I was working at a customer site the other day, and they were looking over my shoulder as I was typing in some Repadmin commands. They asked me the question what is your recommended top 5 Repadmin (plus friends) commands. Well that got me thinking and here is my top 5 list. BTW the numbering does not indicate whether one is more important than another, they are just the ones I seem to use the most.
The important thing to remember about AD replication, is that everything is pulled not pushed. So if there are problems with replication this normally shows itself on the replication partners that pull from a particular DC which is uncontactable for some reason.
1. Repadmin /replsummary - (This is a great command it gives you a snapshot of the state of your active directory replication and how long it is since a partition has replicated). You should not expect to see longer than 1 hour since last replication if everything is healthy. The number of dots at the beginning of this command -3 denotes a DC. This lists up to 50 DCs per line, so two lines is 97 !
2. Repadmin /showrepl /repsto - This command displays the replication partners for each naming context that is held on the specified DC. This enables you to build a picture of the replication topology.
3. Repadmin /queue - This command run against a particular DC determines if its replication queue has any entries. If it has it may indicated it is overloaded.
4. Repadmin /viewlist * - this lists all the DCs in the Forest. This is limited at this point to output upto 999 DCs
5. Netdom /query FSMO - This is a nice quick command to display which server(s) currently hold the Forest and Domain Wide FSMO roles.
This list is by no means exhaustive and there are a numerous switches and other commands to also be aware of. A good place to start is Active Directory Operations Troubleshooting Guide, which puts the commands in context.