Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary solution for load balancing and redundancy in an IP network. Other known first-hop redundancy protocols (FHRPs) are Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP).
Gateway Load Balancing Protocol (GLBP) has two roles:
- Active Virtual Gateway (AVG) – One AVG per group, and it’s responsible for replying to initial ARP requests for the Virtual IP address (VIP).
- Active Virtual Forwarder (AVF) – AVG assigns different virtual MAC addresses to the Active Virtual Forwarders (AVFs). This is done when AVG replies to the initial ARP request from a host. ARP replies are unicast and are not heard by other hosts on the same broadcast segment. AVF is responsible for routing traffic received from assigned hosts. A host can use the same virtual MAC address of an AVF as long as it is reachable.
For the GLBP configuration example, we will use the topology below.
First, we need to define the instance ID and VIP.
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#int gi0/1 R1(config-if)#ip address 10.10.10.3 255.255.255.0 R1(config-if)#glbp 10 ip 10.10.10.1 R1(config-if)# *Mar 7 16:46:31.376: %GLBP-6-STATECHANGE: GigabitEthernet0/1 Grp 10 state Speak -> Active *Mar 7 16:46:42.603: %GLBP-6-FWDSTATECHANGE: GigabitEthernet0/1 Grp 10 Fwd 1 state Listen -> Active
R2#conf t Enter configuration commands, one per line. End with CNTL/Z. R2(config)#int gi0/0 R2(config-if)#ip address 10.10.10.2 255.255.255.0 R2(config-if)#glbp 10 ip 10.10.10.1 R2(config-if)# *Mar 7 16:51:56.659: %GLBP-6-FWDSTATECHANGE: GigabitEthernet0/0 Grp 10 Fwd 2 state Listen -> Active
For our verification, let’s use the ‘show glbp brief’ command for the high-level status and the ‘show glbp’ for the detailed status.
R1#show glbp brief Interface Grp Fwd Pri State Address Active router Standby router Gi0/1 10 - 100 Active 10.10.10.1 local 10.10.10.2 Gi0/1 10 1 - Active 0007.b400.0a01 local - Gi0/1 10 2 - Listen 0007.b400.0a02 10.10.10.2 -
R1#show glbp GigabitEthernet0/1 - Group 10 State is Active 1 state change, last state change 00:09:47 Virtual IP address is 10.10.10.1 Hello time 3 sec, hold time 10 sec Next hello sent in 2.272 secs Redirect time 600 sec, forwarder timeout 14400 sec Preemption disabled Active is local Standby is 10.10.10.2, priority 100 (expires in 9.440 sec) Priority 100 (default) Weighting 100 (default 100), thresholds: lower 1, upper 100 Load balancing: round-robin Group members: 5000.0008.0001 (10.10.10.3) local 5000.0009.0000 (10.10.10.2) There are 2 forwarders (1 active) Forwarder 1 State is Active 1 state change, last state change 00:09:35 MAC address is 0007.b400.0a01 (default) Owner ID is 5000.0008.0001 Redirection enabled Preemption enabled, min delay 30 sec Active is local, weighting 100 Forwarder 2 State is Listen MAC address is 0007.b400.0a02 (learnt) Owner ID is 5000.0009.0000 Redirection enabled, 599.456 sec remaining (maximum 600 sec) Time to live: 14399.456 sec (maximum 14400 sec) Preemption enabled, min delay 30 sec Active is 10.10.10.2 (primary), weighting 100 (expires in 10.624 sec)
For other optional configurations, we can use the commands below:
- Preemption – Allow a more preferred router to take AVG status from another router.
glbp <instance-id> preempt
- Priority – Default priority is 100. A higher priority number is preferred.
glbp <instance-id> priority <1-255>
- Timers – The default hello timer is 3 seconds. The default hold timer is 10 seconds.
R1(config-if)#glbp 10 timers ? <1-60> Hello interval in seconds msec Specify hello interval in milliseconds redirect Specify timeout values for failed forwarders
- Authentication – A router will ignore incoming GLBP packets from routers that do not have the same authentication configuration for a GLBP group.
R1(config-if)#glbp 10 authentication ? md5 MD5 authentication text Plain text authentication
Gateway Load Balancing Protocol Load Balancing
By default, GLBP will load balance traffic using the round-robin method. But we can change it by using the following command:
R1(config-if)#glbp 10 load-balancing ? host-dependent Load balance equally, source MAC determines forwarder choice round-robin Load balance equally using each forwarder in turn weighted Load balance in proportion to forwarder weighting
Let’s try changing it from round-robin to weighted.
R1(config-if)#glbp 10 load-balancing weighted R1(config-if)#glbp 10 weighting 50
For verification, let’s use the ‘show glbp’ command.
R1#sh glbp | inc weight Load balancing: weighted Active is local, weighting 150 Active is 10.10.10.2 (primary), weighting 100 (expires in 11.424 sec)
Increasing the weight of newer and bigger routers allows them to take more traffic than older and smaller routers.
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