Oracle Goldengate - ERROR: sending message to REPLICAT (Timeout waiting for message).

ORACLE 11gR2 - GoldenGate

Recently we observed  lag in one of the Replicat in Golden Gate. Tried to get 'stats','lag','stop' this replicat. However all these commands failed with 
"ERROR: sending message to REPLICAT <Replicat Name> (Timeout waiting for message)."

Identified Unix PID for this replicat and killed and restarted it. 
ps -ef| grep <replicat name>; kill -9 <pid>

This resolved the issue and we see laggap decreasing after this.

ORA-01555 on Standby

DB - 11gR2

Users were running select queries on Primary where as the same select was failing with ORA-01555 in Active standby. 

Undo_retention =   900 sec --Primary
Undo_retention = 21600 sec --Standby

Inspite of higher retention at standby users were facing ORA-01555. 

Later realized that undo_retention at stantby does not have any significance. Oracle Dataguard just replicates the undo_retention value from Primary. Increased the Undo_retention in Primary to get rid of this error.

Below note  really helped in understanding this.
http://alexeymoseyev.wordpress.com/2013/10/24/ora-01555-on-standby/

ORA-12502: TNS:listener received no CONNECT_DATA from client.

Users were able to connect from One client where as they were having issues from another client.
Jobs were failing with ORA-12502 error.

This is RAC env. 11gR2

From both clients
1. we can telnet,ping to db hosnames
2. we can telnet,ping to scan

however telnet to VIP was not working for problematic client. Informed apps team to Open firewall request for VIPs from client. This resolved the issue.

Below two links helped me in troubleshooting the issue.

http://levipereira.wordpress.com/2011/05/03/configuring-client-to-use-scan-11-2-0/
http://stelliosdba.blogspot.sg/2012/02/ora-12502-tnslistener-received-no.html