.Part 1 Architecture HP SAN Design Reference Guide 785351-001

Table 1 Core-edge fabric topology types
DescriptionTopology type
At least 50% of edge ports are dedicated as ISLs, resulting in an ISL ratio of 1:1.Fat tree
Less than 50% of edge ports are dedicated as ISLs, resulting in an ISL ratio of x:1, where
x is 2 or more.
Skinny tree
Recommended ISL ratios
The core-edge fabric type has a high fabric cross-sectional bandwidth (the maximum amount of
data that can pass through ISLs at the fabric midpoint, which is the central connection or core of
the fabric). The higher the ISL ratio, the lower the cross-sectional bandwidth and the more prone
a topology is to ISL oversubscription. Oversubscription occurs when traffic is blocked due to
insufficient ISL bandwidth.
NOTE: When determining the ideal ISL ratio, you must consider the speed of the server, storage,
and ISL ports.
The minimum ISL ratio for an implementation depends on several factors, including:
• Location of server and storage fabric connection
• Server and storage hardware performance
• Data access type (see “Topology data access” (page 32))
• Server application performance requirements
Table 2 (page 31) describes the recommended core-edge fabric ISL ratios.
Table 2 Recommended core-edge fabric ISL ratios
Recommended ratiosI/O workload
1:1 to 3:1
Higher I/O data intensive application requirements (> 70 MB/s at 2 Gb/s, > 140
MB/s at 4 Gb/s, > 280 MB/s at 8 Gb/s)
7:1 to 15:1
Lower I/O data intensive application requirements (< 70 MB/s at 2 Gb/s, < 140
MB/s at 4 Gb/s, < 280 MB/s at 8 Gb/s)
NOTE: HP recommends a ratio of 7:1 for typical distributed data access.
Numeric representation
Core-edge fabrics can also be represented in numeric terms, such as n1 x n2, where n1 represents
the number of core switches and n2 represents the number of edge switches.
For example, a 4 x 24 core-edge fabric indicates 4 core switches and 24 edge switches, for a
total of 28 switches.
Figure 9 (page 32) shows a 4 x 12 core-edge fabric with 4 core switches and 12 edge switches,
each connected to the core with 4 ISLs.
Core-edge fabric 31