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:

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.

Configuration

I will use the following topology to configure GLBP:

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 192.168.1.0 /24 subnet. Let’s enable GLBP:

SW1(config)#interface Vlan1           
SW1(config-if)#glbp 1 ip 192.168.1.254
SW1(config-if)#glbp 1 priority 150

SW2(config)#interface Vlan1
SW2(config-if)#glbp 1 ip 192.168.1.254

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   192.168.1.254   local           192.168.1.2
Vl1         1    1   -   Active   0007.b400.0101  local           -
Vl1         1    2   -   Listen   0007.b400.0102  192.168.1.2     -

SW2#show glbp brief
Interface   Grp  Fwd Pri State    Address         Active router   Standby router
Vl1         1    -   100 Standby  192.168.1.254   192.168.1.1     local
Vl1         1    1   -   Listen   0007.b400.0101  192.168.1.1     -
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 (192.168.1.2) 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 192.168.1.254 address for the default gateway.