[cisco-voip] CUCM split brain question

[Ovidiu Popa]

Hello Wes

Excellent information. I always had a difficulty understanding the 
insides of the CUCM box. Thank you.

One question remains:
Imagine a WAN failure for 30 minutes. CM1 and CM2 are working without 
the SDL layer. CM2 (branch) sees the branch phone as Registered, CM1 
(hq) sees the phone as unregistered or unknown?

Thanks.
Ovidiu


On 06/Apr/11 9:22 PM, Wes Sisk wrote:
> For example sake:
> CM1: headquarters
> CM2: branch
>
> When IP connectivity exists between CM1 and CM2 SDL TCP sessions are 
> established between the 2 ccm processes. Through SDL each server tells 
> every other server about registered devices.  There is opportunity for 
> duplicate registration and some propagation time so there is a window 
> of convergence involved.  Once things are in sync each CM node tells 
> every other node about all significant local state changes.  Make 
> sense?  This usually helps folks understand why QoS is so critical on 
> SDL links.  SDL links are the vehicle for synchronization for 2 real 
> time processes.  This isn't quite as sensitive as parallel graphics 
> processing but it's not far off.
>
> When SDL link goes down each node forgets about all entities it 
> learned from the remote node.  It literally purges them.  Devices have 
> to register to their local node (even the best network admins miss 
> some especially when it comes to virtual devices like hunt pilots, 
> route lists, and software media resources).  Again there is 
> opportunity for some duplicate registration.  SDL links detect outage 
> on the order of 10 seconds or less.  SCCP devices do keepalives on the 
> order of 30 seconds with allowance for 1-2 missed keepalives.  
> 10seconds vs 60-90seconds creates a window of overlap.  If a duplicate 
> registration is detected then CM resets both device processes.  This 
> extends downtime but there really is no way of knowing which is the 
> "right" registration.  This is another window of convergence.
>
> So, after convergence the device appears unregistered on the remote 
> node.  For an interesting dig into this scenario take a look at
> CSCsc62081    CCM SDL Out of Service / In Service causes Unexpected 
> Unity Failover.
>
> and similarly related to realtime synchronization of state machines:
> CSCsc62073    Locations Out of Bandwidth causes unexpected Unity Failover
>
> It was the same customer who originated both of these.  This customer 
> had truly the worst luck with timing that I have ever seen.
>
> Regards,
> Wes
>
>
> On 4/6/2011 11:51 AM, Ovidiu Popa wrote:
>> Hello everyone
>>
>> Here's an unusual scenario that kind of puzzles me.
>>
>> Here are the details:
>> - 2 CUCM with Clustering over WAN (HQ and Branch)
>> - Centralized PSTN Access at HQ (DID numbers routed to HQ)
>> - 1 phone with the Branch CUCM as primary and the HW CUCM as secondary
>>
>> Disaster strikes, the WAN link goes down and we have a split brain 
>> condition.
>>
>> What is the state to the phone on HQ CUCM? Will the phone be 
>> Unregistered or state Unknown?
>>
>> And the most important question is will the HQ CUCM follow the CFUR 
>> if the state is Unknown?
>>
>> Thanks for your input.
>>
>> Regards,
>> Ovidiu
>>
>>
>>
>>
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