How to Configure GLBP? – Router Switch Blog

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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 224.0.0.102, 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 172.16.1.0 network with 172.16.1.1 as the virtual gateway address.This is the default gateway that would be configured on all the hosts in the 172.16.1.0 network.

The configuration can be:

R1(config)#interface vlan110

R1(config-if)#ip address 172.16.1.10 255.255.255.0

R1(config-if)#glbp 1 ip 172.16.1.1

R1(config-if)#glbp 1 preempt

R1(config-if)#glbp 1 priority 150

R2(config)#interface vlan110

R2(config-if)#ip address 172.16.1.11 255.255.255.0

R2(config-if)#glbp 1 ip 172.16.1.1

Other useful commands can be:

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 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:

Commands	Explaination
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 1	Displays GLBP group 1 information on interface fastethernet 0/0

Debugging GLBP:

Commands	Explaination
Router# debug condition glbp	Displays GLBP condition messages
Router# debug glbp errors	Displays all GLBP error messages
Router# debug glbp events	Displays all GLBP event messages
Router# debug glbp packets	Displays messages about packets sent and received