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Gateway Load Balancing Protocol or GLBP also a group of routers to provide redundancy and load-balancing services.

Theory

Much like other first-hop redundancy protocols, routers in a GLBP group provide redundant pathway services to a LAN by listening to a virtual ip address. GLBP differs from the other protocols in that in provides built-in load balancing via virtual mac addresses.

Routers in a GLBP group have a configurable priority value. The group member with the highest priority (a higher value is better) will be elected the active virtual gateway or (AVG) for the group. The other routers will serve as backups to the AVG, again n order of priority. The AVG has two main responsibilities:

1. Assign virtual mac addresses to members of the GLBP group. Up to four virtual macs can be assigned.
2. Respond to host ARP requests from the subnet. The AVG will respond to the ARPs with different virtual mac addresses, thus providing load balancing by assigning different hosts to different members of the GLBP group.

GLBP, like HSRP and VRRP has a preemption feature that allows a backup or newly implemented GLBP router to take over AVG duties from the current master if it has a higher priority value. Preemption is disabled by default, but when enabled, has a default 30-second delay before activating.

When the AVG fails the backup forwarder with the highest priority value will assume the duties of the AVG. How it handles the switchover is governed by two timers:

1. redirect timer - Specifies the amount of time that the AVG will continue to respond to ARP requests using the old virtual forwarder MAC addresses. After this expires, the AVG no longer uses the old virtual forwarder addresses. The GLBP forwards will continue to forward any traffic that is already being sent to these addresses, however.
2. secondary holdtime - When this timer expires, the virtual mac addresses are removed from all gateways in the GLBP group and become eligible to be reassigned by the AVG.

GLBP group members exchange info (like priority values) and maintain their connectivity through the exchange of hello messages. Hellos occur every 3 seconds by default. The period a GLBP member will be declared dead is through the use of a holdtime value, which is 10 seconds. GLBP uses the address of 224.0.0.102 and UDP port 3222 to send hello messages over.

GLBP has three options for load-balancing:

1. round-robin - The default method, each virtual mac address in turn is specified in the AVGs ARP request replies.
2. host-dependent - Based on mac address of the host - the same host is always assigned the same virtual mac address
3. weighted - Load balancing is based on a configurable weight value. Allows you to assign a higher percentage of hosts to any forwarder(s) you specify. For instance, if one forwarder had a weight of 150 and the other had a weight of 50, the forwarder assigned 150 would receive 3 times the number of hosts that the 50-weighted forwarder would receive.

Authentication (clear text or MD5) and object tracking (tracked objects can lower the weight value) are both supported by GLBP.

Commands

• glbp group ip ip-address (interface) - Enables GLBP on an interface and configures it to use a virtual-ip with the address specified.
• glbp group priority level (interface) - Sets the GLBP priority level.
• glbp group preempt [delay minimum seconds] (interface) - Enables the preempt feature and allows you to configure the delay timer.
• glbp group-number name group-name (interface) - Enables IP redundancy. and names the GLBP group.
• glbp group timers [msec] hellotime [msec] holdtime (interface)
• glbp group timers redirect redirect timeout (interface)
• glbp group load-balancing [host-dependent | round-robin | weighted] (interface)
• glbp group-number authentication {text string | md5 {md5 options}} (interface)
• glbp group weighting maximum [lower lower] [upper upper] (interface)
• glbp group weighting track object-number [decrement value] (interface) - Configures object tracking to decrease the weight value based on the object-number being tracked. See the object tracking section for more info.

Default Settings

• Priority is 100
• Hello interval is 3 seconds
• Holdtime is 10 seconds
• Preemption is off, when enabled there is a 30-second delay.
• Weight value is 100

Verification

• show glbp [interface-type interface-number] [group] [state] [brief] - Shows info about GLBP groups and forwarders. The brief keyword limits the output to a single line for each forwarder.

Troubleshooting, Tips, and Tricks

• Nothing listed yet

Online Resources

• Cisco IOS IP application services configuration guide on configuring GLBP.

• Cisco Learning Module presentation on configuring layer 3 redundancy with VRRP and GLBP (must be registered to view).

• GLBP explained from Petr of Internetwork Expert.

• Fabio has a nice overview of GLBP over at CiscoZine.

• Aaron's Worthless Words takes us through configuring GLBP.

• Another overview of GLBP with notes and configuration examples.

• Jeremy Stretch of Packetlife.net shares a very useful first-hop redundancy cheatsheet.

• TCPMag.com's Scott Morris explains how GLBP operates.

• A nice wikipedia entry on Gateway Load Balancing Protocol.

• CCIE notes for GLBP from Cisco Tips & Tricks.

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