Simon's SQL

select SUSER_ID(), SUSER_NAME()
go
select * from server_A.master.dbo.sysobjects
go

(1 row(s) affected)
Msg 18452, Level 14, State 1, Line 0
Login failed for user ‘(null)’. Reason: Not associated with a trusted SQL Server connection.

execute as login = ‘sql_admin’  –who has the permission to login the server.
go
select SUSER_ID(), SUSER_NAME()
go
select * from server_A.master.dbo.sysobjects
go

(1 row(s) affected)

(1277 row(s) affected)

revert;
go
select SUSER_ID(), SUSER_NAME()
go