Gateway Load Balancing Protocol

Gateway Load Balancing Protocol (GLBP) protects data traffic from a failed router or circuit, like HSRP and VRRP, while allowing packet load sharing between a group of redundant routers.

Configuring GLBP

Router(config)#interface fastethernet 0/0

Moves to interface config mode

Router(config-if)#ip address 172.16.100.5 255.255.255.0

Assigns IP address and netmask

Router(config-if)#glbp 10 ip 172.16.100.1

Enables GLBP for group 10 on this interface with a virtual address of 172.16.100.1. The range of group numbers is from 0 to 1023.

Router(config-if)#glbp 10 preempt

Configures the router to preempt, or take over, as the active virtual gateway (AVG) for group 10 if this router has a higher priority than the current AVG

Router(config-if)#glbp 10 preempt delay minimum 60

Configures the router to preempt, or take over, as AVG for group 10 if this router has a higher priority than the current active virtual forwarder (AVF) after a delay of 60 seconds

Router(config-if)#glbp 10 forwarder preempt

Configures the router to preempt, or take over, as AVF for group 10 if this router has a higher priority than the current AVF. This command is enabled by default with a delay of 30 seconds.

Router(config-if)#glbp 10 forwarder preempt delay minimum 60

Configures the router to preempt, or take over, as AVF for group 10 if this router has a higher priority than the current AVF after a delay of 60 seconds

NOTE: Members of a GLBP group elect one gateway to be the AVG for that group. Other group members provide backup for the AVG in the event that the AVG becomes unavailable. The AVG assigns a virtual MAC address to each member of the GLBP group. Each gateway assumes responsibility for forwarding packets sent to the virtual MAC address assigned to it by the AVG. These gateways are known as AVFs for their virtual MAC address.

Virtual forwarder redundancy is similar to virtual gateway redundancy with an AVF. If the AVF fails, one of the secondary virtual forwarders in the listen state assumes responsibility for the virtual MAC address.

Router(config-if)#glbp 10 priority 150

Sets the priority level of the router

NOTE: The range of the priority argument is 1 to 255. The default priority of GLBP is 100. A higher priority number is preferred.

Router(config-if)#glbp 10 timers 5 15

Configures the hello timer to be set to 5 seconds and the hold timer to be 15 seconds

Router(config-if)#glbp 10 timers msec 20200 msec 60600

Configures the hello timer to be 20,200 milliseconds and the hold timer to be 60,600 milliseconds.

NOTE: The default hello timer is 3 seconds. The range of the hello timer interval is 1 to 60 seconds. If the msec argument is used, the timer will be measured in milliseconds, with a range of 50 to 60000.

NOTE: The default hold timer is 10 seconds. The range of the hold timer is 19 to 180 seconds. If the msec argument is used, the timer will be measured in milliseconds, with a range of 18020 to 180000.

The hello timer measures the interval between successive hello packets sent by the AVG in a GLBP group. The holdtime argument specifies the interval before the virtual gateway and the virtual forwarder information in the hello packet is considered invalid. It is recommended that unless you are extremely familiar with your network design and with the mechanisms of GLBP that you do not change the timers. To reset the timers back to their default values, use the no glbp x timers command, where x is the GLBP group number.

Router(config-if)#glbp 10 load-balancing host-dependent

Specifies that GLBP will load balance using the host-dependent method

Router(config-if)#glbp 10 load-balancing weighted

Specifies that GLBP will load balance using the weighted method

Router(config-if)#glbp 10 weighting 80

Assigns a maximum weighting value for this interface for load-balancing purposes. The value can be from 1 to 254.

Router(config-if)#glbp 10 load balancing round robin

Specifies that GLBP will load balance using the round-robin method

NOTE: There are three different types of load balancing in GLBP:

• Host-dependent uses the MAC address of a host to determine which VF MAC address the host is directed toward. This is used with stateful Network Address Translation (NAT) because NAT requires each host to be returned to the same virtual MAC address each time it sends an ARP request for the virtual IP address. It is not recommended for situations where there are a small number of end hosts (fewer than 20).

• Weighted allows for GLBP to place a weight on each device when calculating the amount of load sharing. For example, if there are two routers in the group, and router A has twice the forwarding capacity of router B, the weighting value should be configured to be double the amount of router B. To assign a weighting value, use the glbp xweighting y interface configuration command, where x is the GLBP group number, and y is the weighting value, a number from 1 to 254.

• Round-robin load balancing occurs when each VF MAC address is used sequentially in ARP replies for the virtual IP address. Round robin is suitable for any number of end hosts.

If no load balancing is used with GLBP, GLBP will operate in an identical manner to HSRP, where the AVG will only respond to ARP requests with its own VF MAC address, and all traffic will be directed to the AVG.

Verifying GLBP

Router#show running-config

Displays contents of dynamic RAM

Router#show glbp

Displays GLBP information

Router#show glbp brief

Displays a brief status of all GLBP groups

Router#show glbp 10

Displays information about GLBP group 10

Router#show glbp fastethernet 0/0

Displays GLBP information on interface fastethernet 0/0

Router#show glbp fastethernet 0/0 10

Displays GLBP group 10 information on interface fastethernet 0/0

Debugging GLBP

Router#debug condition glbp

Displays GLBP condition messages

Router#debug glbp errors

Displays all GLBP error messages

Router#debug glbp events

Displays all GLBP event messages

Router#debug glbp packets

Displays messages about packets sent and received

Router#debug glbp terse

Displays a limited range of debugging messages

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