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So when enabling object tracking on an AVF for an ISP uplink (for example), while running GLBP on another interface of the router, we specify a weight range and tell the AVF that if object loses its IP reachability, the weight is dropped below a certain limit and if that is below the lower limit specified, that router can no longer be part of the GLBP process (or does it become an SVF?).

I'm not even sure if my question makes sense but, why is this range even required? Is there any other use of enabling this range? If the uplink goes down, why not just directly take the router out of the AVF role, what's the need for checking with a range?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. – Ron Maupin Aug 7 '17 at 15:22
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First, we need to understand that AVFs are always “active” in the sense that they are always used by a load-balancing algorithm. (However, by setting an AVG weight value below threshold, we may effectively take the AVF out of service. The weight value could be combined with object tracking to bring powerful traffic manipulation options). Next, with respect to redundancy, all AVFs backup each other. For instance, take any AVF: with respect to the other AVFs it is “Active”, and the remaining AVFs are in “Listen” state. If the AVF would fail, other gatewyas will detect the event using Hold timer expiration, and immediately try to take over the failed AVF virtual MAC address. Among the competitors, the AVF with highest weight value would win, and the remaining AVFs will switch back to “Listen” state. At this point, the “winner” will start accepting packets for two virtual MAC addresses: it’s own, and the one it has obtained from the failed AVF. At the same moment, two timers would start: Redirect and Secondary Hold. The Redirect timer determines how long will AVG continue to respond to ARP requests with the virtual MAC of the failed AVF. The Secondary Hold timer sets the amount of time the backup AVF will continue to accept packet for the virtual MAC address taken from the failed AVF.

In essence yes we only have one AVG and one AVF designated at one time, I misunderstood your question. But yes I would agree to me it seems like a valid question. If I'm not mistaken a 3rd or 4th member of the group which had lower values assigned would still be 3rd in order and 4th in order in the event of an AVF failure. They are always "active" in a standby capacity also, preemption of a new AVF IS determined by the weight assigned in the configuration. Priority (AVG) weight+timers (AVF election).

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The use of object tracking for a glbp group enables an engineer or administrator to set priority based upon weight, and priority. Highest priority amongst routers in a glbp group equals the Active virtual gateway...In addition, weight added to the config of the other members of a GLBP group determines which router will become the AVG instead of an AVG and become the primary. All routers share one vMAC for the group and one VIP in addition to also maintaining their hardware MAC. Each router also has its unique IP address in the GLBP group in addition to the VIP which is used for the entire group as an identifier at layer 3

I guess to answer your question, one reason we would want to use tracking is for autonomy, its much easier for our network to pick up after itself without a hitch versus manually logging into a device and changing priorities. Secondly, without tracking how would the group know if an interface or router was to go down....its similar to the concept of HSRP except GLBP allows four router to be part of a group versus HSRP which only allows two. Another function of weight or "tracking" is the ability to shape data traffic based on the weight assigned to routers in the group, IE R1 has a weight of 50 R2 has a weight of 30 so that means R1 in theory gets 50% of the traffic for the group and R2 would get 30%.

Some other features that help with traffic shaping and load balancing are round robin algorithms and source mac address. Also another important point to remember is the preempt command which is used in GLBP, as with other HA protocols, it is a good idea to stagger your timers on your routers in order to determine when a router will attempt to preempt and become AVG. I like to set one at 30 seconds preempt and the desired next AVG at 0 seconds for immediate failover or preemption. Depending on the weight and priority along with the order of the preempt command in a configuration it is possible to designate a desired router to become AVG based on the chosen values. Also as with many other protocols GLBP uses hello and hold down timers these can be configured on the interface when it is added to a GLBP group.

Here is a sample config I borrowed from a CCIE study guide. As you can see R2 is currently the active virtual gateway.

*config:

2.    +R1:
      +interface FastEthernet0/0
      +ip address 174.1.123.1 255.255.255.0
      +glbp 123 ip 174.1.123.254
      +glbp 123 preempt
      +glbp 123 weighting 50
      +glbp 123 load-balancing weighted
!
!
!
11. -R2:
      +interface FastEthernet0/0
      +ip address 174.1.123.2 255.255.255.0
      +glbp 123 ip 174.1.123.254
      +glbp 123 priority 50
      +glbp 123 preempt
      +glbp 123 weighting 30
!
!
!
23. +R3:
      +interface Ethernet0/0
      +ip address 174.1.123.3 255.255.255.0
      +glbp 123 ip 174.1.123.254
      +glbp 123 priority 25
      +glbp 123 preempt
      +glbp 123 weighting 20
*

Lastly, To summarize.... GLBP is a virtual gateway protocol, with built-in load-balancing capabilities. Load balancing is based on manipulating ARP responses to the requests sent to the virtual gateway IP address. AVG role is used to load-balance and respond to ARP requests. AVF role manages one or more virtual MACs and is responsible for packet forwarding. AVG redundancy is controlled by GLBP priority and AVF redundancy is implemented using AVF weight value and two additional timers.

Hope this helps!

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  • For configurations, you should add four spaces to the beginning of the lines, or highlight the block and select the Preformatted Text button. It is difficult to read configuration text as regular text. – Ron Maupin Jul 1 '16 at 1:11
  • Ron thanks for the tip, I'm still getting used to some of the formatting but Ill get it down in short notice =). – Ty Smith Jul 1 '16 at 1:12
  • Hey, thanks for the reply Ty! But I feel you've missed the point of my question. My point was why does GLBP insist on having a range for checking the weight of tracked objects, because I don't see any use of a range. I understand the use of object tracking but if you instruct GLBP to reduce the weight of an object tracked and the weight is lower than the others, it automatically is removed from being an AVF. What's the point of a weight range to check with when it's only locally significant? – Izy- Jul 1 '16 at 1:12
  • Also, few more things regarding your answer. You said "weight added to the config of the other members of a GLBP group determines which router will become an AVG instead of an AVG". I believe that AVG selection is done via priority and highest physical IP, while AVF selection is done via weight and then highest physical ip. Ml – Izy- Jul 1 '16 at 1:15
  • I see what you mean =). – Ty Smith Jul 1 '16 at 1:16

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