HP Brocade 4Gb SAN Switch for HP BladeSystem p-Class - Manual (Page 114)

114 Administering ISL trunking
Fabric considerations
The ISL Trunking feature is provided with the Fabric OS and can be activated by entering a license key,
which is available from the switch supplier. When the ISL Trunking license is activated, trunking is
automatically implemented for any eligible ISLs.
A license must be activated on each switch that participates in trunking. For the Core Switch 2/64, a
single license key enables the feature on both logical switches.
To use ISL Trunking in the fabric, the fabric must be designed to allow trunking groups to form. To identify
the most useful trunking groups, evaluate the traffic patterns before designing/redesigning the fabric. This
also applies to the SAN Director 2/128 configured with two domains.
ISL Trunking can be used to simplify SAN design and improve performance. When designing the SAN,
consider the following recommendations in addition to the standard guidelines for SAN design:
• Evaluate the traffic patterns within the fabric.
• Place trunking-capable switches adjacent to each other.
This maximizes the number of trunking groups that can form. If you are using a core/edge topology,
place trunking-capable switches at the core of the fabric and any switches that are not
trunking-capable at the edge of the fabric.
• Activate an ISL Trunking license on each switch that is to participate in a trunking group.
• Cable lengths for participating links should differ by no more than 30 meters.
• When connecting two switches with two or more ISLs, ensure that all trunking requirements are met to
allow a trunking group to form.
• Determine the optimal number of trunking groups between each set of linked switches, depending on
traffic patterns and port availability.
The goal is to avoid traffic congestion without unnecessarily using ports that could be used to attach
other switches or devices. Consider these points:
• Each physical ISL uses two ports that could otherwise be used to attach node devices or other
switches.
• Trunking groups can be used to resolve ISL oversubscription if the total capability of the trunking
group is not exceeded.
• Consider how the addition of a new path affects existing traffic patterns:
• A trunking group has the same link cost as the master ISL of the group, regardless of the number of
ISLs in the group. This allows slave ISLs to be added or removed without causing data to be
rerouted, because the link cost remains constant.
• The addition of a path that is shorter than existing paths causes traffic to be rerouted through that
path.
• The addition of a path that is longer than existing paths may not be useful because the traffic
chooses the shorter paths first.
• Plan for future bandwidth addition to accommodate increased traffic.
For trunking groups over which traffic is likely to increase as business requirements grow, consider
leaving one or two ports in the group available for future nondisruptive addition of bandwidth.