GLBP Gateway Load Balancing Protocol

By | March 11, 2020

GLBP is a Cisco proprietary default gateway redundancy protocol. It provides the load balancing between virtual routers without using any trick.

“Default gateway load balancing with HSRP and VRRP is a trick of multiple groups.”

GLBP uses one router as an AVG and up to 4 routers as AVF. Every host sends the ARP request to AVG and AVG replies with a virtual MAC address, assigned to an AVF. Host receives this virtual MAC address in ARP reply and used to forward the traffic as a gateway.

AVG replies the virtual MAC address of AVF routers as per the load balancing method. GLBP uses up to 3 load balancing methods and AVF virtual MAC address distribution depends on these load balancing methods. GLBP uses a series of virtual MAC addresses for AVF routers.

GLBP uses the multicast address 224.0.0.102 and every router in a group sends the hello packets in every 3 seconds, using this multicast IP address. If three hello missed from a router then it will be consider as down.

Default GLBP priority is 100 and can be set from 1 to 255.

GLBP group number range is 0 to 1023.

GLBP Virtual MAC address range is 0007.b400.XXYY (X = Group number , Y = AVF number).

GLBP multicast address is 224.0.0.102

GLBP uses UDP port number 3222.

AVG (Active Virtual Gateway)

A router with highest GLBP priority elects as an AVG. If GLBP priority is same on all the routers then highest router ID will be use as tie breaker. AVG is used as default gateway for all the hosts and contains the Virtual MAC address. It receives the ARP replies from all the hosts and send the ARP reply with a virtual MAC address as per load balancing method. AVG also plays a role of an AVF.

AVF (Active Virtual Forwarder)

AVF are routers which are having lower GLBP priority than AVG router. It is used to receive the traffic from hosts on its virtual MAC address and forward it to uplinks. GLBP can use up to 4 routes as AVF including AVG. An AVF router can also be a AVF for more than one virtual MAC address and to avoid this situation, AVG uses two timers: Redirect Timer and Timeout Timer.

Redirect Timer

Redirect timer is used to determine the old MAC addresses which should not be use and AVG should stop to advertise these virtual MAC addresses. Default redirect time is 600 seconds (10 minutes) and can be set from 0 to 3600 seconds (1 hour).

Below is the command to alter the redirect timer:

Switch(config-if)# glbp 1 timers redirect 600 14400

Below is the syntax:

Switch(config-if)# glbp 1 timers redirect redirect_timer timeout_timer

Timeout Timer

Timeout timer is used to specify the time for flush the old virtual MAC addresses from all GLBP groups. Default timeout timer is 14400 seconds (4 hours) and it can be set from 700 to 64800 seconds (18 hours).

Below is the command to alter the Timeout timer:

Switch(config-if)# glbp 1 timers redirect 600 14400

Hello Timer

This is the interval between hello packets. Default hello time is 3 seconds and can be set from 1 to 60 seconds or 50 to 60000 milliseconds.

Below is the command to alter the timers

Switch(config-if)#glbp 1 timers 3 10

Below is the syntax:

Switch(config-if)#glbp group_number timers Hello_time Hold_time

Hold Timer

It specifies that how long a router will wait for hello packet before declare the router as dead.

Default hold time is 10 seconds and can be set 3 to 180 seconds.

Switch(config-if)#glbp 1 timers 3 10

GLBP Load Balancing Methods

GLBP makes the load balancing by distributing the different virtual MAC addresses in ARP replies which are already assigned to AVF routers. Below are the load-balancing methods:

  1. Round Robin

It is the default load balancing method in GLBP. With this method, AVG sends the next available virtual MAC address in ARP reply. So every ARP reply gets the different virtual MAC address. It distributes the equal load on all the AVF routers without look into traffic amount.

Switch(config-if)# glbp 1 load-balancing round-robin
  • Weighted

In this method, AVG replies a virtual MAC address of an AVF router more frequently that has higher weighting results. If interface tracking is not configured then configured maximum weight value will be use.

Switch(config-if)# glbp 1 load-balancing Weighted
  • Host Dependent

In this method, a host always gets the same virtual MAC address in ARP replies. So a host can use the same gateway continue.

Switch(config-if)# glbp 1 load-balancing host-dependent

Example:

GLBP protocol
SwitchA(config)# interface vlan 50
SwitchA(config-if)# ip address 192.168.2.10 255.255.255.0
SwitchA(config-if)# glbp 1 priority 200
SwitchA(config-if)# glbp 1 preempt
SwitchA(config-if)# glbp 1 ip 192.168.2.1
SwitchB(config)# interface vlan 50
SwitchB(config-if)# ip address 192.168.2.11 255.255.255.0
SwitchB(config-if)# glbp 1 priority 150
SwitchB(config-if)# glbp 1 preempt
SwitchB(config-if)# glbp 1 ip 192.168.2.1
SwitchC(config)# interface vlan 50
SwitchC(config-if)# ip address 192.168.2.12 255.255.255.0
SwitchC(config-if)# glbp 1 preempt
SwitchC(config-if)# glbp 1 ip 192.168.2.1

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