GLBP Simplified: Your Shortcut To Superior Network Fail-over

Introduction

GLBP or Gateway Load Balancing Protocol is developed by Cisco in 2005, provides both redundancy and load balancing for IP traffic. It can allow multiple routers to share the outgoing traffic against a single virtual gateway. Unlike HSRP or VRRP, which provide only redundancy, but don’t distribute traffic across the routers. While the GLBP has the capability of not only forwarding the IP traffic, but it can distribute the same traffic across multiple routers simultaneously. It helps optimize bandwidth usage and better manage resources.

How Does GLBP work?

Just like HSRP or VRRP, GLBP also uses a virtual IP address to represent the group of routers. It uses the concept of AVG and AVF, but what are these?
• AVG: AVG stands for the active virtual gateway responsible for assigning virtual MAC addresses to each router in a group. GLBP elects only one AVG router, while all the other routers will be backup if AVG becomes unavailable. AVG monitors and receives all the traffic. It gets the traffic and then sends it to those routers (AVF) that are currently free. All the process is transparent for the internal client PCs.
• AVF: AVF is the abbreviation of active virtual forwarders which sends the forwarding traffic across the network.

GLBP Operation Modes

GLBP uses three operating modes for load-balancing modes:
1) Round-Robin: This is the default method of GLBP load-balancing. In this method, each router in the GLBP group gets traffic from different clients sequentially. There are no specific criteria for routers to get traffic from each client.
2) Weighted: Sometimes traffic is distributed among the routers, based on the weight. A router with a higher weight will handle more traffic, while a low-weight configured router will handle lower traffic.
3) Host-dependent: This is the third method of GLBB in which we bind different clients to a particular type of router based on their MAC address. This is beneficial for applications that require session maintenance.

Lab Topology for GLBP

We have designed the following lab topology in GNS3. Configure all the IP addresses in it and then run the EIGRP protocol on it for communication.

Also Configure IP addresses on all VPCS;

Configure GLBP on R3 and R4;

Run the command “show glbp” command both on R3 and R4;

Similarly, run the “show glbp active” command for checking active forwarders;

We are going to test the connectivity between PC1 and PC3 (connected to R5) through the use of the ping command;

Shutdown the interface fastethernet 0/1 of R3;

Again check connectivity between PC1 and PC3;

The PC1 is still pinging PC3 without no interruption.

Now run the “show glbp” command both on R3 and R4;

R4 has taken over charge as an active router.

Again, enable R3’s fast ethernet 0/1 interface by issuing “no shutdown” command.

Run the show command both on R3 and R4 to check active routers.