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Level 10
Level 10



Gateway Load Balancing Protocol (GLBP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures, while allowing packet load sharing between a group of redundant routers.


GLBP provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. 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).


This feature was integrated into Cisco IOS  Software Release 12.2(15)T. Use Cisco Feature Navigator to find the latest information about platform support and Cisco IOS Software image support.


Before configuring the GLBP feature, ensure that the routers can support multiple MAC addresses on the physical interfaces. Several Ethernet (LANCE and QUICC) controllers in low-end products can only have a single unicast MAC address in their address filter. To verify this, issue the show interfaces command, and ensure that the displayed hardware is neither LANCE nor QUICC. These controllers do not support association of multiple MAC addresses with the interface.


Configuration Example:

GLBP works by making use of a single virtual IP address, which is configured as the default gateway on the hosts. The different routers that assume the forwarding role use different virtual MAC addresses for the same virtual IP address to which packets are forwarded by the hosts.


The following sample configuration sets up two routers R1 and R2 as GLBP, gateways serving the network with as the virtual gateway address.This is the default gateway that would be configured on all the hosts in the network.


The configuration can be:


R1(config)#interface vlan110

R1(config-if)#ip address

R1(config-if)#glbp 1 ip

R1(config-if)#glbp 1 preempt

R1(config-if)#glbp 1 priority 150


R2(config)#interface vlan110

R2(config-if)#ip address

R2(config-if)#glbp 1 ip


Other useful commands can be:


-> You can specify three different types of load balancing in GLBP using following command:



Router(config-if)# glbp <Group no> load balancing round robin / Host-dependent  /  Weighted


Host-dependent uses the MAC address of a host to determine which VF MAC address the host is directed toward. This is used with stateful Network Address Translation (NAT) because NAT requires each host to be returned to the same virtual MAC address each time it sends an ARP request for the virtual IP address. It is not recommended for situations where there are a small number of end hosts (fewer than 20).


Weighted allows for GLBP to place a weight on each device when calculating the amount of load sharing.


Round-robin load balancing occurs when each VF MAC address is used sequentially in ARP replies for the virtual IP address. Round robin is suitable for any number of end hosts.


-> "glbp authentication" =The authentication string is sent in plain text in all GLBP messages. The same authentication string must be configured on all the routers that are configured to be members of the same GLBP group, to ensure interoperation. A router will ignore all GLBP messages that contain the wrong authentication string.


-> "glbp forwarder preempt" =Use to configure a router to take over as AVF for a GLBP group if it has higher priority than the current AVFRouter to take over as AVF for a GLBP group if it has higher priority than the current AVF


-> " glbp <Group No> preempt delay minimum <Delay Time>" = To Configures the router to preempt, or take over.


-> "glbp timer" = The time between hello packets sent by the GLBP gateway and the time that the virtual gateway and virtual forwarder information is considered valid.


-> "glbp timers redirect" =The time during which the AVG for a GLBP group continues to redirect clients to a secondary active virtual forwarder (AVF),


-> "glbp weighting" =To specify the initial weighting value of the GLBP gateway,


-> "glbp weighting track" =By using  the "glbp weighting track" and "track object-number interface type number {line-protocol | ip routing}" commands to configure parameters for an interface to be tracked. If an interface on a router goes down, the weighting for the router can be reduced by a specified value.

Verifying GLBP:



Router# show glbpDisplays GLBP information
Router# show glbp brief  Displays a brief status of all GLBP groups
Router# show glbp 1   Displays information about GLBP group 1
Router# show glbp fastethernet 0/0Displays GLBP information on interface fastethernet 0/0
Router# show glbp fastethernet 0/0 1Displays GLBP group 1  information on interface fastethernet 0/0

Debugging GLBP:


Router# debug condition glbp   Displays GLBP condition messages
Router# debug glbp errorsDisplays all GLBP error messages
Router# debug glbp eventsDisplays all GLBP event messages
Router# debug glbp packetsDisplays messages about packets sent and received
Router# debug glbp terseDisplays a limited range of debugging messages


Also See:

GLBP - Gateway Load Balancing Protocol

GLBP overview and features


Router# show glbp


Displays GLBP information


Router# show glbp brief


Displays a brief status of all GLBP groups


Router# show glbp 1


Displays information about GLBP group 1


Router# show glbp fastethernet 0/0


Displays GLBP information on interface fastethernet 0/0


Router# show glbp fastethernet 0/0 10  


Displays GLBP group 1 information   on interface fastethernet 0/0

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