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Hello Wes<br>
<br>
Excellent information. I always had a difficulty understanding the
insides of the CUCM box. Thank you. <br>
<br>
One question remains:<br>
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?<br>
<br>
Thanks.<br>
Ovidiu<br>
<br>
<br>
On 06/Apr/11 9:22 PM, Wes Sisk wrote:
<blockquote cite="mid:4D9CBD7C.408@cisco.com" type="cite">
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For example sake:<br>
CM1: headquarters<br>
CM2: branch<br>
<br>
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.<br>
<br>
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.<br>
<br>
So, after convergence the device appears unregistered on the
remote node. For an interesting dig into this scenario take a
look at<br>
CSCsc62081 CCM SDL Out of Service / In Service causes
Unexpected Unity Failover.<br>
<br>
and similarly related to realtime synchronization of state
machines:<br>
CSCsc62073 Locations Out of Bandwidth causes unexpected Unity
Failover <br>
<br>
It was the same customer who originated both of these. This
customer had truly the worst luck with timing that I have ever
seen.<br>
<br>
Regards,<br>
Wes<br>
<br>
<br>
On 4/6/2011 11:51 AM, Ovidiu Popa wrote:
<blockquote
cite="mid:BANLkTimjKJr7LESdFC2B=Yt9A0mXYqm-pA@mail.gmail.com"
type="cite">Hello everyone
<div><br>
</div>
<div>Here's an unusual scenario that kind of puzzles me.</div>
<div><br>
</div>
<div>Here are the details:</div>
<div>- 2 CUCM with Clustering over WAN (HQ and Branch)</div>
<div>- Centralized PSTN Access at HQ (DID numbers routed to HQ)</div>
<div>- 1 phone with the Branch CUCM as primary and the HW CUCM
as secondary</div>
<div><br>
</div>
<div>Disaster strikes, the WAN link goes down and we have a
split brain condition.</div>
<div><br>
</div>
<div>What is the state to the phone on HQ CUCM? Will the phone
be Unregistered or state Unknown?</div>
<div><br>
</div>
<div>And the most important question is will the HQ CUCM follow
the CFUR if the state is Unknown?</div>
<div><br>
</div>
<div>Thanks for your input.</div>
<div><br>
</div>
<div>Regards,</div>
<div>Ovidiu</div>
<div> <br>
</div>
<div><br>
</div>
<div><br>
</div>
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