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Purpose of GLBP

The purpose of GLBP is to ensure network connectivity in case of Router or access circuit failure. GLBP automatically Load balances the load from the IP traffic sources.
Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol that attempts to overcome the limitations of existing redundant router protocols by adding basic load balancing functionality.

Other high availability protocols

GLBP functions

  • Active virtual gateway (AVG): Members of a GLBP group elect one gateway to be the AVG for that group. Other group members provide backup for the AVG if the AVG becomes unavailable. The AVG assigns a virtual MAC address to each member of the group.
  • Active virtual forwarder (AVF): Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as AVFs for their virtual MAC address.
  • Communication: GLBP members communicate with each other using hello messages sent every 3 seconds to the multicast address, User Datagram Protocol (UDP) port 3222.

GLBP features

  • Load sharing: Traffic from LAN clients can be shared by multiple routers.
  • Multiple virtual routers: Up to 1,024 virtual routers (GLBP groups) can be on each physical interface of a router, and there can be up to four virtual forwarders per group.
  • Preemption: You can preempt an AVG with a higher priority backup virtual gateway. Forwarder preemption works in a similar way, except that it uses weighting instead of priority and is enabled by default.
  • Efficient resource utilization: Any router in a group can serve as a backup, which eliminates the need for a dedicated backup router because all available routers can support network traffic.

How does GLBP work

Picture 1: GLBP group with Router R1 as AVG and AVF and Router R2 and R3 as AVF

GLBP Roles

AVG - Active Virtual Gateway

The Router with the highest priority is chosen as the AVG. The AVG has the responsebility of assigning MAC addresses to the AVF Routers.

AVF - Active Virtual Forwarder

The AVF Routers Route traffic from the connected clients. Each AVF is assigned its own MAC-Address. The MAC-Address is assigned from the AVG.

Load Balancing

When the clients broadcast ARP requests to get the MAC Address of the default gateway - - the AVG responds with a MAC-Address of one of the AVF's. load balancing can be either

  • Round Robin: The AVG responds to ARP requests in sequence first AVF, second AVF, third AVF, first AVF.... (Default)
  • Host dependent: Specifies a load balancing method based on the MAC address of a host where the same forwarder is always used for a particular host while the number of GLBP group members remains unchanged.
  • Weighted: Specifies a load balancing method that is dependent on the weighting value advertised by the gateway.


Default the hello time for GLBP is 3 seconds and holdtime is 10 seconds. In the example hello time is set to 0.25 second and holdtime is 0.7 second.

hostname R1
interface FastEthernet1/0  
  ip address
  glbp 7 ip
  glbp 7 priority 130
  glbp 7 timers msec 200 msec 700
hostname R2
interface FastEthernet1/0 
  ip address
  glbp 7 ip
  glbp 7 priority 120
  glbp 7 timers msec 200 msec 700
hostname R3
interface FastEthernet1/0 
  ip address
  glbp 7 ip
  glbp 7 priority 110
  glbp 7 timers msec 200 msec 700