slave to slave zone transfer

[Quadri, Jay]

On cisko.com DNS server

# Primary forward addressing files
#
zone "cisko.com" {
        type master;
        file "
";
};   
# Secondary  files
zone "usa.cisko.com" {
        type slave;
        file "sec.usa.cisko.com";
        masters {
                10.10.10.2;
        };
};
zone "uk.cisko.com" {
        type slave;
        file "sec.uk.cisko.com";
        masters {
                10.10.10.3;
        };
};
----------------------------------------------------------------------------
-----
On usa.cisko.com DNS server 
zone "usa.cisko.com" {
 type            master ;
 file            "usa.cisko.com.db" ;
 notify          YES ;
 allow-update    {
  none ;
 } ;
 maintain-ixfr-base yes ;
} ;
zone "cisko.com" {
 type            slave ;
 file            "cisko.com.db.bak" ;
 masters         {
  10.10.10.1 ;
 } ;
 max-transfer-time-in 120 ;
} ;
----------------------------------------------------------------------------
----
On uk.cisko.com DNS server :
zone "uk.cisko.com" {
 type            master ;
 file            "uk.cisko.com.db" ;
 notify          YES ;
 allow-update    {
  none ;
 } ;
 maintain-ixfr-base yes ;
} ;
zone "cisko.com" {
 type            slave ;
 file            "cisko.com.db.bak" ;
 masters         {
  10.10.10.1 ;
 } ;
 max-transfer-time-in 120 ;
} ;

----------------------------------------------------------------------------
-------------------
; named.root
; all DNS server has the same named.root file
;
.                                3600000  IN NS    cisko.com.
cisko.com                      3600000  IN A     10.10.10.1
----------------------------------------------------------------------------
-------------------


why can't uk.cisko.com DNS resolve hosts within usa.cisko.com through
cisko.com, how can I do it without defining usa.cisko.com as slave in the
uk.cisko.com server.