GLBP (Gateway Load Balancing Protocol)


GLBP (Gateway Load Balancing Protocol)


GLBP Overview

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

GLBP has a key benefit that HSRP and VRRP do not have. This benefit 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. According to the assigned MAC, each one does the forwaring.

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.

Main Differences Between GLBP and HSRP/VRRP

In this lesson, 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 becomes down, the Standby Router becames active and it is used for traffic flow. But with GLBP, the situation is different and this is 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 a Standby Router for 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 GLBP (Gateway Load Balancing Protocol) works.

How GLBP Works?

GLBP mechanims has different routers that has different roles like other First Hop Redundancy Protocols (FHRPs). These GLBP routers types are:

• AVG(Active Virtual Gateway)
• AVF(Active Virtual Forwarder)

And one addition name can be used as SVG(Standby Virtual Gateway) for backup AVG.

To start with AVG(Active Virtual Gateway), AVG is the head of this system. In GLBP, one AVG is elected according to the GLBP Priority values. After this electioni the selected device, AVG, manages the GLBP system. The default GLBP Priority is 100. If the priority values are the same, the tie breaker is the Highest IP address. The router that has the Highest IP address is elected as AVG.

There is a Virtual IP Address in GLBP like other First Hop Redundancy Protocols (FHRPs). The traffic comes to this Virtual IP Address is replied by AVG with different MAC Addresses. AVG assigns these Virtual MAC Addresses to the members of GLBP group. Each router on the GLBP group forwards the traffic that came to their Virtual MAC Address.

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

There is also another router type used in GLBP. This is AVF(Active Virtual Forwarder). There can be many AVF in a GLBP Group. 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, SVG become Active. And whenever the old AVG become online, it will not be AVG again, because by default the preempt is off. This can be changed by configuring preemption.

On the other hand, this feature is on for AVFs by default.If an AVF fails and becomes online again, it will do his job again like before. For AVFs the preemption mechanims uses Weight instead of priority.

GLBP AVG and AVF States

There are different states for AVG and AVF in GLBP.These states are given below.

For AVG there are six states. These states 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 states 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.

GLBP Messages

GLBP has three message types. The GLBP messages are:

• Hello
• Request
• Reply

Hello Packet is used for advertising protocol information, while Request/Reply messages are used for Virtual MAC Address assignment. Hello Packets are sent as Multicast messages while Request/Reply packets are sent as Unicast messages.

The routers in GLBP group sends hello messages for each other as keepalive mechanism. This hellos are send every 3 seconds.

GLBP Multicast address is Hellos are sent to this multicast address. Beside, GLBP uses UDP port 3222.

Load Balancing in GLBP

There are three Load Balancing mechanism that is used with GLBP. These Load Balancing mechanism are :

• Round-robin
• Host-dependent
• Weighted

Round-robin is the default Load Balancing mechanism. In this mechanism, the traffic coming to the Virtual IP Address, is replied each AVF in turn.

Host-dependent is the Load Balancing mechanism based on the MAC address of a host where the same forwarder is always used for a particular host.

Weighted is the depentant Load Balancing mechanims.

redundancy protocols GLBP (Gateway Load Balancing Protocol)

GLBP (Gateway Load Balancing Protocol)

Other First Hop Redundancy Protocols (FHRPs) Lessons

First Hop Redundancy Protocols (FHRPs) – Overview
First Hop Redundancy Protocols (FHRPs) – HSRP
First Hop Redundancy Protocols (FHRPs) – HSRP Configuration on Cisco
First Hop Redundancy Protocols (FHRPs) – VRRP
First Hop Redundancy Protocols (FHRPs) – VRRP Configuration on Cisco
First Hop Redundancy Protocols (FHRPs) – GLBP
First Hop Redundancy Protocols (FHRPs) – GLBP Configuration on Cisco

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