Cisco MDS 9000 Series Configuration Manual page 244

Managing FLOGI, Name Server, FDMI, and RSCN Databases
VMID
• Physical Fabric Port (PF_Port)—A physical fabric port of a switch.
When an HBA Port Comes Up
After an HBA driver has logged a physical or virtual HBA port into the fabric, the driver may request Local
VE IDs from the fabric through the port. The Virtual Machine Identification Server (VMIS) on the locally
attached switch provides the range of Local VE IDs (up to 255) in the response. The driver then assigns the
identifiers to the port FCID.
When a VM Initially Accesses a Disk
Outside of a fabric, a VM is identified by a Global VE ID. Within a fabric, a VM is identified by a Fabric VE
ID. When a VM initially accesses a virtual disk, the hypervisor starts accessing the corresponding physical
disk through an HBA port. For each path to the physical disk an unused Local VE ID from the pool for the
FCID is assigned. The FCID and Local VE ID are combined to create a unique Fabric VE ID by the HBA
driver. The HBA driver then notifies the locally attached VMIS about the assigned VE ID to Global VE ID
mapping. This mapping is done for each VM path to the fabric through the hypervisor and allows all VM
traffic to be identified by SAN tools by path, such as Cisco MDS SAN Analytics.
When a VM Goes Down or Moves Within a Fabric
The difference between a Global VE ID and Local VE ID is seen when a VM migrates between VEMs. When
a VM is deinstantiated or migrated between VEMs, the Local VE ID is returned to the pool by the HBA driver,
but does not notify the switch VMIS. The switch will time out the Local VE ID or VM mapping after 1 to 4
hours of no traffic. If a VM reinstantiates on the same VEM, it may get a different Local VE ID from the pool
of the same FCID because the previously assigned Local VE ID may have been allocated to another VM when
this VM was down. When a VM migrates to a different VEM, the VM will likely utilize a different FCID and
likely be assigned a different Local VE ID from the FCID's pool. Thus, when a VM reboots or migrates
between VEMs, the Global VE ID remains constant but Local VE ID may change.
Figure 52: VMID Components, on page 223 show the components of VMID:
Cisco MDS 9000 Series Fabric Configuration Guide, Release 8.x
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