Multiple first hop routers on the LAN combine to offer a single virtual first hop IP router while sharing the IP packet forwarding load. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using.

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In this tutorial, we will learn how GLBP works. If one group fails, we must reconfigure the default gateways on the hosts, which results in extra administrative burden.

The election is based on the priority of each gateway highest priority wins. If all of them have the same priority then the gateway with the highest real IP address becomes the AVG. For example in the topology above suppose all of the gateways have the same priority and GLBP is turned on at the same time on all gateways or they are configured with the preempt feature , R4 will be elected AVG because of its highest IP address In this case we only have four gateways so surely they are all elected AVFs.

R4 assigned the MAC addresses of And load balancing is achieved! What will happen now? Communication between R4 continues without disruption or change at the host side. Maybe you have a question to ask here. So how about the Switch? How can the switch forward the frames to the new SVG on another port? Well, the answer here is when the standby becomes the active it will send a gratuitous ARP reply to flush the CAM tables of the switches and the ARP cache of the hosts.

So the switch will learn the new port for MAC To detect a gateway failure, GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address This is the default load balancing mode in GLBP and is suitable for any number of end hosts. Weighted load-balancing algorithm: Traffic is balanced proportional to a configured weight. For example, if there are two routers in a group and R1 has double the forwarding capacity of router B, the weighting value of router A should be configured to be double the amount of R2.

Host-dependent load-balancing algorithm: A given host always uses the same router. The second router then becomes primary. This transition is transparent for the hosts. With this method, the same keys are configured on both ends. One end will send the encrypted key called hash, using MD5 to the other.

At the other side, the same key is also encrypted and compared with the receiving encrypted key. If the two encrypted keys are the same then authentication is approved. The advantage of this method is only the encrypted key is sent through the link. The key for the MD5 hash can either be given directly in the configuration using a key string or supplied indirectly through a key chain.


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