We have problem where 2 DC's will replicate and communicate fine with other DC's but not with each other. Nothing has changed on either server since this problem appeared, and in total across all sites there are about 25 DC's.
I will try and explain further, but will only talk about 2 sites.
Site A has 2 DC's which are as follows: dca.blah.local and also dca2.sub.blah.local (second dc in a child domain)
Site B has 2 DC's which are as follows: dcb.blah.local and also dcb2.sub.blah.local (again second dc in child domain)
dca2 can communicate and replicate info with dca and dcb and all other dc's, but not dcb2. dcb2 can also replicate and communicate fine with dca and dcb and all other dc's, but will not replicate with dca2.
On the DNS side of things we are able to ping dca2 from dcb2 and vice versa. When running replmon.exe on either dca2 or dcb2 we can connect to other dc's but not each other. When trying to connect using replmon from dca2 to dcb2 or the other way around we get the error RPC Server Unavailable, yet we can connect from either of these to other dc's fine.
It is possible to telnet to port 135 from dca2 or dcb2 to any other dc, but not each other.
dca2 is running Windows 2003 Server SP2 and dcb2 is running Windows 2003 Server SP1 - this is how these servers have been for a long time, and no patches have been applied around when this problem started happening.
Any help would be appreciated.