When you'll see this error
You're updating the Active Directory schema—maybe adding a new Exchange server or a third-party application that extends the schema—and the update fails with ERROR_DS_LINK_ID_NOT_AVAILABLE (0X00002181). The exact message reads: "Schema update failed: There are no link identifiers available." This usually happens on Windows Server 2016 or 2019 domain controllers that have been running for years, especially in large environments with tons of custom schema extensions. The trigger is almost always an LDIFDE import or an ADPrep command that tries to create a new linked attribute—like a forward link or back link—but the forest has already used up all 32767 available link IDs.
What's actually going on
Active Directory stores linked attributes (like manager-directReports) using a pair of numbers called link IDs. Each linked attribute gets a forward link ID and a back link ID. The system has a hard limit: 32767 total link IDs. That's not a lot when you consider Exchange, Skype for Business, and third-party apps each consume dozens. When the counter hits 32767, any new schema update that tries to create a linked attribute will fire 0X00002181. You haven't broken anything—you've just hit a ceiling Microsoft never expected anyone to hit outside of test labs. The good news? You don't need to rebuild the forest.
The fix: reclaiming unused link IDs
Skip any suggestions about increasing the limit—there isn't one. The real fix is identifying and removing orphaned link IDs from defunct schema objects. Here's the process I've used successfully on multiple forests.
Step 1: Find the current link ID count
Log into a domain controller with Schema Admin credentials. Open an elevated PowerShell prompt and run:
$schema = [ADSI]"LDAP://cn=schema,cn=configuration,dc=yourdomain,dc=com"
$schema.Get("linkIDNextAvailable")If the value is 32767 or higher, you're out. If it's close, you're about to hit the wall. Write this number down.
Step 2: Find orphaned link IDs
Orphaned IDs come from schema objects that were created then deleted, but the link IDs weren't reclaimed. Run this to list all link IDs in use:
$searcher = [ADSISearcher]"(&(objectCategory=attributeSchema)(linkID=*))"
$searcher.SearchRoot = [ADSI]"LDAP://cn=schema,cn=configuration,dc=yourdomain,dc=com"
$searcher.FindAll() | ForEach-Object { $_.Properties }Look for attributes with isDefunct: TRUE that still have a linkID assigned. These are your orphans. Common culprits include old Exchange versions, retired Lync/Skype schemas, and abandoned application extensions.
Step 3: Manually reclaim the orphans
For each defunct attribute with a link ID, you need to set the linkID attribute to an empty or null value. Use ADSI Edit to do this cleanly:
- Open ADSI Edit and connect to the Schema partition.
- Navigate to
CN=Schema,CN=Configuration,DC=yourdomain,DC=com. - Find the defunct attribute (for example,
CN=ms-Exch-OldAttribute). - Right-click, select Properties, and locate
linkID. - Clear the value and click OK.
You can also do this via PowerShell if you're comfortable with direct LDAP writes:
$defunctAttr = [ADSI]"LDAP://cn=OldAttribute,cn=schema,cn=configuration,dc=yourdomain,dc=com"
$defunctAttr.PutEx([System.DirectoryServices.PropertyAccess]::Set, "linkID", @())
$defunctAttr.SetInfo()Step 4: Reset the next available link ID
After clearing the orphans, you need to tell AD the IDs are free. Run:
$schema.Put("linkIDNextAvailable", $newCount) # $newCount is the highest used ID + 1
$schema.SetInfo()If you cleared IDs 100-150, set it to 100. This forces AD to re-use those slots.
Step 5: Re-run the schema update
Now try your LDIFDE import or schema extension again. It should complete without error. If you still get 0X00002181, you missed some orphans—go back to Step 2 and double-check all attributes with isDefunct: TRUE.
If it still fails
Three things to check. First, verify no other schema extensions are running simultaneously—they'll compete for IDs. Second, confirm your new attribute actually needs a link ID. Not all attributes do. If you're just adding a single-value string property, skip the linked attribute flag. Third, look at the forest functional level—Windows Server 2008 R2 and older have a lower ID pool. Upgrade to at least 2012 R2 if you can.
One last thing: backup the schema partition before you start. I've seen admins accidentally clear the wrong linkID and break Exchange. Take a system state backup or export the schema using ldifde -f schema.ldf. You'll thank yourself later.