> # How CA Certificates Are Used by IPsec Devices

> When two IPsec switches want to exchange IPsec-protected traffic passing between them, they must first authenticate each other—otherwise, IPsec protection cannot occur. The authentication is done with IKE.

> IKE can use two methods to authenticate the switches, using preshared keys without a CA and using RSA key-pairs with a CA. Both methods require that keys must be preconfigured between the two switches.

> Without a CA, a switch authenticates itself to the remote switch using either RSA-encrypted preshared keys.

> With a CA, a switch authenticates itself to the remote switch by sending a certificate to the remote switch and performing some public key cryptography. Each switch must send its own unique certificate that was issued and validated by the CA. This process works because the certificate of each switch encapsulates the public key of the switch, each certificate is authenticated by the CA, and all participating switches recognize the CA as an authenticating authority. This scheme is called IKE with an RSA signature.

> Your switch can continue sending its own certificate for multiple IPsec sessions, and to multiple IPsec peers until the certificate expires. When the certificate expires, the switch administrator must obtain a new one from the CA.

> CAs can also revoke certificates for devices that will no longer participate in IPsec. Revoked certificates are not recognized as valid by other IPsec devices. Revoked certificates are listed in a certificate revocation list (CRL), which each peer may check before accepting a certificate from another peer.

> Certificate support for IKE has the following considerations:

> - The switch FQDN (host name and domain name) must be configured before installing certificates for IKE.
> - Only those certificates that are configured for IKE or general usage are used by IKE.
> - The first IKE or general usage certificate configured on the switch is used as the default certificate by IKE.
> - The default certificate is for all IKE peers unless the peer specifies another certificate.
> - If the peer asks for a certificate which is signed by a CA that it trusts, then IKE uses that certificate, if it exists on the switch, even if it is not the default certificate.
> - If the default certificate is deleted, the next IKE or general usage certificate, if any exists, is used by IKE as the default certificate.
> - Certificate chaining is not supported by IKE.
> - IKE only sends the identity certificate, not the entire CA chain. For the certificate to be verified on the peer, the same CA chain must also exist there.

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