Routing

Do you think what is difference for following two bgp configurations? Thank you

Sometimes I see command : 

set routing-instances DOC protocols bgp group ABC neighbor 19.9.9.9 local-address 66.3.3.3

and other time I cannot see the command when confuguring bgp

I see only one command here

I beleive , your query is with or without local- addres configuration

"If you omit the local-address statement, the expected source of BGP update messages is based on the device’s source address selection rules, which normally result in the egress interface address being the expected source of update messages. When this happens, the peering session is not established because a mismatch exists between the expected source address (the egress interface of the peer) and the actual source (the loopback interface of the peer). To ensure that the expected source address matches the actual source address, specify the loopback interface address in the local-address statement"

The above is applicable for your loopback peering especially in ebgp mulihop loopback peering this statement is mandatory.

I mean sometime they do not use the command "set routing-instances DOC protocols bgp group ABC neighbor 19.9.9.9 local-address 66.3.3.3" when they configure bgp. Do you think it is correct? 

In other word, one is using the commad above, and other is not using the command. What is diffenence bwteeen the two? 

If your EBGP speaker is your direct interface(peering with direct interface ) , mentioning without local-address is correct. if you are having a peering with loopback address of your neighbor then local-address config is required

You are right. Thank you so much