http://msdn.microsoft.com/ru-ru/library/8xx3tyca.aspx

When a connection is first opened, a connection pool is created based on an exact matching algorithm that associates the pool with the connection string in the connection. Each connection pool is associated with a distinct connection string. When a new connection is opened, if the connection string is not an exact match to an existing pool, a new pool is created. Connections are pooled per process, per application domain, per connection string and when integrated security is used, per Windows identity. Connection strings must also be an exact match; keywords supplied in a different order for the same connection will be pooled separately.