Gateway Load Balancing Protocol

This guide explains the functionality of GLBP and shows how to use it in a simple network environment and also in an enterprise network environment.

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Introduction
GLBP States
Basic Lab
Enterprise Lab

Introduction

GLBP is a protocol used to provide redundancy for IP addresses of gateways in a network. It allows multiple routers to share the load of routing traffic to a particular network by creating a virtual gateway IP address that multiple routers can respond to. In GLBP, there are multiple active virtual routers, each of which has its own virtual MAC address and IP address. When a client sends a packet to the virtual IP address, the GLBP routers share the traffic load by using a round-robin algorithm or an algorithm based on a weighting system. One router, the active virtual gateway, is responsible for forwarding the traffic, while the other routers act as backups. If the active virtual gateway fails, one of the backup routers takes over the responsibility of forwarding traffic. GLBP provides redundancy and load balancing, and can help improve network availability and performance. It is often used in networks with high availability requirements, such as data centers, where downtime can have serious consequences.

Description
GLBP allows multiple routers to share the load of routing
It creates a virtual gateway IP address that multiple routers can respond to
Multiple active virtual routers, each of which has its own virtual MAC address and IP address
GLBP routers share the traffic load by using a round-robin algorithm
GLBP provides redundancy and load balancing
GLBP support the maximum number of 4 routers in one group
The Active Virtual Gateway (AVG) responds to ARP queries asking for the MAC addres of a default gateway
The AVG is the active router in the group
All routers in the group can be Active Virtual Forwarders (AVF) and forward traffic
Load balancing is realized by providing different MAC addresses as gateways for clients
Clients can use the same gateway IP, but different routers receive the packets to forward because of the different MAC addresses
The default hello timer is 3 seconds
The default hold time is 10 seconds
GLBP supports MD5 authentication for communication between all routers
The multicast group for GLBP is 224.0.0.102
The default priority is 100
The network protocol is UDP 3222

GLBP States

State
Initial state: In this state, the router initializes and starts the GLBP protocol
Listen state: In this state, the router listens for hello messages from other GLBP routers in the group
Speak state: In this state, the router sends hello messages to the other GLBP routers in the group
Standby state: In this state, the router has received hello messages from the active virtual gateway and is waiting for a higher priority router to become the active virtual gateway
Active state: In this state, the router is the active virtual gateway, forwarding traffic to the virtual IP address
Speak-learn state: In this state, the router can be both a speaker and a listener at the same time. The router can learn the virtual MAC address of the active virtual gateway in this state
Learn state: In this state, the router learns the virtual MAC address of the active virtual gateway

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