## 3.6.1 Physical topology 

The switch configuration in a SAN Volume Controller fabric must comply with the switch manufacturer’s configuration rules, which can impose restrictions on the switch configuration. For example, a switch manufacturer might limit the number of supported switches in a SAN. Operation outside of the switch manufacturer’s rules is not supported. 

The hardware compatible with V8.2 supports 8 Gbps and 16 Gbps FC fabric, depending on the hardware platform and on the switch to which the SAN Volume Controller is connected. In an environment where you have a fabric with multiple-speed switches, the preferred practice is to connect the SAN Volume Controller and back-end storage systems to the switch operating at the highest speed. 

You can use the lsfabric  command to generate a report that displays the connectivity between nodes and other controllers and hosts. This report is helpful for diagnosing SAN problems. 

SAN Volume Controller nodes are always deployed in pairs (I/O Groups). An odd number of nodes in a cluster is a valid standard configuration only if one of the nodes is configured as a hot spare. However, if there is no hot spare node and a node fails or is removed from the configuration, the remaining node operates in a degraded mode, but the configuration is still valid. 

If possible, avoid communication between nodes that route across ISLs. Connect all nodes to the same Fibre Channel or FCF switches. 

No ISL hops are permitted among the nodes within the same I/O group, except in a stretched system configuration with ISLs. 

For more information, search for Stretched system configuration details  in IBM Knowledge Center: 

https://www.ibm.com/support/knowledgecenter/ 

However, no more than three ISL hops are permitted among nodes that are in the same system but in different I/O groups. If your configuration requires more than three ISL hops for nodes that are in the same system but in different I/O groups, contact your support center. 

Avoid ISL on the path between nodes and back-end storage. If possible, connect all storage systems to the same Fibre Channel or FCF switches as the nodes. One ISL hop between the nodes and the storage systems is permitted. If your configuration requires more than one ISL hop, contact your support center. 

In larger configurations, it is common to have ISLs between host systems and the nodes. 

To verify the supported connection speed for FC links to the SAN Volume Controller, use the IBM System Storage Interoperation Center (SSIC) site: 

https://www.ibm.com/systems/support/storage/ssic/interoperability.wss 

In an Enhanced Stretched Cluster or HyperSwap setup, the two nodes forming an I/O Group can be colocated (within the same set of racks), or can be placed in separate racks, separate rooms, or both. For more information, see IBM System Storage SAN Volume Controller and Storwize V7000 Best Practices and Performance Guidelines , SG24-7521. 

