Domain IDs uniquely identify a switch in a VSAN. A switch may have different domain IDs in different VSANs. The domain ID is part of the overall FCID.
The configured domain ID can be preferred or static. When you assign a static domain ID type, you are requesting a particular domain ID. If the switch does not get the requested address, it will isolate itself from the fabric. When you specify a preferred domain ID, you are also requesting a particular domain ID; however, if the requested domain ID is unavailable, the switch will accept another domain ID. By default, the configured domain ID is 0 (zero) and the configured type is preferred. The 0 (zero) value can be configured only if the preferred option is used. If a domain ID is not configured, the local switch sends a random ID in its request. Static domain IDs are recommended.
When a subordinate switch requests a domain, the following process takes place:
The local switch sends a configured domain ID request to the principal switch.
The principal switch assigns the requested domain ID if available. Otherwise, it assigns another available domain ID.
The behavior for a subordinate switch changes based on three factors: the allowed domain ID lists, the configured domain ID, and the domain ID that the principal switch has assigned to the requesting switch. The valid range for an assigned domain ID list is from 1 to 239. You can specify a list of ranges to be in the allowed domain ID list and separate each range with a comma. The principal switch assigns domain IDs that are available in the locally configured allowed domain list.
When the received domain ID is not within the allowed list, the requested domain ID becomes the runtime domain ID, and all interfaces on that VSAN are isolated. When the assigned and requested domain IDs are the same, the preferred and static options are not relevant, and the assigned domain ID becomes the runtime domain ID.
When the assigned and requested domain IDs are different, the following cases apply:
If the configured type is static, the assigned domain ID is discarded, all local interfaces are isolated, and the local switch assigns itself the configured domain ID, which becomes the runtime domain ID.
If the configured type is preferred, the local switch accepts the domain ID assigned by the principal switch, and the assigned domain ID becomes the runtime domain ID.
Note
A static domain is specifically configured by the user and may be different from the runtime domain. If the domain IDs are different, the runtime domain ID changes to take on the static domain ID after the next restart, either disruptive or nondisruptive. If you perform a disruptive restart, reconfigure fabric (RCF) frames are sent to other switches in the fabric, and data traffic is disrupted on all the switches in the VSAN (including remotely segmented ISLs). If you perform a nondisruptive restart, build fabric (BF) frames are sent to other switches in the fabric, and data traffic is disrupted only on the switch.
If you change the configured domain ID, the change is accepted only if the new domain ID is included in all the allowed domain ID lists currently configured in the VSAN. Alternatively, you can also configure a zero-preferred domain ID.
Refer to Figure 8-16 for domain ID distribution phases. In the domain ID assigned (DIA) phase, the principal switch assigns itself a domain ID (1a) and floods the DIA frames with this information in the fabric. Each switch in the fabric that received the DIA frames from the principal switch requests a domain ID using a request domain identifier (RDI). The RDI request can contain a preferred (or static) domain ID depending on the configuration of the switch. In the next phase, the principal switch grants the requested domain ID or another from its domain list to the subordinate switch (SS). Lastly, the downstream switch accepts the domain ID (2b) if it is the same as the static ID or it belongs to its domain list. Finally, the fabric is formed. If the downstream switch doesn’t accept the domain ID, it becomes an isolated switch.
In Figure 8-17, the principal switch, switch 1, has a runtime domain ID of 3. The subordinate switch, switch 2, is configured with a domain ID of 7 as preferred. Switch 2 requests configured domain ID 7 from the principal switch. Because the assignment of domain ID 7 is not restricted by the Unallowed domain IDs list, the principal switch assigns domain ID 7 to switch 2. If domain ID 7 were in the Unallowed domain ID list, the principal switch would assign another domain ID, in this example 51, to the subordinate switch.