Cisco Tips & Tricks

November 8, 2009

CCIE notes for GLBP

Filed under: ccie, cisco, Technology and Software — ciscotips @ 5:47 am

Gateway load balancing protocol performs similar function to HSRP and VRRP. In both HSRP and VRRP,  group of routers participating in first hop-redundancy has one Active and can have multiple Client routers. At one single time, traffic is being passed through Active router, leaving client routers with unused bandwidth. Client routers will only become active once Active router in a group fails. We can create multiple groups and create different active routers but it results in extra administrative burden.

GLBP on the other hand can provide load balancing over multiple routers (gateways) using a Single Virtual IP and multiple Virtual mac-addresses. The bandwidth/traffic load is shared between multiple routers participating in the group rather than being handled by a single active router.

Following are the important points conceptually for GLBP.

  1. GLBP uses single Virtual IP and multiple mac-addresses to provide first-hop Gateway redundancy.
  2. In GLBP, there can be four routers/gateways in a group
  3. Hello messages are used to communicate with in the group destined to, udp port 3222 and they will be sent every 3 secs by default.
  4. initially group members will elect one AVG ( Active Virtual Gateway) and other routers will act as backup AVG’s incase the active AVG fails
  5. AVG will assign Virtual mac-addresses to other routers, they are known as AVF’s ( Active Virtual Forwarders)
  6. Each AVF assumes responsibility for  forwarding  packets sent to Virtual Mac’s assigned by AVG.
  7. AVG is responsible for answering ARP requests for Virtual IP’s

Configuring GLBP

R2(config-if)#glbp 1 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

There are three different types of Load balancing algorithms in GLBP.


  1. The Mac-address of the host is used to determine which AVF’s  mac is the host directed towards.
  2. A given host is guaranteed to use the same Virtual Mac as long as number of VF’s in the GLBP group are constant
  3. Host dependant GLBP is not recommended in situation where there are small number of hosts, for example, less than 20


  1. GLBP places a weight on each device to calculate the amount of load sharing that will occur through MAC assignment
  2. Each GLBP router in a group will advertise its weight and AVG will act based on that value
  3. For example  we have two routers, Router A and Router B. If router A has double the bandwidth capacity then router B. Router A will be configured with the double weighting value of router B


  1. With Round-robin VF mac-address is used sequentially in ARP replies for the virtual IP
  2. This is the default type of GLBP algorithm
  3. It is suitable for any number of hosts.

Steps for  configuring GLBP

  1. enable GLBP with glbp 1 load-balancing
  2. glbp 1 priority ( Higher is better, default is 100)
  3. glbp 1 ip x.x.x.x
  4. glbp 1 preempt < To enable preempt, by default its disabled>
  5. glbp 1 authentication  ( Enabling authentication with in a group)


Show glbp



  1. is it called Global load balancing protocol or Gateway load balancing protocol?


    Comment by johannes — November 8, 2009 @ 8:37 pm

  2. thanks for correcting me, I have edited the post!

    Comment by ciscotips — November 8, 2009 @ 10:19 pm

  3. Thanks for the explaination! I bookmarked this and will be back! Thanks

    Comment by Joe Wilson — December 2, 2009 @ 2:14 am

  4. Question? AVG preemption min delay. The default is 30 secs, in a flat 802.3 network, 3 routers, what variables are taken into consideration when changing the default minimum delay. I have read dozens of Cisco docs and posts and there really isn’t anything outside of the command reference guide summary?

    Comment by Bill — January 26, 2010 @ 4:36 pm

  5. i am trying to achive load balancing round robin but my windows xp computers send a broadcast arp (who has tell wireshark output) and each time my computers ask this my connection to my citrix server goes down cause it changes its public ip adddress. by the way i am doing nat/PAT should i use round robin?

    Comment by Ricardo — February 22, 2010 @ 9:32 pm

    • @Ricardo

      Per Cisco:

      Host dependent load balancing will need to be used when using stateful Network Address Translation (NAT) because it requires each host to be returned the same virtual MAC address each time it sends an ARP request for the virtual IP address

      What this means is that Host-dependent ensures a host uses one and only one gateway for its traffic. In case of round-robin/weighted, a host sends some packets through 1st gateway and some packets of the same session to the 2nd gateway and this doesn’t work well with NAT.

      Comment by Dave — April 19, 2010 @ 8:21 pm

  6. is a good Cisco CCNA CCNP CCIE reference blog. But we are not seeing any new recent posts. Request the blog owner to kindly add new useful Cisco resources. Your blog is a good resource for those who prepare for Cisco CCNA CCNP CCIE examinations.

    Kindly add new technical posts.

    Thanks a lot for your blog.

    Comment by CCNA CCNP Chennai — August 12, 2012 @ 5:51 am

  7. Why is Host dependant GLBP is not recommended for a small number of Hosts? How do I manage the Client Macs on the router?

    Comment by blub — August 22, 2013 @ 1:05 pm

  8. Host dependant may not work well with a small number of hosts because you could have large amounts of traffic going to one host and not much to another. So it won’t be very balanced, the more hosts you connect to the more likely it is that the load will be more evenly spread across the routers.

    Comment by Steve — October 29, 2013 @ 9:11 pm

  9. Tablet popust

    CCIE notes for GLBP | Cisco Tips & Tricks

    Trackback by Tablet popust — September 22, 2014 @ 7:11 pm

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