GLBP protocol

GLBP protocol

GLBP (Gateway Load Balancing Protocol ) as other fhrp  VRRP and HSRP provides gateway redundancy for hosts. This protocol gives more load sharing then other fhrp with condition that clients will have just one default gateway configured. It is more flexible.

GLBP routers exchange messages which contain the information providing Active Virtual Gateway election, determines GLBP router roles and tracking their states. GLBP provides up to four gateways in the group. One router in the group is elected as AVG (Active Virtual Gateway) and one router is elected as standby virtual gateway, other routers within the group are placed in "Listening" state. 

Active Virtual Gateway is responsible to reply for the ARP requests to virtual IP. AVG sends one of the forwarder's virtual MAC address inside the ARP reply. AVG maintaining virtual mac to IP mapping in the client cache. 

All routers of the group are forwarders and each router is the owner of the virtual mac-address assigned by the AVG and responsible for sending packets to its virtual address. 

Protocol does load sharing by responding to ARP request sent to virtual IP  with different virtual mac-addresses assigned to all GLBP routers. There are three different load sharing schemes: round-robin (default), weighted and host dependent.

GLBP:

-hello timer - 3 sec by default

-hold timer 10 sec by default

-GLBP group number available: 1-1023

-redirect timer -  AVG will reply to arp with failed forwarder's mac address due redirect time

-time to live - time while failed forwarder's virtual mac address is staying alive

-Virtual mac address has the following form: 0004.b4xx.xxyy where xx.xx - 16 bits where 6 bits are empty and next 10 bits are reserved for GLBP group number. yy - are the forwarder number.

-GLBP packets are sent to destination UDP port 3222 multicast address 224.0.0.102 to destination multicast MAC address 0105.5e00.0006 from source primary IP address and source virtual MAC address.

-Router with the priority equal to AVG and higher IP address didn't preempt the active GLBP router.

-GLBP is not IPv6 compatible

-GLBP doesn't support stacking feature

AVG election

AVG election is provided by priority (default value is 100). Router with the higher priority becomes AVG.

AVG fails

When AVG fails, the standby router becomes AVG after hold down timeout and new standby router would be elected.

AVF fails

After AVF fails it stops  to send it's GLBP messages, One of the GLBP forwarders pick up this failed  AVF's virtual mac address and this mac address become "Active" for it ----> now this backup forwarder has primary virtual address and secondary. this secondary virtual address is staying alive within minimum 600 seconds (default ARP timeout) and then flushes from arp table of clients. Due time to live backup forwarder sends additional TLV where specifyed onemore secondary virtual mac address.  

GLBP scenario or what are we going to do:

Our GLBP mini-lab include the next two parts related to Active Virtual Gateway (AVG) and Active Virtual Forwarding (AVF).

AVG discovering:

- Enable GLBP on one router, looking for information related to AVG
- explore TCP dump with GLBP packets

- explore debugging
- Enable GLBP on the neighboring router
- Tuning AVG timers
- enable AVG preemption
- exploring AVG election procedure

AVF discovering:

- configure 3 GLBP routers
- configure simple topology for sample traffic forwarding and testing
- testing failover with default timers
- tuning timers
- configure weighting
- explore tracking and weights thresholds

As known GLBP datagrams are sent over UDP port 3222.  GLBP information is transmitted inside the specified TLVs:

We enabled GLBP only on one router that is why the same router is will be the AVG and AVF.

R1(config-if)#glbp 1 ip 10.12.10.100

*Mar  1 01:56:31.767: %GLBP-6-STATECHANGE: FastEthernet0/0 Grp 1 state Standby -> Active

*Mar  1 01:56:41.767: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 1 state Listen -> Active

All GLBP parameters are default.

