An application keeps the configuration synchronized in a fabric through CFS. Two such fabrics might merge as a result of an ISL coming up between them. These two fabrics could have two different sets of configuration information that need to be reconciled in the event of a merge. CFS provides notification each time an application peer comes online. If a fabric with M application peers merges with another fabric with N application peers, and if an application triggers a merge action on every such notification, a link-up event results in M*N merges in the fabric.
CFS supports a protocol that reduces the number of merges required to one by handling the complexity of the merge at the CFS layer. This protocol runs per application per scope. The protocol involves selecting one switch in a fabric as the merge manager for that fabric. The other switches do not play any role in the merge process.
During a merge, the merge manager in the two fabrics exchange their configuration databases with each other. The application on one of them merges the information, decides if the merge is successful, and informs all switches in the combined fabric of the status of the merge.
In case of a successful merge, the merged database is distributed to all switches in the combined fabric, and the entire new fabric remains in a consistent state.
CFS Regions
A CFS region is a user-defined subset of switches for a given feature or application in its physical distribution scope. When a SAN is spanned across a vast geography, you may need to localize or restrict the distribution of certain profiles among a set of switches based on their physical proximity. CFS regions allow you to configure multiple islands of distribution within the fabric, for a given CFS feature or application. CFS regions are designed to restrict the distribution of a feature’s configuration to a specific set or grouping of switches in a fabric.
Note
You can only configure a CFS region on physical switches in a SAN. You cannot configure a CFS region in a VSAN.
CFS regions are identified by numbers ranging from 0 through 200. Region 0 is reserved as the default region, and it contains every switch in the fabric. You can configure regions from 1 through 200. If the feature is moved—that is, assigned to a new region—its scope is restricted to that region; it ignores all other regions for distribution or merging purposes. The assignment of the region to a feature has precedence in distribution over its initial physical scope. You can configure a CFS region to distribute configurations for multiple features. However, on a given switch, you can configure only one CFS region at a time to distribute the configuration for a given feature. Once you assign a feature to a CFS region, its configuration cannot be distributed within another CFS region.
Let’s discuss a scenario where CFS regions will be useful. Call Home is an application that triggers alerts to network administrators when a situation arises or something abnormal occurs. When the fabric covers many geographies and with multiple network administrators who are each responsible for a subset of switches in the fabric, the Call Home application sends alerts to all network administrators regardless of their location. For the Call Home application to send message alerts selectively to network administrators, the physical scope of the application has to be fine-tuned or narrowed down, which is achieved by implementing CFS regions.