Cisco Gateway load balancing protocol GLBP, How GLBP works

Gateway Load Balancing Protocol GLBP

A Cisco-proprietary protocol called Gateway Load Balancing Protocol GLBP allows two or more routers to share the duties of serving as a subnet’s default router. Its capacity to balance the packet load (also called load sharing) among a number of redundant routers and protect data traffic from router or circuit failures is one of its special features. Better load-balancing choices than the per-subnet load balancing provided by HSRP and VRRP were the reason behind the introduction of this capability.

How GLBP Works

In order to balance the packet load per host inside each subnet, Gateway Load Balancing Protocol GLBP uses an active/active paradigm.

Virtual Router and Roles: All client devices are given a virtual IP address by GLBP, which they utilize as their default gateway. One router is chosen by the other routers in a Gateway Load Balancing Protocol GLBP group to be the Active Virtual Gateway (AVG). The router in the group with the highest IP address is the AVG, or the router with the higher priority value if priorities are equal. Other routers in the group either go into a Listen state or serve as backup virtual gateways.

Virtual MAC Address Assignment: In load balancing, the AVG is essential. Every ARP request for the virtual IP address is answered by it. The AVG gives each member of the Gateway Load Balancing Protocol GLBP group a distinct virtual MAC address rather than responding with its own MAC address each time. These routers are known as Active Virtual Forwarders (AVFs), and they are in charge of forwarding packets submitted to the virtual MAC addresses they have been assigned. A maximum of four virtual MAC addresses can be supported by a GLBP group.

Load Balancing Process (Per-Host): The AVG uses the virtual MAC addresses of alternating routers (AVFs) to respond to each new ARP request for the virtual IP address in order to balance the load.

• For instance, the AVG may respond with Router 1’s Virtual MAC 1 (VMAC1) if Host A sends an ARP request for the virtual IP. Router 1 will thereafter receive more packets from Host A.
• The AVG may then respond using Router 2’s Virtual MAC 2 (VMAC2) if Host B initiates an ARP request. After that, Router 2 will receive more packets from Host B.
• Traffic from various hosts is efficiently distributed throughout the group’s physical routers via this procedure.

Communication: Gateway Load Balancing Protocol GLBP gateways use UDP port 3222 to send Hello messages to the multicast address 224.0.0.102 every three seconds.

Failover (Redundancy):

• The standby virtual gateway takes control of the virtual IP address in the event that the AVG fails, and a new standby virtual gateway is chosen from among the routers in the Listen state.
• One of the secondary virtual forwarders in the Listen state assumes control of the failing AVF’s virtual MAC address in the event that it fails. To guarantee that hosts move away from the previous MAC address, Gateway Load Balancing Protocol GLBP employs timers (forwarder timeout and redirection time).

Benefits of GLBP

True Load Balancing: By allocating client traffic among several physical routers in a group on a per-host basis, Gateway Load Balancing Protocol GLBP offers active/active load balancing in contrast to HSRP and VRRP, which normally function in an active/standby manner. Better use of WAN lines and network resources results from this.

High Availability and Redundancy: Like HSRP and VRRP, it safeguards data transmission against router or circuit failures, guaranteeing uninterrupted network functioning. The function of an AVG or AVF router is effortlessly assumed by another router in the event of its failure.

Transparent Failover: Since end-user devices continue to utilize the same virtual IP address as their default gateway, the failover procedure is invisible to them and doesn’t require client reconfiguration.

Simplified Client Configuration: Network administration is made easier by the hosts’ single, static virtual IP address configuration.

Cost-Effective Resource Utilization: By having many routers actively forward traffic, it makes better use of the hardware already present in routers and may eliminate the need for specialised load-balancing hardware.

Drawbacks of GLBP

Cisco Proprietary: Since Gateway Load Balancing Protocol GLBP is a Cisco-proprietary protocol, only Cisco devices may use it. Interoperability in mixed-vendor settings is thus constrained.

Configuration Complexity: Although the fundamental setup is similar to HSRP, using more sophisticated features like weighting and various load-sharing algorithms could make things more complicated.

No Longer a CCNA Objective: Gateway Load Balancing Protocol GLBP is not a major topic for the current CCNA 200-301 exam; instead, the curriculum emphasizes HSRP as an illustration of FHRP capabilities.

Examples

GLBP Group and Election

• Up to four routers can serve as forwarders in a Gateway Load Balancing Protocol GLBP group.
• The greatest priority determines who gets to choose the Active Virtual Gateway (AVG). The router with the highest IP address becomes AVG if priorities are equal.
• Similar to HSRP, Gateway Load Balancing Protocol GLBP preemption is by default deactivated for the AVG role. This means that a backup AVG, regardless of its greater priority, will only take over in the event that the primary AVG fails. Preemption can be activated, though.
• By default, Virtual Forwarder Preemption is activated with a 30-second lag, enabling a backup AVF to assume the virtual MAC address of a failed AVF.

GLBP Virtual MAC Address

• Each group can have up to four virtual MAC addresses assigned by the AVG.
• Example virtual MAC addresses might be aaaa.aaaa.aaaa, bbbb.bbbb.bbbb, cccc.cccc.cccc. The output of show glbp brief shows these MAC addresses.

Load-Sharing Methods

Three load-sharing strategies are supported by Gateway Load Balancing Protocol GLBP:

• Round-robin: Traffic is distributed equally among all Active Virtual Forwarders (AVFs) using this default technique.
• Weighted: This approach calculates the percentage of traffic routed to each AVF using a weighting technique. ARP responses with that gateway’s virtual MAC address are more common when the weighting values are higher. Depending on weighting, thresholds can be adjusted to either disable or re-enable forwarding.
• Host-dependent: The same virtual MAC address is returned to each client that sends an ARP request, giving that client a consistent gateway MAC address.

Configuration and Verification

• Basic GLBP configuration uses the glbp group-number ip virtual-ip-address interface subcommand.
• Example: glbp 1 ip 10.1.1.1.
• Setting load balancing mode: glbp 10 load-balancing round-robin.
• The show glbp and show glbp brief commands are used to verify Gateway Load Balancing Protocol GLBP status, including the AVG, AVFs, their states, virtual IP/MAC addresses, timers, and load-balancing method.

GLBP Timers

• Hello time: 3 seconds.
• Hold time: 10 seconds.
• Redirect time: 600 seconds (maximum).
• Forwarder timeout: 14400 seconds (maximum).

Applications of GLBP

When network administrators require both default gateway redundancy and effective use of multiple router links for egress traffic, Gateway Load Balancing Protocol GLBP is the best option. The load-balancing and transparent failover features of Gateway Load Balancing Protocol GLBP will be advantageous to any application or service that needs continuous network connectivity outside of its local subnet. All client-server communications, web-based apps, and real-time services will benefit from improved performance and continuous service as a result.