SRX5600 source NAT pool unevenly used

Hi!

Using snmp looking at the jnxJsNatSrcNumSessions it seems very un-balanced for our nat source pool user-shared-pool (x.y.142.0-x.y.142.255). For some (individual) IP addresses, the number of sessions are 0, for others the number is 8000.

Name/OID: jnxJsNatSrcNumSessions.16.117.115.101.114.45.115.104.97.114.101.100.45.112.111.111.108.0.x.y.142.224; Value (Integer): 0

Name/OID: jnxJsNatSrcNumSessions.16.117.115.101.114.45.115.104.97.114.101.100.45.112.111.111.108.0.x.y.142.254; Value (Integer): 7979

Is this an expected behavior or should the sessions be more evenly distributed? In CGNAT in the MX I understand that there is a "round-robin" option, but that doesn't seem to be available in the SRX5k. Perhaps the SRX wants to fill upp one IP with sessions and only uses the next IP in case of port number collisions.

I appreciate any input!

/Fredrik