R1(config-if)#do sh glbp
FastEthernet0/0 - Group 1
  State is Active
    2 state changes, last state change 00:00:05
  Virtual IP address is 10.12.10.100
  Hello time 3 sec, hold time 10 sec  <--------- default AVG timer values
    Next hello sent in 0.488 secs
  Redirect time 600 sec, forwarder timeout 14400 sec  
  Preemption disabled
  Active is local
  Standby is unknown
  Priority 100 (default)
  Weighting 100 (default 100), thresholds: lower 1, upper 100
  Load balancing: round-robin
  Group members:
    0011.1111.1111 (10.12.10.1) local
  There is 1 forwarder (0 active)
  Forwarder 1
    State is Listen
    MAC address is 0007.b400.0101 (default)
    Owner ID is 0011.1111.1111
    Redirection enabled
    Preemption enabled, min delay 30 sec
    Active is unknown

HELLO TLV:

Router sends GLBP packet with only HELLO TLV while being starting GLBP process and while transition Init--->Listen and Listen---> Speak occurs.

Source MAC address is a primary mac of the router

Destination MAC address is a multicast MAC  0100.5e00.0066

Source IP address is a primary address of the router and destination IP is multicast IP address 224.0.0.102

PIC.1 Hello GLBP message:

Request/response

The next step after router advertised itself as a AVG, it is time to advertise itself as a AVF and it sends GLBP message with request/response TLV:

GLBP router sends Requet/response message with the virtual assigned source mac address to multicast mac address

Source IP address is a primary IP of the GLBP interface of the router.

GLBP router advertise assigned virtual mac-address.

Virtual mac address has the following form:

0004.b4xx.xxyy where xx.xx - 16 bits where 6 bits are empty and next 10 bits are reserved for GLBP group number. yy - are the forwarder number.

PIC.2 GLBP  Request/response TLV

After advertising itself as a GLBP, router send GLBP messages with both TLVs  with source virtual mac address: 

PIC.3 GLBP message with both TLVs.

GLBP AVG preemption is disabled on the port by default, that is why after enabling GLBP on the neighboring router with higher priority or with the same priority, but higher IP address, new router GLBP state will be "Standby"

AVF preemption is enabled by default.

R3(config-if)#glbp 1 ip 10.12.10.100
R3(config-if)#
*Mar  1 03:15:44.911: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 2 state Listen -> Active
R3(config-if)#do sh glbp
FastEthernet0/0 - Group 1
  State is Standby
    1 state change, last state change 00:51:53
  Virtual IP address is 10.12.10.100
  Hello time 3 sec, hold time 10 sec
    Next hello sent in 0.380 secs
  Redirect time 600 sec, forwarder timeout 14400 sec
  Preemption disabled
  Active is 10.12.10.1, priority 100 (expires in 8.656 sec)
  Standby is local
  Priority 100 (default)
  Weighting 100 (default 100), thresholds: lower 1, upper 100
  Load balancing: round-robin
  Group members:
    0011.1111.1111 (10.12.10.1)
    0033.3333.3333 (10.12.10.3) local
  There are 2 forwarders (1 active)
  Forwarder 1
    State is Listen
    MAC address is 0007.b400.0101 (learnt)
    Owner ID is 0011.1111.1111
    Time to live: 14398.644 sec (maximum 14400 sec)
    Preemption enabled, min delay 30 sec           <--------------------- AVF preemption

As you can see from debugging there are two active forwarders, one active and one standby gateways:

*Mar  1 12:19:32.964: GLBP: Fa0/0 Grp 1 Hello  out VG Active  pri 100 vIP 10.12.10.100 hello 20000, hold 100000 VF 1 Active  pri 167 vMAC 0007.b400.0101
*Mar  1 12:19:33.544: GLBP: Fa0/0 Grp 1 Hello  in  VG Standby pri 100 vIP 10.12.10.100 hello 20000, hold 100000 VF 2 Active  pri 167 vMAC 0007.b400.0102

Let's tune timers on the AVG and look at the timers on standby gateway GLBP router:

R1(config-if)# glbp 1 timers ?
  <1-60>    Hello interval in seconds
  msec      Specify hello interval in milliseconds
  redirect  Specify timeout values for failed forwarders

