GLBP (Gateway Load Balancing Protocol)

GLBP stands for Gateway Load Balancing Protocol and just like HSRP / VRRP it is used to create a virtual gateway that you can use for hosts. If you have no idea what a virtual gateway is then read my Introduction to Gateway Redundancy first. Also I would recommend to look at the HSRP and VRRP lessons before you continue with GLBP.

One of the key differences of GLBP is that it can do load balancing without the group configuration that HSRP/VRRP use (what’s in a name right?).

Let’s take a closer look:

glbp avg avf

All devices running GLBP elect an AVG (Active Virtual Gateway). There will be only one AVG for a single group running GLBP but other devices can take over this rule if the AVG fails. The role of the AVG is to assign a virtual MAC address to all other devices running GLBP. All devices will become an AVF (Active Virtual Forwarder) including the AVG. Whenever a computer sends an ARP Request the AVG will respond with one of the virtual MAC addresses of the available AVFs. Because of this mechanism all devices running GLBP will be used to forward IP packets.

There are multiple methods for load balancing:

  • Round-robin: the AVG will hand out the virtual MAC address of AVF1, then AVF2, AVF3 and gets back to AVF1 etc.
  • Host-dependent: A host will be able to use the same virtual MAC address of an AVF as long as it is reachable.
  • Weighted: If you want some AVFs to forward more traffic than others you can assign them a different weight.

Let’s take a look at a configuration example so you can see how this works.


I will use the following topology to configure GLBP:

Glbp Lab Topology

SW1 and SW2 are multilayer switches, their GigabitEthernet 0/1 interfaces are switchports and in VLAN 1. Their interfaces that connect to R3 are routed ports. We configure SW1 and SW2 so they create a virtual gateway for the hosts in the /24 subnet. Let’s enable GLBP:

SW1(config)#interface Vlan1           
SW1(config-if)#glbp 1 ip
SW1(config-if)#glbp 1 priority 150
SW2(config)#interface Vlan1
SW2(config-if)#glbp 1 ip

I’ll enable GLBP on SW1 and Sw2 using the same group number (1). I changed the priority on SW1 because I want it to be the AVG. Let’s see if this works:

SW1#show glbp brief 
Interface   Grp  Fwd Pri State    Address         Active router   Standby router
Vl1         1    -   150 Active   local 
Vl1         1    1   -   Active   0007.b400.0101  local           -
Vl1         1    2   -   Listen   0007.b400.0102     -
SW2#show glbp brief
Interface   Grp  Fwd Pri State    Address         Active router   Standby router
Vl1         1    -   100 Standby     local
Vl1         1    1   -   Listen   0007.b400.0101     -
Vl1         1    2   -   Active   0007.b400.0102  local           -

Use the show glbp brief command to verify your configuration. There are a couple of things we can see here:

• SW1 has become the AVG for group 1. SW2 ( is standby for the AVG role and will take over in case SW1 fails and group1 has two AVFs:

  • 1: SW1: Virtual MAC address 0007.b400.0101.
  • 2: SW2: Virtual MAC address 0007.b400.0102.

The virtual MAC address that GLBP uses is 0007.b400.XXYY (where X = GLBP group number and Y = AVF number). Let’s take a look at our host, I configured it to use the address for the default gateway.

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Forum Replies

  1. Hi Srini,

    I think it depends on the hardware platform. I just took a look around and the supervisor 720 supports up to 1024 groups.


  2. Hello Staut

    Yes, you are correct. I’ll let Rene know…



  3. Hello Patryk

    Keep in mind that an AVG and an AVF are two different things. The priority value configured will determine which switch will become AVG, but the weighting values will determine if a particular switch will be an AVF. Remember there is only one AVG, while all functioning switches in the group are AVFs.

    Now in your scenario above, SW2 will be AVG. Even if both links to the router fail, it will still remain AVG. This is because even if both links fail, SW2 and SW1 still maintain a connection between them via the subnet, so there is n

    ... Continue reading in our forum

  4. Once again just like with VRRP, HSRP and GLBP any of you that are using GNS3 and happen to be having trouble getting the devices to talk to one another and become active/standby and it does not work despite having layer 3 connectivity and everything being configured correctly.

    You need to use the global config command:

    no ip igmp snooping

    This needs to be on all switches the mutlicast traffic passes through

    I had to do this on IOU L2 images, show version output here:

    IOU1#show version
    Cisco IOS Software, Solaris Software (I86BI_LINUXL2-IPBASEK9-M), Experimen

    ... Continue reading in our forum

  5. Hello Helen

    Let me clarify. Take a look at the topology of the lab:

    SW1 and SW2 can communicate with each other over their Gi0/1 interfaces in order to determine the AVG, and the AVF. Even if the links towards R3 fail, this link remains active.

    The role of the AVF is affected directly by any failure in the uplinks to R3. If Gi0/2 goes down on SW1 for example, SW1 can no longer play the role of the AVF.

    However, Unlike the AVF, the role of the AVG d

    ... Continue reading in our forum

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