Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol that attempts to overcome the limitations of existing redundant router protocols by adding basic load balancing functionality.
In addition to being able to set priorities on different gateway routers, GLBP allows a weighting parameter to be set. Based on this weighting (compared to others in the same virtual router group), ARP requests will be answered with MAC addresses pointing to different routers. Thus, load balancing is not based on traffic load, but rather on the number of hosts that will use each gateway router. By default GLBP load balances in round-robin fashion.
GLBP elects one AVG (Active Virtual Gateway) for each group. Other group members act as backup in case of AVG failure. In case there are more than two members, the second best AVG is placed in the Standby state and all other members are placed in the Listening state. This is monitored using hello and holdtime timers, which are 3 and 10 seconds by default. The elected AVG then assigns a virtual MAC address to each member of the GLBP group, including itself, thus enabling AVFs (Active Virtual Forwarders). Each AVF assumes responsibility for forwarding packets sent to its virtual MAC address. There could be up to four active AVFs at the same time.
By default, GLBP routers use the local multicast address 224.0.0.102 to send hello packets to their peers every 3 seconds over UDP 3222 (source and destination).
Cisco implemented IPv6 support for GLBP in IOS release 12.2(33)SXI.
- (Example)Load Balancing
- (Example)Load Sharing
- GLBP weighting has the ability to place a weight on each device when calculating the amount of load sharing that will occur through MAC assignment. Each GLBP router in the group will advertise its weighting and assignment. The AVG will act based on that value.
- The only reason you would use this is if you have a larger circuit on the primary router than on the backup router. So the higher weight of 160 will take twice as much traffic as the lower weight of 80. If the weights are 120 and 80 respectively, the primary router will take a quarter more traffic than the backup.
Show GLBP
To display Gateway Load Balancing Protocol (GLBP) information, use the show glbp command in privileged EXEC mode.
show glbp
The following is sample output from the show glbp command:
Router# show glbp FastEthernet0/0 - Group 10 State is Active 2 state changes, last state change 23:50:33 Virtual IP address is 10.21.8.10 Hello time 5 sec, hold time 18 sec Next hello sent in 4.300 secs Redirect time 600 sec, forwarder time-out 7200 sec Authentication MD5, key-string Preemption enabled, min delay 60 sec Active is local Standby is unknown Priority 254 (configured) Weighting 105 (configured 110), thresholds: lower 95, upper 105 Track object 2 state Down decrement 5 Load balancing: host-dependent There is 1 forwarder (1 active) Forwarder 1 State is Active 1 state change, last state change 23:50:15 MAC address is 0007.b400.0101 (default) Owner ID is 0005.0050.6c08 Redirection enabled Preemption enabled, min delay 60 sec Active is local, weighting 105The following is sample output from the show glbp command with the brief keyword specified:
Router# show glbp brief Interface Grp Fwd Pri State Address Active router Standby router Fa0/0 10 - 254 Active 10.21.8.10 local unknown Fa0/0 10 1 7 Active 0007.b400.0101 local -The following is sample output from the show glbp command that displays GLBP group 10:
Router# show glbp 10 FastEthernet0/0 - Group 10 State is Active 2 state changes, last state change 23:50:33 Virtual IP address is 10.21.8.10 Hello time 5 sec, hold time 18 sec Next hello sent in 4.300 secs Redirect time 600 sec, forwarder time-out 7200 sec Authentication MD5, key-string Preemption enabled, min delay 60 sec Active is local Standby is unknown Priority 254 (configured) Weighting 105 (configured 110), thresholds: lower 95, upper 105 Track object 2 state Down decrement 5 Load balancing: host-dependent There is 1 forwarder (1 active) Forwarder 1 State is Active 1 state change, last state change 23:50:15 MAC address is 0007.b400.0101 (default) Owner ID is 0005.0050.6c08 Redirection enabled Preemption enabled, min delay 60 sec Active is local, weighting 105The following output shows that the redundancy name has been assigned to the "glbp1" group:
Router# show glbp ethernet0/1 1 Ethernet0/1 - Group 1 State is Listen 64 state changes, last state change 00:00:54 Virtual IP address is 10.1.0.7 Hello time 50 msec, hold time 200 msec Next hello sent in 0.030 secs Redirect time 600 sec, forwarder time-out 14400 sec Authentication text, string "authword" Preemption enabled, min delay 0 sec Active is 10.1.0.2, priority 105 (expires in 0.184 sec) Standby is 10.1.0.3, priority 100 (expires in 0.176 sec) Priority 96 (configured) Weighting 100 (configured 100), thresholds: lower 95, upper 100 Track object 1 state Up decrement 10 Load balancing: round-robin IP redundancy name is "glbp1" Group members: 0004.4d83.4801 (10.0.0.0) 0010.7b5a.fa41 (10.0.0.1) 00d0.bbd3.bc21 (10.0.0.2) localThe following output shows GLBP support for SSO mode on an active RP:
Router# show glbp Ethernet0/0 - Group 1 State is Standby 1 state change, last state change 00:00:20 Virtual IP address is 172.24.1.254 Hello time 3 sec, hold time 10 sec Next hello sent in 0.232 secs Redirect time 600 sec, forwarder time-out 14400 sec Preemption disabled Active is 172.24.1.2, priority 100 (expires in 7.472 sec) Standby is local Priority 100 (default) Weighting 100 (default 100), thresholds: lower 1, upper 100 Load balancing: round-robin Group members: aabb.cc00.0100 (172.24.1.1) local aabb.cc00.0200 (172.24.1.2) There are 2 forwarders (1 active) Forwarder 1 State is Listen MAC address is 0007.b400.0101 (learnt) Owner ID is aabb.cc00.0200 Time to live: 14397.472 sec (maximum 14400 sec) Preemption enabled, min delay 30 sec Active is 172.24.1.2 (primary), weighting 100 (expires in 9.540 sec) Forwarder 2 State is Active 1 state change, last state change 00:00:28 MAC address is 0007.b400.0102 (default) Owner ID is aabb.cc00.0100 Preemption enabled, min delay 30 sec Active is local, weighting 100The following output shows GLBP support for SSO mode on a standby RP:
RouterRP-standby# show glbp Ethernet0/0 - Group 1 State is Init (standby RP, peer state is Standby) Virtual IP address is 172.24.1.254 Hello time 3 sec, hold time 10 sec Redirect time 600 sec, forwarder time-out 14400 sec Preemption disabled Active is unknown Standby is unknown Priority 100 (default) Weighting 100 (default 100), thresholds: lower 1, upper 100 Load balancing: round-robin Group members: aabb.cc00.0100 (172.24.1.1) local aabb.cc00.0200 (172.24.1.2) There are 2 forwarders (0 active) Forwarder 1 State is Init (standby RP, peer state is Listen) MAC address is 0007.b400.0101 (learnt) Owner ID is aabb.cc00.0200 Preemption enabled, min delay 30 sec Active is unknown Forwarder 2 State is Init (standby RP, peer state is Active) MAC address is 0007.b400.0102 (default) Owner ID is aabb.cc00.0100 Preemption enabled, min delay 30 sec Active is unknownFamous quotes containing the words gateway, load and/or balancing:
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