Understanding Gateway Load Balancing Protocol (GLBP)
Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary solution for redundancy and load balancing in an IP network. GLBP allow automatic selection and simultaneous recovery from first hop router failures.
GLBP is designed to overcome some of the limitations of earlier First Hop Redundancy Protocols (FHRPs) such as Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP). GLBP enables automatic, efficient load balancing of traffic from hosts across multiple gateways (routers or multilayer switches) in a local area network (LAN), in addition to providing redundancy.
GLBP provides load balancing over multiple (router) 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.
How GLBP Works
GLBP operates by electing one router as the Active Virtual Gateway (AVG) and one or more routers as Active Virtual Forwarders (AVFs). The AVG is responsible for assigning a virtual MAC address to each member of the GLBP group, which then allows those routers to forward packets as AVFs. This mechanism ensures that multiple routers can actively participate in forwarding traffic, which enhances both the bandwidth utilization and redundancy of the network.
Election of the Active Virtual Gateway (AVG): When GLBP is enabled on a network, the routers participate in an election process to choose one router as the AVG. The election is based on the priority assigned to each router; the router with the highest priority becomes the AVG. If priorities are equal, the router with the highest IP address is elected as the AVG.
Assignment of Virtual MAC Addresses: The AVG assigns a unique virtual MAC address to each router in the GLBP group, making them Active Virtual Forwarders (AVFs). These AVFs are responsible for forwarding the packets sent to the virtual IP address of the GLBP group.
Load Balancing: GLBP supports three types of load balancing methods: i). round-robin, ii). weighted, and iii). host-dependent.
The round-robin method distributes packets evenly across all AVFs. The weighted method distributes packets according to the weight assigned to each AVF, allowing for more flexible load distribution based on the capacity of each router. The host-dependent method ensures that a host uses the same AVF as long as it is up, which can be useful for maintaining session consistency.
Failover Mechanism: If an AVF fails, the AVG automatically redistributes the load among the remaining AVFs, ensuring uninterrupted network service. Similarly, if the AVG fails, a new AVG is elected among the AVFs, and the new AVG continues the operation without disrupting the traffic flow.
Advantages of GLBP
Load Balancing: GLBP allows multiple gateways to participate in forwarding traffic, which efficiently utilizes network resources and increases bandwidth.
Redundancy: Provides redundancy for the first hop internal network, ensuring network availability even if one or more routers fail.
Single Virtual IP: Like other FHRPs, GLBP uses a single virtual IP address, simplifying network configuration and management for connected hosts.
Types of GLBP load Balancing Mechanism.
There is two load-balancing mechanism that is used with GLBP. These including :
1. Round-robin: The default one. Each AVF, in turn, is included in address resolution replies for the virtual IP address.
2. Host-dependent: Based on the MAC address of a host where the same forwarder is always used for a particular host.
– Weighted: Based on weight dependent share of a user between routers.
GLBP Load Balancing mechanism States.
There are different states for AVG and AVF in a GLBP group.
AVG is having six states. These including:
1- Disabled: means no Virtual IP address configured.
2- Initial: means the virtual IP address configured but virtual gateway configuration is incomplete.
3 – Listen: receiving hello messages and ready to “speak” state if AVG unavailable.
4 – Speak: means the Virtual gateway is attempting to become the AVG.
5 – Standby: ready to become the next AVG.
6 – Active: means the current AVG and responsible for responding to ARP requests for the virtual IP address.
AVF is having four states. These including:
1- Disabled : means no Virtual MAC address assigned.
2 – Initial : The virtual MAC address is OK but virtual forwarder configuration is incomplete.
3 – Listen : Virtual forwarder is receiving hello and ready to “active” state if AVF unavailable.
4 – Active : current AVF and responsible for forwarding packets sent to the virtual forwarder MAC address.
Benefits of GLBP
* Allows full use of resources on all devices without the administrative burden of creating multiple groups
* Provides a single virtual IP address and multiple virtual MAC addresses
* Routes traffic to single gateway shared evenly across multiple routers
* Provides automatic rerouting in the event of any failure
In Conclusion
GLBP stands out among FHRPs for its unique ability to provide both load balancing and redundancy through the use of multiple active gateways. This not only maximizes the utilization of network resources but also ensures higher availability and reliability of the network infrastructure. By leveraging GLBP, organizations can enhance their network performance and resilience, ensuring efficient and continuous operation of their critical services.
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