Redundancy Protocols – Part 5 (GLBP) - :
Content Protection by

GLBP (Gateway Load Balancing Protocol)

GLBP (Gateway Load Balancing Protocol) is an another redundancy protocol.
It is a Cisco proprietary redundancy protocol like HSRP. So ,you can only use this protocol on Cisco devices.

GLBP has a key benefit that HSRP and VRRP do not have. This is load-balancing.

GLBP provides load balancing over multiple routers 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.

GLBP(Gateway Load Balancing Protocol) supports up to 1024 virtual routers meaning GLBP groups on each physical interface of a router. And for each GLBP group, 4 virtual forwarders are supported.

In this article, I think it will be better to compare GLBP with HSRP and VRRP. So, you can easily understand the difference between GLBP and the other two protocol.

First of all, while using HSRP and VRRP, there is only one active router at the same time as I mentined before. The traffic only goes through on this active router. Whenever the active router become down, the the standby router became active and used for traffic. But with GLBP, the situation is different and more efficient. In GLBP, there can be more than one active router. So you can use these routers at the same.

Secondly, we can configure groups for HSRP and VRRP on routers, and we can configure one router as an active router for one group and standby of another group. So it looks like a basic load-balancing. But this is not widely used in reality. With GLBP, we can use more active routers and we can use them with different load balancing adjustments. So the traffic goes through all of them with different ratios.

At this point, let’s explore how this GLBP works.

In GLBP there are two types routers. One of them is AVG(Active Virtual Gateway) and the other is AVF(Active Virtual Forwarder). And one addition name can be used as SVG(Standby Virtual Gateway) for backup AVG.

To start with AVG, AVG is the head of this system. In GLBP, one AVG is choosen according to the priority values and then this AVG manages the system. The default priority is 100 but can be changed to a different value. If the priority values are the ame, the the highes ip addressed router become AVG.

AVG assignes virtual MAC address to the member of GLBP group. Each router on the group forwards the traffic that came to their virtual MAC address. There is also a virtual ip address that GLBP configured on. AVG replies with ARP packets to the traffic came to this ip address with different MAC address. This different MAC address are the virtual MAC addresses assigned to the routers as I mentioned above.

There is only one AVG in one group. If it fails, the one that is determined before as a SVG, is become AVG.

But in a GLBP grup there are many AVFs. Each of these AVFs is the primary AVF for the MAC address that is assigned to them. And for other ones, this AVF become secondary AVF if there will be a failure on primary AVF of another MAC address.

In this point it is important to mention one additional point about AVG and AVF routers. If AVG fails the SVG become active. And whenever the old AVG become online, it will not be AVG again, because by default the preempt is off. But this can be changed. On the other hand, this feature is on for AVFs by default.If an AVF fails and comes again, then it will do his job again like before. For AVFs the preemption is uses weight instead of priority.

The routers in GLBP group sends hello messages for each other as keepalive mechanism. This helleos are send every 3 seconds. These routers are communicate eachother with multicast IP address on UDP 3222 port.

In GLBP tracking mechanism can be also used for decrementing weight values.

There are different states for AVG and AVF in GLBP.

For AVG there are six states. These are :

– Disabled : Virtual IP address has not been configured…
– Initial : The virtual IP address is OK but virtual gateway configuration is not complete…
– Listen : Receiving hello and ready to “speak” state if AVG or SVG unavailable.
– Speak : Virtual gateway is attempting to become the AVG or SVG.
– Standby : I am the next AVG.
– Active : I am AVG and responsible for responding to ARP requests for the virtual IP address.

And for AVF, there are four states. These are:

– Disabled : Virtual MAC address has not been assigned…
– Initial : The virtual MAC address is OK but virtual forwarder configuration is not complete…
– Listen : Virtual forwarder is receiving hello and ready to “active” state if AVF unavailable.
– Active : I am AVF and I forward packets sent to the virtual forwarder MAC address.

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

– Round-robin: The default one. Each AVF in turn is included in address resolution replies for the virtual IP address.
– 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 user between routers.

GLBP’de kullanılacak load-balancing türünü belirtmek için yapılması gereken “glbp grup_numarası load-balancing [host-dependent | round-robin | weighted]” komutunu girmek olacaktır. Routerlarda istenilen weight değerlerini ayarlamak için “glbp grup_numarası weighting değer(1-255)” komutu girilmelidir.

GLBP Configuration Example on Cisco IOS

Let’s do a basic GLBP configuration for two routers. This will hepl to understand how GLBP configuration commands are like other redundancy protocol configuration commands on cisco routers.

redundancy protocols GLBP (Gateway Load Balancing Protocol)

GLBP (Gateway Load Balancing Protocol)

On Router1:

Router1(config)#interface fastethernet 0/0
Router1(config-if)#ip address
Router1(config-if)#glbp 1 ip
Router1(config-if)#glbp 1 priority 130
Router1(config-if)#glbp 1 preempt
Router1(config-if)#glbp 1 load-balancing round-robin

On Router2:

Router2(config)#interface fastethernet 0/0
Router2(config-if)#ip address
Router2(config-if)#glbp 1 ip
Router2(config-if)#glbp 1 load-balancing round-robin

To sum up this configuration, here, the Router1 will be the AVG. And it will continue also being an AVF. Because it has a higher priority which is 130. The Router 2’s priority is default 100. When an ARP request to the virtual ip address came to Router 1, it will reply with two different MAC address according to round-robin method. Then, two clients’s trafic will go throuhg different routers. First one go throught Router1 and the other will go throuht Router 2.

I hope this article will be helpful for you. Keep on;)

Redundancy Protocols – Part 1
Redundancy Protocols – Part 2 (HSRP)
Redundancy Protocols – Part 3 (HSRP Configuration)
Redundancy Protocols – Part 4 (VRRP)

About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

One comment for “Redundancy Protocols – Part 5 (GLBP)”


verry good.
Thank you.

January 18th, 2017 at 16:01

Leave a Reply

Copy Protected by Chetan's WP-Copyprotect.