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Figure 5 Combination full-mesh core with triangular links to all sites
1. Full mesh core network
Using multiple subnets
IMPORTANT: Best practice is to use a single subnet in most cases.
You can use multiple subnets with a Multi-Site SAN. Multiple subnets let you have multiple VIPs,
one VIP per subnet. The advantage of using multiple VIPs is that you can connect to a volume
without having to leave the local subnet.
Alternatively, you can assign an application server to a site. Application servers assigned to a site
connect only to storage systems in the same site. For more information, see “Best practices for
implementing the Multi-Site SAN network” (page 5).
If you use multiple subnets within the iSCSI cluster and implement one subnet per site, you can take
advantage of the following Multi-Site SAN features:
• One Virtual IP address per site. iSCSI discovery sessions are terminated on a storage system
that is local to the application server in that site.
• Virtual IP addresses are hosted only on storage systems that reside in the same subnet as the
VIP.
• Read and Write I/O requests are forwarded from the application server to a storage system
that is in the same site as the server. This ensures that the I/O path is optimal and multiple
network hops across the network link between the sites are eliminated.
Figure 6 (page 10) shows an example of how the Multi-Site SAN setup maps to subnets, VIPs, and
application servers.
Designing the network for the Multi-Site SAN 9