Back to: Storage Area Network (SAN) Storage
In a Fibre Channel Arbitrated Loop (FC-AL) topology, devices are arranged in a ring-like structure, where each node transmits data to the next node in the sequence.
An FC-AL network can be established using a Fibre Channel hub, enabling device connectivity within the loop.
Since all devices share the available bandwidth, communication is limited to one pair of devices at a time, which can impact overall system efficiency. Although FC-AL supports up to 127 devices, practical implementations typically restrict the number to 10–15 to maintain performance.
Each device in an FC-AL topology is assigned a unique Arbitrated Loop Physical Address (AL_PA) for identification and communication.
🔹 Key Features of Attributed Loop SAN Topology
- Hybrid Architecture
- A mix of loop devices (NL_Ports) and switched fabric devices (F_Ports).
- Nodes communicate via Fibre Channel switches, reducing some limitations of pure FC-AL.
- NL_Ports (Node Loop Ports) & FL_Ports (Fabric Loop Ports)
- NL_Ports: Devices operating in loop mode (e.g., JBODs, Tape Libraries).
- FL_Ports: Switch ports connecting loops to the fabric.
- Improved Performance Over Traditional FC-AL
- Traditional FC-AL is a shared bandwidth topology where devices must arbitrate for access.
- Attributed Loop topology allows better integration with FC switches, improving bandwidth utilization.
- Loop Initialization Protocol (LIP) Management
- LIP occurs when devices enter/leave the loop, potentially causing disruptions.
- Attributed Loop topologies reduce disruptions by limiting loop resets to only the local segment.
- Scalability
- Unlike pure FC-AL, Attributed Loop allows integration with multiple switches, making it more scalable.
- Bridges the gap between low-cost FC-AL storage solutions and high-performance switched SANs.
🔹 Comparison with Other Topologies
| Feature | FC-AL | Attributed Loop | Switched Fabric |
|---|---|---|---|
| Device Connectivity | Loop-based | Loop + Fabric | Fully Switched |
| Performance | Shared bandwidth | Improved with switch connectivity | Dedicated bandwidth |
| Scalability | Limited (126 devices max) | Higher than FC-AL | Very high |
| Disruptions (LIP) | Affects the entire loop | Limited to local segment | Minimal |
🔹 Use Cases
- Cost-effective SAN solutions with moderate performance needs.
- Legacy storage systems that still use FC-AL but require better integration with modern FC fabrics.
- Tape libraries, JBODs, or disk enclosures that traditionally use FC-AL but benefit from fabric interconnects.