtual MAC addresses

* Routes traffic to single gateway shared evenly across multiple routers

* Provides automatic rerouting in the event of any failure


1. Active Virtual Router (AVG)

     > Assigns Mac Address to the member of GLBP group.

     > Responds to ARP requests

2. Virtual Forwarders (AVF)

    > Forwards for given Mac address


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