In this post I will talk about the redundancy protocol GLBP. For the purpose of this post I used the diagram below.

As you can see in the diagram vlan 15 uses GLBP as a first-hop redundancy protocol. When using GLBP there is always one router that acts as the AVG (Active Virtual Gateway). The AVG role is to respond with different virtual MAC-addresses to ARP requests for the virtual GLBP (gateway) address. In this case router R1 will be the AVG. Another thing the AVG does is take care of the load-sharing. For the load-sharing component there are 3 choices, host-dependent, round-robin of weighted (in some next post more on this). The rest of the routers are called AVF’s (Active Virtual Forwarder). In this case routers R2 and R3 will be the AVF routers.

So let’s implement some GLBP here. We will use 10.10.15.123 as the GLBP (gateway) address. Below the configuration of GLBP on routers R1, R2 and R3 :

R1 :

R1#sh run int fa2/0
Building configuration...

Current configuration : 186 bytes
!
interface FastEthernet2/0
 ip address 10.10.15.1 255.255.255.0
 ip access-group 100 in
 duplex auto
 speed auto
 glbp 123 ip 10.10.15.123
 glbp 123 priority 255
 glbp 123 preempt
end
    

R2 :

R2#sh run int fa2/0
Building configuration...

Current configuration : 186 bytes
!
interface FastEthernet2/0
 ip address 10.10.15.2 255.255.255.0
 ip access-group 100 in
 duplex auto
 speed auto
 glbp 123 ip 10.10.15.123
 glbp 123 priority 155
 glbp 123 preempt
end
    

R3 :

R3#sh run int fa2/0
Building configuration...

Current configuration : 185 bytes
!
interface FastEthernet2/0
 ip address 10.10.15.3 255.255.255.0
 ip access-group 100 in
 duplex auto
 speed auto
 glbp 123 ip 10.10.15.123
 glbp 123 priority 55
 glbp 123 preempt
end
    

As you can see the configuration of GLBP is pretty straightforward. I also put in a priority per router, this priority is not like in HSRP or VRRP where the router with the highest priority handles all the traffic. In GLBP gateway priority determines the role that each GLBP gateway plays and what happens if the AVG fails. Priority also determines if a GLBP router functions as a backup virtual gateway and the order of ascendancy to becoming an AVG if the current AVG fails.

By default, the GLBP gateway preemptive scheme is disabled. A backup virtual gateway can become the AVG only if the current AVG fails, regardless of the priorities assigned to the virtual gateways. You can enable the GLBP preemptive scheme using the glbp preempt command. Preemption allows a backup virtual gateway to become the AVG, if the backup virtual gateway is assigned a higher priority than the current AVG.

The Virtual MAC addresses in GLBP will be of the form:
0007.b4yy.yyyy where yy.yyyy equals the lower 24 bits; these bits consist of 6 zero bits, 10 bits that correspond to the GLBP group number, and 8 bits that correspond to the virtual forwarder number.

So the virtual-mac address of router R1 :
0007.b400.7b01 : last 24 bits = 0000 0000 0111 1011 0000 0001 = GLBP group 123 (123 in hexadecimal is 7b), forwarder 1

Now if we go to routers R4, R5 and R9 and do a “sh arp” we can see that the AVG has responded to the ARP requests for 10.10.15.123 with three different MAC addresses :

R4 :

R4#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.15.4              -   cc04.0e54.0020  ARPA   FastEthernet2/0
Internet  10.10.15.3              0   cc01.0e54.0020  ARPA   FastEthernet2/0
Internet  10.10.15.123            0   0007.b400.7b03  ARPA   FastEthernet2/0
    

R5 :

R5#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.15.5              -   cc03.0e54.0020  ARPA   FastEthernet2/0
Internet  10.10.15.3              0   cc01.0e54.0020  ARPA   FastEthernet2/0
Internet  10.10.15.2             43   cc02.0e54.0020  ARPA   FastEthernet2/0
Internet  10.10.15.123           43   0007.b400.7b02  ARPA   FastEthernet2/0
    

R9 :

R9#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  10.10.15.9              -   cc0a.11f4.0010  ARPA   FastEthernet1/0
Internet  10.10.15.1             44   cc00.0e54.0020  ARPA   FastEthernet1/0
Internet  10.10.15.123           44   0007.b400.7b01  ARPA   FastEthernet1/0
    

As you can see above the Gateway address 10.10.15.123 uses three different MAC addresses.