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Cisco Employee

PG Majority Connections for SIDE-A

Hi Members,

In SRND it is mentioned as below,

• The Call Routers detects the failure by missing five consecutive TCP keep-alive messages. The
currently enabled side (side A in most cases) transitions to an isolated-enabled state and continues
to function as long as it is in communication with at least half of the PGs configured in the system.


• The paired-disabled side (side B in most cases) transitions to an isolated-disabled state. This side
will then check for device majority. If it is not communicating with either an Active or Idle DMP to
more than half of the configured PGs in the system, it will stop processing and stay disabled.

For Side A to run in isolated-enabled, does it require the communication to be ACTIVE for the half of the PG's or it also counts the IDLE Connection too.

For Side B it is clearly mentioned that Active and idle will be counted for device majority.

Regards,

Senthil

Everyone's tags (2)
12 REPLIES
Green

PG Majority Connections for SIDE-A

They work the same way - the only difference is the tie breaking mechanism. Side A needs to see 50% or more - side B needs to see strictly more than 50%. Idles and actives are both counted.

Regards,

Geoff

Cisco Employee

PG Majority Connections for SIDE-A

Thank you Geoff, So the SideA and SideB both counts the Active and Idle DMP connections when there is private link failure.

Regards,

Senthil

Green

PG Majority Connections for SIDE-A

Cautious answer - yes.

What exactly are you trying to map out here? How many PGs do you have? Are all the pairs colocated? Are all the pairs geographically isolated from the side A and side B CCs? How far between the CCs?

Regards,

Geoff

Cisco Employee

Re: PG Majority Connections for SIDE-A

Geoff,

In that case, say if i have 1GenericPG and MR PG in SideA and SideB. They have Active Connection to RoggerA and Idle connection to RoggerB.

Rogger A -- 2PG's Active

Rogger B -- 2PG's Idle.

This would be the same everytime and both A and B always more than 50% of the configured PG's.

Do we really require Dummy PG to be added in any Sides ? Kindly clarify.

Am i Right in the above calculation or it will be like below

Rogger A -- 2 Active Connection, 2 Idle Connection

Rogger B -- 4 Idle Connections

Regards,

Senthil

Message was edited by: SenthilKumar Sankar

Cisco Employee

PG Majority Connections for SIDE-A

Geoff,

Is the 50% calculation is based on the Overall connections or number of PG's Configured.

Like totally 8 Connections, 50% on A(4 connections) and 50% on B(4 connections), is that is how it is calculated.

Regards,

Senthil

Green

PG Majority Connections for SIDE-A

A complicated issue.

1. regarding the "number of PG's Configured".

The algorithm is determined on the number of PGs setup in the registry of each Call Router. You can configure 100 PGs in PG Explorer and they are not counted unless they have been "setup" in the registry of the Router.

2. the way the system was designed to deal with the failure of the private connection between the Central Controllers was for a large, distributed system where the side A CC is at one data center and the side B CC was at another data center, but these are not contact centers and don't have agents. Then geographically separate from these data centers we have a number of contact centers - at each is a pair of PGs, connected by a crossover cable.

When the Call Router is in duplex mode, one of the MDS will be the "Enabled Synchronizer" and one will be the "Disabled Synchronizer". You will see it like so:

The side A MDS is Enabled, and the side B MDS is Disabled. This is most often the case.

When the private link is severed, any PG that had an active DMP path to the Disabled Synchronizer (Router B) CCAgent will realign to the A side. The PG does not disconnect - it simple makes the path to Router B CCAgent idle and the path to Router A CCAgent goes active (if it can).

Now Router B has to assess the situation. It checks for device majority and if so, issues TOS (Test Other Side). If Router B can communicate with a majority of the PGs, none of which can see that Router A is enabled, it will promote itself to Isolated-Enabled.

When the private link is severed, the Enabled Synchronizer has a simpler algorithm. If it has device majority, it goes straight to Isolated-Enabled; if it doesn't, it goes to Isolated-Disabled.

In your case, you have the contact centers at the side A and side B sites. If the private link is severed, and the WAN link is not, there won't be a problem because any TOS message issued from B will find that the PG can talk to Router A, so Router B will never go Isolated-Enabled.

Now if you are trying to deal with a failure of both links between the sides, this can be a huge issue if both go Isolated-Enabled and each CC runs - the "split brain" problem - which is a nightmare when the links come back and the system tries to resume duplex operation but the databases are inconsistent.

I never really understood the "Dummy PG" idea.

Regards,

Geoff

Cisco Employee

PG Majority Connections for SIDE-A

Geoff,

Thank you for your response,

Say my Router A and B has their registry setting as 2PG's

1) If my Complete Side A Rogger and Side A PG's are down, at that time will my Side B CC will go to isolated-enabled state.

I think Side B wont send TOS message to the PG's as it has only 50% of DMP connections. Correct me if i am wrong.

Regards,

Senthil

Green

PG Majority Connections for SIDE-A

Why not? Side B can see 2 PGs out of 2.

Regards,

Geoff

Cisco Employee

PG Majority Connections for SIDE-A

Thank You Geoff,

So in my case,

Router B Registry = 2Pg's

PG's = 1Generic & 1MR PG

Rogger B will no go in Isolated enabled only when below scenario occurs,

1) Side A Rogger and Side A PG's down and if my MR PG of side B is stopped

2) Side A Rogger and Side A PG's down and if my Second PG is not configured in Side B

Regards,

Senthil

Green

Re: PG Majority Connections for SIDE-A

Rogger B will no go in Isolated enabled only when below scenario occurs,

1) Side A Rogger and Side A PG's down and if my MR PG of side B is stopped

2) Side A Rogger and Side A PG's down and if my Second PG is not configured in Side B

I think the following applies.

If all of side A is down, and both PGs on side B are up, side B will go into "Isolated-Enabled".

If all of side A is down, and only one PG on side B is up, side B will NOT go into "Isolated-Enabled".

If all of side A is down and side B is setup with just one PG (say no MR PG here) and the PG on side B is up, then device majority with just 1 configured should be achieved if the Disabled Synchronizer can talk to the PG, and side B will go into "Isolated-Enabled".

These cases are difficult to work through, and there can be surprises. I would like to see it work to be sure.

What are you thinking of doing though. You don't want to have the MR PG configured on just one side. What happens if that box breaks?

Regards,

Geoff

Cisco Employee

PG Majority Connections for SIDE-A

Yes, If i dont configure MR PG just on one side, but while configuring Rogger B i will give the PG count as 1-2.

No one will do like above scenario, but just trying to understand is that is the way dummy pg configured.

Geoff, But now i got the point how it is calculating the device majority. Thank you for clarifying.

Regards,

Senthil

Green

PG Majority Connections for SIDE-A

I think that's exactly why dummy PGs are created. But I have never done it or studied the method in great detail. I think this aspect of the platform is not well explained in any Cisco docs and I am on shaky ground. It would be good if there was a nice Wiki doc that explained it all.

Regards,

Geoff

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