Short version: typically you don't have to pay attention to this stat. As long as number of "fails" is zero, AD replication is healthy.
Note: When you have replication fails and subsequently remedy the problem, the number of "fails" in "replsummary" report is not going to change to zeros right away. The report of this command is a snapshot of history, so it takes a bit time for all fails to disappear.
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Repadmin /replsummary result is simple, but yet somewhat confusing. A few notes:
- If you don't specify /bysrc or /bydest, it will list status for both directions. You want to pay attention to Destination DSA as AD replication is pull-based.
- Most critical column is "fails". If there is no fails, obviously you don't have much to worry about
- Most confusing column is "largest delta". It's common misunderstanding (on the Net at least) that value in this column should be less then 1hr. However, depending on how large your AD environment is, and how frequent changes happen in a particular Naming Context, value in this column could be very large (days)
- Microsoft's official interpretation for "largest delta": longest replication gap amongst all replication links for a particular DC", which is not really helpful. I personally had hard time to understand this interpretation itself.
- This value is for the particular DC, among all its replication partners, the longest time that it hasn't replicated anything against whatever NC. This value has to be read together with /showrepl command against that DC.