> # CRL Downloading, Caching, and Checking Support

> Certificate revocation lists (CRLs) are maintained by CAs to give information of prematurely revoked certificates, and the CRLs are published in a repository. The download URL is made public and also specified in all issued certificates. A client verifying a peer’s certificate should obtain the latest CRL from the issuing CA and use it to determine if the certificate has been revoked. A client can cache the CRLs of some or all of its trusted CAs locally and use them later if necessary until the CRLs expire.

> Cisco MDS NX-OS allows the manual configuration of pre-downloaded of CRLs for the trust points, and then caches them in the switch bootflash (cert-store). During the verification of a peer certificate by IPsec or SSH, the issuing CA’s CRL is consulted only if the CRL has already been cached locally and the revocation checking is configured to use CRL. Otherwise, CRL checking is not performed and the certificate is considered to be not revoked if no other revocation checking methods are configured. This mode of CRL checking is called CRL optional.

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