I'm trying to configure a Cisco Nexus 5548 for 802.1x using MAC Authentication Bypass.  I'm finding that there isn't an option to configure the guest VLAN in the event that authentication fails.  According to [this][1] documentation it should be an available feature.  But as you can see below it's not an available option.

    Switch(config-if)# dot1x ?
      default            Configure Dot1x with default values for this port
      host-mode          Role of 802.1x authenticating entity
      mac-auth-bypass    Configure Mac-Auth-Bypass
      max-reauth-req     Maximum Re-authentication Attempts Before Failing
      max-req            Maximum Retries to Initiate Authentication
      pae                Role of 802.1x authenticating entity
      port-control       Port control
      re-authentication  Enable or Disable Reauthentication for this port
      timeout            Various Timeouts

I've enabled feature dot1x.  Configured my radius server and preshared key.  I've added the needed aaa statement as well.

I can even run: "test aaa server radius x.x.x.x testuser password" and the user account authenticates successfully.  It's just the guest-vlan feature that I can't get working.

I noticed in the linked documentation that is says: 

> If MAB is configured, and if there is an authentication failure due to MAB, then the guest VLAN (if available), will be assigned as access VLAN.

What does it mean by (if available)?  Do I need to define a VLAN and then establish it as a Guest VLAN with some sort of parameter?

I should emphasize that this is on a Nexus 5548 and not a Nexus 9000 series switch.  I've read that Nexus 9Ks don't support the guest-vlan feature.
  

[1]: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5500/sw/security/6x/b_5500_Security_Config_6x/b_5500_Security_Config_602N12_chapter_0111.html#task_C51F75F1B3EE498F8EE0251A6660BDC3