GLBP Overview

In this post you’ll read some key points of GLBP and then I’ll invite you to test a sample implementation of GLBP.

GLBP means Gateway Load Balancing Protocol. What an easy definition 😀

It is a VGP (Virtual Gateway Protocol) / FHRP, similar to HSRP and VRRP. It’s one of the strongest kind of VGPs because it is capable of using multiple physical gateways at the same time.

This protocol attempts to overcome the limitations of existing redundant router protocols by adding basic load balancing functionality.

In addition to being able to set priorities on different gateway routers, GLBP allows a weighting parameter to be set. Based on this weighting (compared to others in the same virtual router group), ARP requests will be answered with MAC addresses pointing to different routers. Thus, load balancing is not based on traffic load, but rather on the number of hosts that will use each gateway router. By default GLBP load-balances in round-robin fashion.

GLBP elects one AVG (Active Virtual Gateway) for each group. Other group members act as backup in case of AVG failure. In case there are more than two members, the second best AVG is placed in the Standby state and all other members are placed in the Listening state. This is monitored using hello and holdtime timers, which are 3 and 10 seconds by default. The elected AVG then assigns a virtual MAC address for each member of the GLBP group, including itself, thus enabling AVFs (Active Virtual Forwarders). Each AVF assumes responsibility for forwarding packets sent to its virtual MAC address. There could be up to four active AVFs at the same time.

Here are the key points of GLBP in my point of view:

  • Unequal Load Balancing is supported
  • Single VIP on multiple VMACs
  • GLBP supports IPv6.
  • GLBP uses multicast address to send hello packets to their peers every 3 seconds over UDP 3222.
  • Hello: 3seconds, Hold: 10seconds

My topology is shown below:

The goal of this configuration is that if one of the links to the active of each GLBP group changed state to down,  R1 & R4 continue to be reachable from each other.

Two GLBP groups should be configured, one for each side of the network. Also, we’ve to track both interfaces of GLBP routers to be informed of their link-state and be capable to change the active/standby routers.

Creating two tracking objects of both interfaces for use in GLBP-weighting for both routers:

These commands should be applied to all GLBP interfaces in interface mode:

These commands should be applied just to the interfaces which we want to be the AVG of each GLBP group# in interface mode:

Useful EXEC Commands:

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Author: Mo Moghaddas

Building to give users more time back and make scheduling a pleasant experience.

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