repadmin /replicate now

there is many way you can do this

repadmin /replicate server2.Domain.com server1.Domain.com dc=Domain,dc=com

http://www.windowstricks.in/2009/11/force-active-directory-replication.html

i get an error DsReplicasysc() failed with status 8452.
we have a BH server and Hub-to-spoke teoplogy.

Did you specify the correct DN and domain in the first comment?

Error 8452 is "The naming context is in the process of being removed or is not replicated from the specified server." This means replication is failing, somewhere.

Start with dcdiag and see where the errors are occurring.

I do not get an error when i issue the command between the BH servers. for an example srv-A and srb-B at site 1. srvA is the BH.
srv-c and srv-d are at site 2. srv-c is the BH.
if i issue repadmin replicate srv-B to srv-d then i get error 8453.
My understanding the KCC optimize the path so we can not manually force the replication between non-bridge head, i might be wrong.

so if we have multiple sites (A,B,C), more than two then i have to replicate to transitive (B) site first, though KCC and inter site and intrasite enabled am i correct ?

You shouldn't need to manually replicate except in very rare situations. But yes, what you describe is the data path for hub-and-spoke.

Thank you for the info. The Exchnage team updates the CU from time to time and  they want to replicate manually.
i suggested them to run repadmin /syncall /adep instead if they want . Is there  any problem running that?

You mean /adeP (cap P), right? I don't see a problem with running it, I honestly don't know if it'll do what you want, I don't have your particular type of environment in my lab, but you should be easily able to test that.