CUCM: Understanding Partition and Calling Search Space (CSS)

From the cisco documentation: “Understanding and Using Partitions and Calling
Search Spaces with Cisco CallManager“, it says phones located in different partitions cannot call to each other, phones that reside within the same partition can call to each other.

“Note: Any device that makes a call can explicitly reach any dial plan entry that is left in the partition. In order to avoid unexpected results, Cisco recommends that you do not leave dial plan entries in the partition.”

This is quoted from the Cisco documentation, I felt this is the most important clause and should be located in the first page of the documentation not after several pages.

I actually get the advice from this blog, which clears all my confusion in the past 2 weeks.

Confusing scenarios

Scenario 1:
CIPC uses CSS-BLUE calling search space, 7940 IP Phone uses CSS-RED calling search space, both phones use none partition, both phones can call to each other.

Scenario 2:
CIPC uses none calling search space and none partition, 7940 IP phone uses CSS-BLUE calling search space and PART-BLUE partition. 7940 can call CIPC, CIPC cannot call 7940.

Scenario 3:
CIPC uses none partition and CSS-BLUE calling search space, 7940 IP phone uses none partition and CSS-RED calling search space. Both phones can call to each other.

Scenario 4:
CIPC uses none partition and CSS-BLUE calling search space, 7940 IP phone uses PART-RED partition and CSS-RED calling search space. 7940 can call CIPC but CIPC cannot call 7940.

hence it is advisable not to use none partition at all times, give the phones meaningful partition names to segregate them altogether. My team and I were confused the whole time, because if both phones were in different calling search space and the CSSes do not contain the same partition how could the phones call to each other?

Same partition and same CSS

There are two ways to let phones call to each other or one another.

1. CIPC locates in partition but uses CSS-RED, 7940 uses PART-RED partition and CSS-RED calling search space, both can call to each other.

2. Both CIPC and 7940 use the same partition and calling search space. (Both phones do not use none partition)

Create partition and CSS
From CUCM, Call routing > Class of Control > Partition to create partition.
From CUCM, Call routing > Class of control > Calling Search space to create CSS.

I am not clear whether the CSS should be applied under Device Information or Association Information or both…perhaps Cisco Voice experts can enlighten me…