R1(config-if)# glbp 1 timers 20 ?
  <21-180>  Hold time in seconds
  msec      Specify hold time in milliseconds

R1(config-if)# glbp 1 timers 20 100

Next look at the standby GLBP router's timers:

R3(config-if)#do sh glbp
FastEthernet0/0 - Group 1
  State is Standby
    1 state change, last state change 09:01:27
  Virtual IP address is 10.12.10.100

 Hello time 20 sec, hold time 100 sec   <----------------Changing timers on the AVG 

  automatically changes timers for all GLBP standby gateways in the group

    Next hello sent in 1.888 secs
  Redirect time 600 sec, forwarder timeout 14400 sec
  Preemption enabled, min delay 0 sec
  Active is 10.12.10.1, priority 100 (expires in 81.380 sec)
  Standby is local
  Priority 100 (default)
  Weighting 100 (default 100), thresholds: lower 1, upper 100
  Load balancing: round-robin
  Group members:
    0011.1111.1111 (10.12.10.1)
    0033.3333.3333 (10.12.10.3) local
  There are 2 forwarders (1 active)
  Forwarder 1
    State is Listen
    MAC address is 0007.b400.0101 (learnt)
    Owner ID is 0011.1111.1111
    Time to live: 14381.368 sec (maximum 14400 sec)
    Preemption enabled, min delay 30 sec
    Active is 10.12.10.1 (primary), weighting 100 (expires in 89.940 sec)
  Forwarder 2
    State is Active
      1 state change, last state change 09:02:26
    MAC address is 0007.b400.0102 (default)
    Owner ID is 0033.3333.3333
    Preemption enabled, min delay 30 sec
    Active is local, weighting 100

Let's enable AVG preemption:

R3(config-if)# glbp 1 preempt

R3(config-if)# do sh run int fa 0/0

Building configuration...

Current configuration : 162 bytes
!
interface FastEthernet0/0
mac-address 0033.3333.3333
ip address 10.12.10.3 255.255.255.0
speed 100
full-duplex
glbp 1 ip 10.12.10.100
glbp 1 preempt
end

Router with the priority equal to AVG and higher IP address didn't preempt the active GLBP router.

Now we change priority on the standby gateway:

R3(config-if)#glbp 1 priority ?
  <1-255>  Priority value
 R3(config-if)#glbp 1 priority 105
R3(config-if)#

*Mar  1 12:29:21.072: GLBP: Fa0/0 1 Standby: l/Hello rcvd from lower pri Active router (100/10.12.10.1)
*Mar  1 12:29:21.076: GLBP: Fa0/0 1 Active router IP is local, was 10.12.10.1
*Mar  1 12:29:21.076: GLBP: Fa0/0 1 Standby router is unknown, was local
*Mar  1 12:29:21.076: GLBP: Fa0/0 1 Standby -> Active

*Mar  1 12:29:21.076: %GLBP-6-STATECHANGE: FastEthernet0/0 Grp 1 state Standby -> Active

Look at the Active router debugging:

R1(config-if)#
*Mar  1 12:29:13.084: GLBP: Fa0/0 Grp 1 Hello  out VG Active  pri 100 vIP 10.12.10.100 hello 20000, hold 100000 VF 1 Active  pri 167 vMAC 0007.b400.0101
*Mar  1 12:29:13.248: GLBP: Fa0/0 Grp 1 Hello  in  VG Active  pri 105 vIP 10.12.10.100 hello 20000, hold 100000 VF 2 Active  pri 167 vMAC 0007.b400.0102
*Mar  1 12:29:13.248: GLBP: Fa0/0 1 Active router IP is 10.12.10.3, was local
*Mar  1 12:29:13.252: GLBP: Fa0/0 1 Standby router is unknown, was 10.12.10.3
*Mar  1 12:29:13.252: GLBP: Fa0/0 1 Active: k/Hello rcvd from higher pri Active router (105/10.12.10.3)
*Mar  1 12:29:13.252: GLBP: Fa0/0 1 Active -> Speak
*Mar  1 12:29:13.252: %GLBP-6-STATECHANGE: FastEthernet0/0 Grp 1 state Active -> Speak
R1(config-if)#
*Mar  1 12:29:13.256: GLBP: Fa0/0 Grp 1 Hello  out VG Speak   pri 100 vIP 10.12.10.100 hello 20000, hold 100000 VF 1 Active  pri 167 vMAC 0007.b400.0101

