GLBP Overview and Features

Gateway Load Balancing Protocol (GLBP) can be used to replace HSRP in environments where you would like to load balance between multiple nodes, instead of having an active/standby pair.

In order to enhance on the capabilities of Hot Standby Router Protocol (HSRP), Cisco developed GLBP. GLBP provides automatic, first-hop gateway load balancing, which allows for more efficient resource usage and reduced administrative costs. It is an extension of HSRP and specifies a protocol that dynamically assigns responsibility for a virtual IP address and distributes multiple virtual MAC addresses to members of a Gateway Load Balancing Protocol group.

In campus networks, Layer 3 VLAN interfaces act as the gateway for the hosts. These Layer 3 VLAN interfaces from different switches are load balanced using GLBP. Layer 3 interfaces from multiple switches form one GLBP group. Each group contains one unique virtual IP address.

Supervisor 720 can have a maximum of 1024 GLBP groups (group numbers 0 to 1023). Supervisor 2 supports only one GLBP group. A GLBP group can have a maximum of 4 members. It means that GLBP can load balance up to 4 gateways

Gateway Load Balancing Protocol 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).

Gateway Load Balancing Protocol 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 Gateway Load Balancing Protocol 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 Gateway Load Balancing Protocol, use the “glbp load-balancing[host-dependent | round-robin | weighted]” command in interface configuration mode.

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

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 Gateway Load Balancing Protocol 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 Gateway Load Balancing Protocol group if it has higher priority than the current AVF, use the “glbp forwarder preempt” command in interface configuration mode.

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.

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.

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