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GLBP (Gateway Load Balancing Protocol )

August 9, 2012 6 comments

I will take a little break regarding Multicast 🙂 and this time I will talk about a First Hop Redundancy Protocol (FHRP) which is GLBP (Gateway Load Balancing Protocol).

I have decided to look deeper into GLBP as I am quite familiar with HSRP or VRRP but the other day I had to troubleshoot a network which was configured with GLBP and although I can easily configure it there are some mechanisms of GLBP that are still unclear to me. So let´s clear them out!

Gateway Load Balancing Protocol (GLBP) is one of the First Hop Redundancy Protocol you can use when configuring First hop redundancy gateway for your end hosts. GLBP is Cisco proprietary and the big advantage compare to HSRP and VRRP is that you can load balance between the different gateways without changing the default gateway on the end hosts as you would do with HSRP for example to achieve load balancing which results in an extra administrative burden.

 GLBP is composed of two components:

  • The Active Virtual Gateway (AVG) at the control plane
  • The Active Virtual Forwarders (AVF) at the data plane

The AVG responds to ARP requests sent by end hosts to the virtual gateway IP address, and replies with different virtual MAC addresses that correspond to different active virtual forwarders (AVFs).

The AVF are responsible for sending traffic destined to their Virtual Mac address which has been allocated to them by the AVG. Both the AVG and AVFs are redundant, i.e. if a primary physical router representing the AVG or an AVF fails, another physical router will take its role.

Let´s consider the following topology to demonstrate how GLBP works:

Platform/IOS: Cisco 2691/12.4(15)T11 Adv IP services

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