After some hello messages exchanging we have new active and standby GLBP routers:

R1(config-if)#
*Mar  1 12:30:53.160: GLBP: Fa0/0 Grp 1 Hello  in  VG Active  pri 105 vIP 10.12.10.100 hello 20000, hold 100000 VF 2 Active  pri 167 vMAC 0007.b400.0102
*Mar  1 12:30:53.252: GLBP: Fa0/0 1 Speak: f/Standby timer expired (unknown)
*Mar  1 12:30:53.252: GLBP: Fa0/0 1 Standby router is local
*Mar  1 12:30:53.252: GLBP: Fa0/0 1 Speak -> Standby
*Mar  1 12:30:53.252: GLBP: Fa0/0 Grp 1 Hello  out VG Standby pri 100 vIP 10.12.10.100 hello 20000, hold 100000 VF 1 Active  pri 167 vMAC 0007.b400.0101

 Verify GLBP router's states:

R1(config-if)#do sh glbp bri
Interface   Grp  Fwd Pri State    Address         Active router   Standby router
Fa0/0       1    -   100 Standby  10.12.10.100    10.12.10.3      local
Fa0/0       1    1   -   Active   0007.b400.0101  local           -
Fa0/0       1    2   -   Listen   0007.b400.0102  10.12.10.3      -

Let's configure third router to be in this GLBP group:

R4(config-if)#do sh run int fa 0/0

interface FastEthernet0/0
mac-address 0044.4444.4444
ip address 10.12.10.4 255.255.255.0
speed 100
full-duplex
glbp 1 ip 10.12.10.100
glbp 1 preempt
end

This config caused the router R1 with the lowest IP address to be in the "Listen" state and R3 and R4 in the Active and Standby states.

As conclusion we can say, that Standby GLBP router will preempt only when it's priority becomes higher then priority of the Active router. Routers with lower IP addresses automatically becomes "Listen" GLBP routers. There are only one Active and Only one standby router.

Weighting and tracking

In GLBP can be used three different schemes of load sharing.

Weighting can be assign manually and every GLBP router will advertise it to each other. ARP replies with virtual mac addresses will be sent proportionally weights of GLBP router s in the group.

You can set lower and upper threshold to track the state of GLBP router. You also can configure tracking the way you need to automatically tune GLBP weight depending of operation of your network. It is a very flexible tool to solve specific tasks related with load sharing.

Configuration example:

We need the weight of R1 to be decreased lower the threshold after line protocol of one of specified interfaces goes down.

R1(config-if)#do sh ip inter bri
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            10.12.10.1      YES manual up                    up
FastEthernet1/0            172.20.123.1    YES manual up                    up
R1(config-if)#int fa 0/0
R1(config-if)#glbp 1 weightin ?
  <1-254>  Weighting maximum value
  track    Interface tracking

R1(config-if)#glbp 1 weightin 100 ?
  lower  Weighting lower threshold
  upper  Weighting upper threshold
  <cr>

 R1(config-if)#glbp 1 weightin 100 lower ?
  <1-99>  Weighting lower threshold value          <----------- Possible threshold values lower

R1(config-if)#glbp 1 weightin 100 lower 40 ?
  upper  Weighting upper threshold                    <------------ Possible threshold values upper
  <cr>

R1(config-if)#glbp 1 weightin 100 lower 40 upper 100
R1(config-if)#do sh run int fa 0/0
Building configuration...

