> For the complete documentation index, see [llms.txt](https://help.security.gallagher.com/knowledge-hub/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.security.gallagher.com/knowledge-hub/products/cc-hardware/credentials/credential-information/credentials-ssk-migration-and-the-2-mo.md).

# Site Specific Key migration modes

Understand all-reader and selected-reader modes for Site Specific Key migrations.

When you perform a Site Specific Key migration, or pause one, there are two distinct migration modes. The difference looks subtle, but each mode serves a different purpose and each has implications for whether cards keep working across the site.

### Migration with all readers

All readers take part in the migration. Any supported card presented to any reader has its keys updated. When the migration is paused, cards with either the old or the new keys continue to work across all readers.

### Migration with selected readers

Only the selected readers update keys on cards. Non-selected readers **enforce the new keys**, so cards that have not yet been migrated are **denied access**. This enforcement applies both during an active migration and while the migration is paused.

![SSK migration mode selection dialog](https://328339335-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F6bM8WhlBE9BYizPWJaPN%2Fuploads%2Fgit-blob-679ee4e9fe69eddbc13e0306ae09e80e41bdd3a2%2FSECHUB-CREDENTIALS-SSK-01-ssk-migration-mode-selection.png?alt=media)

### When to use selected readers

Migration with selected readers is typically used to enforce migration at a designated choke point, such as a high security entry or exit. This ensures that all cardholders must pass through a specific reader to have their card updated.

It also gives the site a way to address security concerns. A compromised or non-compliant card is denied access until it passes through the required migration point.

### Pause and stop a migration

| Action    | Effect                                                                                                              |
| --------- | ------------------------------------------------------------------------------------------------------------------- |
| **Pause** | The original site keys stop working. Only the new keys continue to work. You can resume the migration at any point. |
| **Stop**  | The original site keys stop working. Only the new keys continue to work. You can restart the migration at any time. |

***

🔗 **Related**

* [Credentials - Site Specific Key (SSK) vs Default Site Key](/knowledge-hub/products/cc-hardware/credentials/credential-information/credentials-site-specific-key-ssk-vs-d.md). Which card types are affected by a key migration, and what happens to MIFARE Plus cards.
* [Credentials - Rolling keys on cards issued to a new site with a SSK](/knowledge-hub/products/cc-hardware/credentials/credential-information/credentials-rolling-keys-on-cards-issu.md). Running a migration to move cards from Default keys to the SSK.


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