How to configure first Hop-Redundancy with GLBP on Cisco IOS

Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol that attempts to overcome the limitations of existing redundant Router protocols (HSRP, VRRP) by adding basic load balancing functionality. GLBP introduced in the Cisco IOS Release 12.2(15), GLBP is supported by Cisco 1700, 2600, 3620, 3631, 3640, 3660, 3725, 3745, 7100, 7200, 7400, 7500 series.

Gateway load balancing protocol (GLBP) performs similar function to HSRP and VRRP. In both HSRP and VRRP, group of routers participating in first hop-redundancy has one Active Router and can have multiple Standby routers. At one single time, traffic is being passed through Active router, leaving Standby routers with unused bandwidth. Standby routers will only become active once Active router in a group fails. We can create multiple groups and create different active routers but it results in extra administrative burden.

The advantage of GLBP is it provides load balancing over multiple Routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. The forwarding load is shared among all Routers in a GLBP group rather than being handled by a single Active Router while the other Routers stand idle. Each host is configured with the same virtual IP address, and all Routers in the virtual router group participate in forwarding packets. GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address, User Datagram Protocol (UDP) port 3222 (source and destination).

Following are Cisco GLBP important concepts:

  1. GLBP uses single Virtual IP and multiple mac-addresses to provide first-hop Gateway redundancy.
  2. In GLBP, there can be four routers/gateways in a group
  3. Hello messages are used to communicate with in the group destined to, udp port 3222 and they will be sent every 3 secs by default.
  4. Initially GLBP group members will elect one AVG ( Active Virtual Gateway) and other Routers will act as backup AVG’s incase the active AVG fails
  5. AVG will assign Virtual mac-addresses to other routers, they are known as AVF’s ( Active Virtual Forwarders)
  6. Each AVF assumes responsibility for forwarding packets sent to Virtual Mac’s assigned by AVG.
  7. AVG is responsible for answering ARP requests for Virtual IP’s

Cisco GLBP states:

For a virtual gateway (AVG) the state can be one of the following:

  1. Disabled: Indicates that the virtual IP address has not been configured or learned yet, but other GLBP configuration exists.
  2. Initial: The virtual IP address has been configured or learned but virtual gateway configuration is not complete. An interface must be up and configured to route IP, and an interface IP address must be configured.
  3. Listen: Virtual gateway is receiving hello packets and is ready to change to the “speak” state if the active or standby virtual gateway becomes unavailable.
  4. Speak: Virtual gateway is attempting to become the active or standby virtual gateway.
  5. Standby: Indicates that the gateway is next in line to be the active virtual gateway (AVG).
  6. Active: Indicates that this gateway is the AVG, and that it is responsible for responding to Address Resolution Protocol (ARP) requests for the virtual IP address.

For a virtual forwarder (AVF) the state can be one of the following:

  1. Disabled: Indicates that the virtual MAC address has not been assigned or learned. This is a transitory state because a virtual forwarder changing to a disabled state is deleted.
  2. Initial: The virtual MAC address is known but virtual forwarder configuration is not complete. An interface must be up and configured to route IP, an interface IP address must be configured, and the virtual IP address must be known.
  3. Listen: Virtual forwarder is receiving hello packets and is ready to change to the “active” state if the active virtual forwarder (AVF) becomes unavailable.
  4. Active: Indicates that this gateway is the AVF, and that it is responsible for forwarding packets sent to the virtual forwarder MAC address.

Types of Cisco GLBP Load balancing :

Ciscoglbp_1(config-if)#glbp 1 load-balancing ?
host-dependent  Load balance equally, source MAC determines forwarder choice
round-robin     Load balance equally using each forwarder in turn
weighted        Load balance in proportion to forwarder weighting

There are three different types of Load balancing algorithms in GLBP.


  1. The Mac-address of the host is used to determine which AVF’s  mac is the host directed towards.
  2. A given host is guaranteed to use the same Virtual Mac as long as number of VF’s in the GLBP group are constant
  3. Host dependant GLBP is not recommended in situation where there are small number of hosts, for example, less than 20


  1. GLBP places a weight on each device to calculate the amount of load sharing that will occur through MAC assignment
  2. Each GLBP router in a group will advertise its weight and AVG will act based on that value
  3. For example  we have two routers, Router A and Router B. If router A has double the bandwidth capacity then router B. Router A will be configured with the double weighting value of router B


  1. With Round-robin VF mac-address is used sequentially in ARP replies for the virtual IP
  2. This is the default type of GLBP algorithm
  3. It is suitable for any number of hosts.

Configuring Cisco GLBP:

Suppose to we have two Routers (Ciscoglbp_1 and Ciscoglbp_2) and you would create a GLBP group number 1 between the two Routers.

Router Ciscoglbp_1 Configuration:

Ciscoglbp_1(config)#interface fa0/0

Ciscoglbp_1(config-if)#ip address

Ciscoglbp_1(config-if)#glbp 1 ip

Ciscoglbp_1(config-if)#glbp 1 preempt

Ciscoglbp_1(config-if)#glbp 1 preempt delay minimum 60

Ciscoglbp_1(config-if)#glbp 1 load-balancing round-robin

Ciscoglbp_1(config-if)#glbp 1 priority 150

Ciscoglbp_1(config-if)#glbp 1 weighting track 1 100

Ciscoglbp_1(config-if)#track 1 interface s0/0/0 line-protocol or /ip routing


Router Ciscoglbp_2 Configuration:

Ciscoglbp_2(config)#interface fa0/1

Ciscoglbp_2(config-if)#ip address

Ciscoglbp_2(config-if)#glbp 1 ip

Ciscoglbp_2(config-if)#glbp 1 preempt

Ciscoglbp_2(config-if)#glbp 1 preempt delay minimum 60

Ciscoglbp_2(config-if)#glbp 1 load-balancing round-robin

Ciscoglbp_2(config-if)#glbp 1 priority 140

Ciscoglbp_2(config-if)# glbp 1 weighting track 1 100

Ciscoglbp_2(config-if)# track 1 interface s0/0/1 line-protocol or /ip routing



Verification and Troubleshooting Cisco GLBP:

Following are useful commands for verification and troubleshooting Cisco GLBP.

Show glbp

debug condition glbp

debug glbp errors

debug glbp events

debug glbp packets

debug glbp terse




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