Site Replication Service is always a confusing topic for some in both E2K and E2K3. Sometimes our customers see some Exchange 200x have SRS turned on in one mixed site by setup, while other Exchange 200x servers in the same site does not have SRS, therefore they want to get clear in what situation the SRS get enabled on the server by setup.
 
And here is the answer, SRS are enabled during the setup under following three scenarios:
 
A. Install E2K or E2K3 server into a Pure 5.5 site. The first E2K/E2K3 server will get its SRS enabled.
B. The first in-place upgraded E2K server in a Pure 55 site get its SRS enabled. And later if this E2K server upgrades to E2K3 server, the SRS will keep enabled on the E2K3 server.
C. If a 5.5 server is a bridgehead server for any kind of connectors (SC, X400, DRC...) in the site (either pure 5.5 site or mixed site), and this 5.5 server is upgraded into E2K, then the SRS is also enabled on this E2K.
 
Therefore, in a scenario like this, you might get multiple SRS servers in the site:
 
1. You have a 4 5.5 servers 5.5 site, with names of 55server1 to 55server4. Among them, 55server1 and 55server2 are bridgehead servers for an X400 and a Site Connector with other 5.5 sites.
2. Install an E2K3 server TI1 into the site, the TI1 server get its SRS on because it’s the first E2K/E2K3 server in the site.
3. Install another E2K3 server TI2 into the site. This time the TI2 won’t get SRS enabled, because it’s not the first E2K/E2K3 server in the site.
4. You upgrade 55server1 to E2K server, let call it PT1 now. PT1 server get its SRS enabled, because it’s the bridgehead server of a connector. 5. You upgrade the 55server3 to E2K server, this time no SRS is enabled.

I also wanted to answer a few questions from previous post on SRS that seems like did not get an answer before:

Q: How does one force SKCC to run on demand?
A: An easy way to force SKCC to run on demand, is to right click the configCA, and choose ‘replicate now’. By forcing the configCA to replicate, this will trigger the SKCC to act.

Q: Does it mean we cannot remove SRS until there are no more 5.5 in the whole org, or just no 5.5 in that AG?
A: Even though there are no more 55 in that AG, the configCA/SRS may still be responsible for replicate other 55 sites to the AD, if there is a connector and DRC configured between this AG and other 55 sites. In that case, you still can not remove SRS even though there are no more 5.5 in that AG.

Hope this post can get you cleared on why some E2K/E2K3 servers in your site have SRS enabled by setup, while some are not.

- Stanley Yuan