our isp use mikrotik based BRAS as a pppoe server for give connectivity to clients. most of client have private ip adress & some have public ip address framed by radius server. somehow i manged to get a read only account on isp BRAS for learning purpose. the question appears here, there is no masquerading rule for pppoe clients,ive bit confused how masquerading is done for pppoe clients
sorry for bad english..
It’s done in the same way for all IPs, by programming nat rules.
If you mean if there’s a mechanism to dynamically create NAT rules, there’s not, AFAIK.
Not sure if that answers your question.
That’s why I’m confused there is no Nat rule doing masquerade functions.
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Maybe that BRAS server isn’t the Provider Edge router (another router is doing the masquerading)
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I think language barriers are getting in the way 
Could you post the output of
/ip export
/ip route print
on the BRAS? (you can trim pppoe-interfaces from the outputs of these commands to a handful representative ones)
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These are the rules (31,32) on IP > firewall > Nat doing the NAT for private 192.168.250.0/24 and 10.3.42.0/24 ranges:
add action=src-nat chain=srcnat src-address=192.168.250.0/24 to-addresses=\
112.133.xxx.yyy
add action=src-nat chain=srcnat src-address=10.3.42.0/24 to-addresses=\
112.133.xxx.yyy
These rules are only for port fowarding clients.
JUNip.JPG
but there is a Juniper M120 Multi-service Edge Router behind BRAS doing masquerading .