Gateway load balancing protocol (GLBP)

This is cisco proprietary and communicates via UPD multicast at port 3222. It provides load balancing between routers in a group. It uses one virtual IP address but multiple MAC address. The active virtual agteway (AVG) responds to ARP request with the virtual MAC address, Active virtual Forwarder(AVF) associated to the virtual mac address forwards the traffic.

GLBP supports three load balancing options:

Weighted load balancing – preconfigured weights assigned to gateways
host-dependent load balancing – hosts will be use the same virtual MAC of an AVF as long as it is reachable
Round-robin – Rotates assigning MAC address between AVF’s equally.

  • The virtual MAC address that GLBP uses is 007.b400.XXYY (XX GLBP group number and YY is AVF number)
  • GLBP is only supported on Cisco‚Äôs 4500, 6500, and Nexus lines

GLBP commands

glbp 1 ip
glbp 1 priority 150
glbp 1 prempt
glbp 1 authentication md5 key-string xyz

track 2 interface fa 0/16 line-protocol
glbp 1 weighting track 2 decrement 40
glbp 1 weighting 100 lower 70 upper 90 (default weighting. If weighting is less than 70, that router cannot be AVF. When the weighting goes above 90 the router will become an AVF again

show glbp