• Special Enhanced ISL (EISL) header is attached to every frame by ingress port:
– EISL header contains 12-bit VSAN ID
• Header is stripped off at the edge of the VSAN-aware part of the fabric:
– Other vendors’ switches do not see the EISL header – VSAN tagging is transparent to end devices
SOF
Fibre Channel Frame with EISL Header
EISL
Header
(8 bytes)
FC Header EISL Header
R_CTL
(8 bits)
Version
(2)
Fr. Type
(2)
User Prio
(3)
VSAN-id
(12)
MPLS
(1)
Rsvd
(4)
TTL
(8)
Rsvd
(24)
Payload CRC EOF
To recognize and isolate the traffic traveling in a VSAN, each frame must be uniquely labeled with the VSAN ID of the VSAN to which it belongs. The labeling of frames is called tagging. Inside MDS switches, each frame is tagged with a VSAN ID that is stored in a special header called the Extended Inter-Switch Link (EISL) header. The EISL header is attached to the frame by the ingress port.
The EISL header is stripped off at the other edge of the VSAN-aware fabric. The edge of the VSAN-aware fabric is any port that does not support EISL headers. Currently, only Cisco switches support EISL headers. Frames passed between MDS switches and end devices do not carry the EISL header, and neither do frames passing between MDS switches and non-MDS switches. EISL headers are added and stripped off at the ingress and egress TE_Ports.
Frames are tagged without the explicit involvement of the end device. VSAN tagging is transparent to end devices.