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Calling Search Space Clauses and Partition Name Limitations

You would be wrong if you're thinking that calling search space (CSS) clauses have something to do with Christmas.  Actually, a "CSS clause" is used internally within a call manager (CUCM) during call processing.  It is a text string that contains the ordered list of partitions found within both the device CSS and the partitions found within the CSS for the directory number (DN) or route pattern associated by the device.

Ultimately, the contents of the CSS clause is the ordered list of partitions available to be considered when a call routing decision must be made.  This order is the sequence in which they are configured within the CSS AND the sequence in which they are considered for matches.

If a duplicate route pattern is found within two different partitions contained within a singular calling search space clause, the order in which the partition is listed will be the tiebreaker.  Cisco recommends that this situation be avoided as it can create confusion when anticipating where a call would be routed.

How Many Partitions Can Be In A CSS?

The maximum size of the combined calling search space clause is 1024 characters, including a character used to separate each partition.  Also, since the clause consists of two different calling search spaces, the maximum length for an individual CSS is 512 characters.

Diagram 1: Calling Search Space Clause

As you can see from the constraint imposed by the CSS clause, the actual number of partitions that can be added to a single CSS is variable.  To estimate the maximum number of partitions possible, consider the following formula:

Max Number of Partitions = [ 512 / [ Average Length of the Partitions + 1 ] ]

Length of Partitions Maximum Number of Partitions
2 characters 170
3 characters 128
4 characters 102
5 characters 86
... ...
10 characters 46
15 characters 32
20 characters 24

Tutorial:  Partitions and Calling Search Spaces

Perhaps you've reached this page and you'd like some additional information about partitions and calling search spaces.  If that sounds like you, I recommend that you watch the following video which does a very good job explaining partitions and calling search spaces.  Furthermore, review the "Additional Information" links at the bottom of the post.

Additional Information

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