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GLBP overview and features




The Gateway Load Balancing Protocol feature provides automatic router backup for IP hosts configured with a single default gateway on an IEEE 802.3 LAN. Multiple first hop routers on the LAN combine to offer a single virtual first hop IP router while sharing the IP packet forwarding load. Other routers on the LAN may act as redundant GLBP routers that will become active if any of the existing forwarding routers fail.

GLBP performs a similar, but not identical, function for the user as the HSRP and the VRRP. HSRP and VRRP protocols allow multiple routers to participate in a virtual router group configured with a virtual IP address. One member is elected to be the active router to forward packets sent to the virtual IP address for the group. The other routers in the group are redundant until the active router fails. These standby routers have unused bandwidth that the protocol is not using. Although multiple virtual router groups can be configured for the same set of routers, the hosts must be configured for different default gateways, which results in an extra administrative burden. 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).


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.

There are two types of routers in a GLBP group which make redundancy and load balancing work. 

Active Virtual Gateway (AVG): One virtual gateway within a GLBP group is elected as the active virtual gateway, and is responsible for the operation of the protocol. This router has the highest priority value, or the highest IP address in the group, if there is no highest priority. The AVG answers all ARP requests for the virtual router address. Which MAC address it returns depends on which load-balancing algorithm it is configured to use.

Active Virtual Forwarder (AVF): One virtual forwarder within a GLBP group is elected as active virtual forwarder for a specified virtual MAC address, and is responsible for forwarding packets sent to that MAC address. Multiple active virtual forwarders can exist for each GLBP group.

GLBP features

A) Load Sharing: You can configure GLBP in such a way that traffic from LAN clients can be shared by multiple routers, thereby sharing the traffic load more equitably among available routers. The load sharing available are:

-> 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.

-> Round-robin: Specifies a load balancing method where each virtual forwarder in turn is included in address resolution replies for the virtual IP address. This method is the default.

-> Weighted: Specifies a load balancing method that is dependent.

-> To specify the load-balancing method used by the AVG of the GLBP, use the "glbp  load-balancing[host-dependent | round-robin | weighted]" command in  interface configuration mode.

B) Multiple Virtual Routers: GLBP supports up to 1024 virtual routers (GLBP groups) on each physical interface of a router, and up to 4 virtual forwarders per group.

C) Preemption: The redundancy scheme of GLBP enables you to preempt an active virtual gateway with a higher priority backup virtual gateway that has become available. Forwarder preemption works in a similar way, except that forwarder preemption uses weighting instead of priority and is enabled by default.

->To configure the gateway to take over as AVG for a GLBP group if it has higher priority than the current AVG, use the "glbp preempt" command in interface configuration mode.

-> To configure a router to take over as AVF for a GLBP group if it has higher priority than the current AVF, use the "glbp forwarder preempt" command in interface configuration mode.

D) Authentication: You can use a simple text password authentication scheme between GLBP group members to detect configuration errors. A router within a GLBP group with a different authentication string than other routers will be ignored by other group members.

To configure an authentication string for the GLBP, use the "glbp group authentication text string" command in interface configuration mode.

E) Tracking: You can track different interfaces to decrement the GLBP weighting by varying amounts.

-> To configure an interface to be tracked where the GLBP  weighting changes based on the state of the interface, use the "track object-number interface type number {line-protocol | ip routing}" command in global configuration mode.

By using  the "glbp weighting track" and "track" 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.

Error - Debug glbp errors

Router# debug glbp errors
GLBP Errors debugging is on 1d19h:
GLBP: Fa0/0 API active virtual address not found 1d19h:
GLBP: Fa0/0 API active virtual address not found 1d19h:
GLBP: Fa0/0 API active virtual address not found 

GLBP generates the GLBP: API active virtual address x.x.x.x not found message if the x.x.x.x address being queried by the IP Routing process is not in use by GLBP. This message is seen if the debug glbp command is issued on the router. This is just an informational message indicating a normal situation, that GLBP is not already using the address being queried by the IP routing process, and it does not require any corrective action.