I just tried this:
$> cf service q name=isakmp
Which gave me the output:
cf service modify name=isakmp agent='ISAKMP Server' udp_ports=500,4500
So I tried modifying it like this (changing the port to 4501):
$> cf service modify name=isakmp agent='ISAKMP Server' udp_ports=500,4501
But I received an error:
service modify error: ParameterError: Can't modify udp_ports
It looks like you can't modify the port (at least via the 'cf service' command). I'll try and find out if this is possible via other means.
support gave me the same idea. But - Yes it is impossible to modify isakmpd ports this way.
I also tried sysctl parameter net.inet.ipsec.natt_port: 4500 but I can not see that it reflects on isakmpd work.
I asked your question to some others and got an answer from Development: you cannot change the NAT-T port on the Sidewinder.