GLBP MAC Addresses

I understand the concept of how GLBP works and the role of the AVG. However,how does it know which MAC addresses the other AVFs are using? Is this negotiated when the AVF joins the group? I imagine they must be in sync as the AVF would have to answer ARP requests from hosts.

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davenull

I believe the AVG assigns virtual MAC addresses to the AVFs, then only the AVG replies to ARP requests and hands out those addresses in the way it's configured - round-robin, host-dependent or weighted.

fitzybhoy

davenull wrote: »

I believe the AVG assigns virtual MAC addresses to the AVFs, then only the AVG replies to ARP requests and hands out those addresses in the way it's configured - round-robin, host-dependent or weighted.

Ok, so these MAC Adresses are negotiated at some point. If AVG is answers ARP requests, how does a switch associate MAC with correct port? Does AVF sent out gratuitous ARP?

davenull

The MAC addresses are not negotiated, the AVG assigns them. I'm not sure I understand the question though. Here's some further reading if maybe it will help:

GLBP - Gateway Load Balancing Protocol - Cisco Systems

theodoxa

FHRPs (HSRP, VRRP, and GLBP) all use a fixed format of MAC Address. If I remember correctly, for GLBP, the last 4 digits are the only ones that change. The first two digits are the GLBP Group Number in Hex and the Last two are the Gateway Number in Hex.

[EDIT] Couldn't find the specific format on the web, so I had to dig into my SWITCH notes. Apparently, the last 5 digits are variable (4 if the Group Number is less than 255). The first 3 are the group number (GLBP only uses the low order 10 bits giving a range of 000 to 3FF), and the last 2 are the Virtual Forwarder Number.

0007.B40_.____

For example, Group 33, Virtual Forwarder 3 would be 0007.B400.2103