IDS does not have some transparent mechanism for redirecting client requests to different database servers in replication pair.
But three different redirection mechanism exists (on client side):
1)Automatic redirection with the DBPATH environment variable -(INFORMIXSERVER for regularly database server, DBPATH for alternative database server)
2) Administrator-controlled redirection with the connectivity information (changing connectivity information, Connection-Redirection Option + group option)
3) User-controlled redirection with the INFORMIXSERVER environment variable (just changing INFORMIXSERVER environment variable after failure).
For detail description of each mechanism look at the documentation.