Current configuration : 208 bytes
!
interface FastEthernet0/0
mac-address 0011.1111.1111
ip address 10.12.10.1 255.255.255.0
speed 100
full-duplex
glbp 1 ip 10.12.10.100
glbp 1 weighting 100 lower 40
end

Configure tracking

R1(config-if)#glbp 1 weighting track 1 decrement ?
  <1-255>  Decrement value
R1(config-if)#glbp 1 weighting track 1 decrement 70
R1(config-if)#exit
R1(config)#track 1 interface fa 1/0 line-protocol

Now we manually shut tracked interface and look at the debufg output:

R1(config-if)#do deb glbp terse <----- turn on brief debugging of GLBP 
GLBP:
  GLBP Errors debugging is on
  GLBP Events debugging is on
    (protocol, redundancy, track)
  GLBP Packets debugging is on
    (Request, Reply)
R1(config-if)#
R1(config-if)#int fa 1/0

R1(config-if)#shut              <----Manually shutting interface
*Mar  2 13:06:34.674: %TRACKING-5-STATE: 1 interface Fa1/0 line-protocol Up->Down
*Mar  2 13:06:34.678: GLBP: Fa0/0 1 Track 1 object changed, state Up -> Down
*Mar  2 13:06:34.678: GLBP: Fa0/0 1 Weighting 100 -> 30
*Mar  2 13:06:36.674: %LINK-5-CHANGED: Interface FastEthernet1/0, changed state to administratively down
*Mar  2 13:06:37.674: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0, changed state to down
*Mar  2 13:07:06.118: GLBP: Fa0/0 1.1 Active: i/Hello rcvd from higher pri Active router (135/10.12.10.3)
*Mar  2 13:07:06.118: GLBP: Fa0/0 1.1 Active -> Listen
R1(config-if)#
*Mar  2 13:07:06.118: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 1 state Active -> Listen   <---Forwarder becomes "Listen"
R1(config-if)#

Let's look at debugging on ohter GLBP routers :

Here is debugging from the router that became backup forwarder after R1 failed:

R4(config-if)#
*Mar  2 13:06:43.794: GLBP: Fa0/0 1.1 Preemption delayed, 30 secs remaining <----default preemption delay
 *Mar  2 13:07:13.806: GLBP: Fa0/0 1.1 Listen: k/Hello rcvd from lower pri Active router (39/10.12.10.1) <---- R1 became a lower priority GLBP forwarder

*Mar  2 13:07:13.810: GLBP: Fa0/0 1.1 Listen -> Active

*Mar  2 13:07:13.810: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 1 state Listen -> Active

PIC.4 R4 becomes a backup forwarder fot R1 and carry two TLVs:

If you disable forwarding preemprtion on all GLBP routers then after time to live timer expire --> virtual mac address will become disable:

R4(config-if)#do sh run int fa 0/0 | i glbp
glbp 1 ip 10.12.10.100
glbp 1 timers redirect 10 610
glbp 1 preempt                        <-------------AVG preemption is enabled
glbp 1 load-balancing weighted
no glbp 1 forwarder preempt     <------------ AVF preemption is disabled

R4(config-if)#do sh glbp | i live
    Time to live: 0.568 sec (maximum 609 sec)  <--------time to live timer expires
    Time to live: 607.848 sec (maximum 610 sec)
R4(config-if)#
*Mar  1 18:36:48.839: GLBP: Fa0/0 1.1 Active: c/Secondary timer expired
*Mar  1 18:36:48.839: GLBP: Fa0/0 1.1 Active -> Disabled
*Mar  1 18:36:48.839: %GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 1 state Active -> Disabled
R4(config-if)#

%GLBP-6-FWDSTATECHANGE: FastEthernet0/0 Grp 1 Fwd 1 state Active -> Disabled   <----AVF's virtual mac is disabled

I the next part of GLBP protocol notes I will test GLBP in action.