Schema Level Problem !

Question

{color:#000080}Hi,{color}
{color:#000080}I have successfully configured the schema level replication, but some issues are front of me to struck me.
So, I am going to explain my scenario as following.{color}
{color:#000080}1: Site1 having oracle 10g DB and having two schema level processes: Capture and Propagate
2: Site2 having oracle 10g DB and having two schema level processes: Capture and Propagate
3: Central_DB having oracle10g DB and having two APPLY processes one for site1 and second for site2.{color}
{color:#000080}I have a plan to connect the total 18 branches to our Central_DB in future.
From all sites to Central_DB we will configure the Schema Level Replication using streams.(All sites have same schema)(DML + DDL)
But From Central_DB to all sites we will configure Table Level Rule-Based replication.(Only DML's){color}
{color:#000080}My Questions are:{color}
{color:#000080}1: Please recommend me that it's a good model or not ?
2: When site1 doing DML or DDL changes locally then changes are captured by capture process and
After that changes are propagated from source queue(Site1) to destination queue(CEntral_DB) and
Applied by apply process of Central_DB.
     > If site1 doing changes and at the same time Central_DB goes shutdown and
restart again then changes are applied from site1 to Central_DB.(Normal Working)
     > If site1 doing changes and at the same time Central_DB goes shutdown and
site1 doing more changes locally, after few minutes site1 also goes shutdown , Now site1 and Central_DB both are down.
     > After one day when both machines are upped and running, but propagation process
     contains some connection errors and no further changes are replicated, even status of processes are enabled.{color}
{color:#ff0000}ERROR: {color}
{color:#ff0000}     ORA-12545: Connect failed because target host or object does not exis {color}
{color:#000080}Please assist me to troubleshoot this problem why no further changes are replicated?{color}
{color:#000080}Thanks,
Fazi {color}
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Answer 1

That would be if an IP or DNS lookup (depending on what is specified as HOST in the tnsnames entry) is failing.
Possibly, the lookups before the servers went down were via DNS and now the DNS is no longer available (it has gone down ?). OR that someone has recreated a hosts file and the earlier entry for the target host is missing.