GLBP

GLBP provides load balancing over multiple routers (gateways) using a single virtual IP addressand multiple virtual MAC addresses. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets. GLBP members communicate between each other through hello messages sent every 3 seconds to the multicast address 224.0.0.102, User Datagram Protocol (UDP) port 3222 (source and destination).



GLBP uses a weighting scheme to determine the forwarding capacity of each router in the GLBP group. The weighting assigned to a router in the GLBP group determines whether it will forward packets and, if so, the proportion of hosts in the LAN for which it will forward packets. Thresholds can be set to disable forwarding when the weighting falls below a certain value, and when it rises above another threshold, forwarding is automatically reenabled.


The GLBP group weighting can be automatically adjusted by tracking the state of an interface within the router. If a tracked interface goes down, the GLBP group weighting is reduced by a specified value. Different interfaces can be tracked to decrement the GLBP weighting by varying 

GLBP states:

For a virtual gateway the state can be one of the following:

1. Disabled: Indicates that the virtual IP address has not been configured or learned yet, but other GLBP configuration exists.
2. Initial: The virtual IP address has been configured or learned but virtual gateway configuration is not complete. An interface must be up and configured to route IP, and an interface IP address must be configured.
3. Listen: Virtual gateway is receiving hello packets and is ready to change to the “speak” state if the active or standby virtual gateway becomes unavailable.
4. Speak: Virtual gateway is attempting to become the active or standby virtual gateway.
5. Standby: Indicates that the gateway is next in line to be the active virtual gateway (AVG).
6. Active: Indicates that this gateway is the AVG, and that it is responsible for responding to Address Resolution Protocol (ARP) requests for the virtual IP address

 Ref :: 
http://www.ciscozine.com/2008/11/18/configuring-redundancy-with-glbp/


Example 

Configuration


R1
interface FastEthernet0/0
 ip address 192.168.1.252 255.255.255.0
 duplex auto
 speed auto
 glbp 1 ip 192.168.1.254
 glbp 1 priority 150
 glbp 1 preempt
end

R2
interface FastEthernet0/0
 ip address 192.168.1.253 255.255.255.0
 duplex auto
 speed auto
 glbp 1 ip 192.168.1.254
 glbp 1 preempt

Verify Configuration

R1#show glbp

FastEthernet0/0 - Group 1

  State is Active

    2 state changes, last state change 00:04:26

  Virtual IP address is 192.168.1.254

  Hello time 3 sec, hold time 10 sec

    Next hello sent in 0.392 secs

  Redirect time 600 sec, forwarder timeout 14400 sec

  Preemption enabled, min delay 0 sec

  Active is local

  Standby is 192.168.1.253, priority 100 (expires in 8.344 sec)

  Priority 150 (configured)

  Weighting 100 (default 100), thresholds: lower 1, upper 100

  Load balancing: round-robin

  Group members:

    c201.0548.0000 (192.168.1.253)

    c203.0548.0000 (192.168.1.252) local

  There are 2 forwarders (1 active)

  Forwarder 1

    State is Active

      1 state change, last state change 00:04:16

    MAC address is 0007.b400.0101 (default)

    Owner ID is c203.0548.0000

    Redirection enabled

    Preemption enabled, min delay 30 sec

    Active is local, weighting 100

  Forwarder 2

    State is Listen

    MAC address is 0007.b400.0102 (learnt)

    Owner ID is c201.0548.0000

    Redirection enabled, 597.444 sec remaining (maximum 600 sec)

    Time to live: 14397.444 sec (maximum 14400 sec)

    Preemption enabled, min delay 30 sec

    Active is 192.168.1.253 (primary), weighting 100 (expires in 7.440 sec)

Debug 

R1#debug ip udp

UDP packet debugging is on

R1#

*Mar  1 00:15:18.187: UDP: rcvd src=192.168.1.253(3222), dst=224.0.0.102(3222), length=68

*Mar  1 00:15:21.175: UDP: rcvd src=192.168.1.253(3222), dst=224.0.0.102(3222), length=68

*Mar  1 00:15:24.203: UDP: rcvd src=192.168.1.253(3222), dst=224.0.0.102(3222), length=68

*Mar  1 00:15:27.179: UDP: rcvd src=192.168.1.253(3222), dst=224.0.0.102(3222), length=68