Definition of Gateway Load Balancing Protocol
Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol designed to provide automatic gateway redundancy and load balancing in a network. Unlike other redundancy protocols like HSRP or VRRP, which only allow for a single active gateway at any given time, GLBP utilizes multiple available gateways simultaneously to distribute traffic load more efficiently.
Detailed Explanation of Gateway Load Balancing Protocol
GLBP allows multiple routers (known as GLBP group members) to participate in a single virtual router setup with each router able to forward packets. Here’s how it achieves redundancy and load balancing:
- Virtual Router Identity: GLBP creates a single virtual IP address that is shared among all group members. This virtual IP is used as the default gateway by devices on the LAN.
- Multiple Virtual MAC Addresses: Unlike HSRP or VRRP, which use a single virtual MAC address, GLBP assigns a unique virtual MAC address to each member of the GLBP group, allowing multiple routers to forward packets in parallel.
- Roles within GLBP:
- Active Virtual Gateway (AVG): One GLBP member is elected as the AVG, responsible for assigning virtual MAC addresses to the other members (known as Active Virtual Forwarders or AVFs) and responding to ARP requests for the virtual IP address.
- Active Virtual Forwarders (AVFs): These are GLBP members that actually forward the packets. Each AVF is assigned a unique virtual MAC address by the AVG.
- Load Balancing Methods: GLBP supports three types of load balancing:
- Round-robin: Each AVF in turn is given the next new request, distributing the load equally.
- Weighted: Each AVF is assigned a weight based on its processing capacity, and the AVG distributes clients proportionally to these weights.
- Host-dependent: A client always uses the same AVF for as long as it is reachable.
- Health Check and Redundancy: GLBP monitors the health of each AVF. If an AVF fails, the AVG automatically redirects traffic to the remaining AVFs, ensuring continuous availability.
Example of GLBP in Action
Consider a corporate network that includes two routers, Router A and Router B, connected to the same LAN. The network administrator sets up GLBP to ensure that both routers can share the load of routing traffic to the internet and provide backup for each other.
- Configuration:
- Routers A and B are configured as a GLBP group with a virtual IP address of 192.168.1.1.
- Router A is elected as the AVG because it has a higher priority.
- Router A assigns one virtual MAC address to itself and another to Router B.
- Operation:
- Devices on the LAN are configured with the default gateway IP address 192.168.1.1.
- When a device sends an ARP request for 192.168.1.1, Router A (as AVG) responds with one of the virtual MAC addresses. Assume it uses round-robin, so the first request gets Router A’s MAC, and the second gets Router B’s MAC.
- Traffic from the LAN devices is then balanced between Router A and Router B, reducing the load on each router and utilizing both their capacities.
- Failover Scenario:
- If Router A fails, Router B takes over as the AVG in addition to being an AVF. It continues to handle traffic originally directed to Router A, ensuring no interruption in service.
Summary of Gateway Load Balancing Protocol
GLBP is particularly useful in environments where network traffic load is heavy, and downtime cannot be tolerated. By distributing the load and providing redundancy, GLBP enhances both the efficiency and the reliability of the network. This protocol is ideal for businesses requiring high availability combined with effective utilization of networking resources.
Useful Links
- https://www.cisco.com/c/en/us/td/docs/switches/connectedgrid/cg-switch-sw-master/software/configuration/guide/availability/Availability/red_hsrp.html
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