HP Cisco MDS 9124 Cisco MDS 9000 Family Fabric Manager Configuration Guide, Re - Page 427

Chapter 22 Configuring Generation 2 Switches and Modules Buffer Credit Allocation Send documentation comments to [email protected] Ports configured in dedicated mode are allocated the required bandwidth to sustain a line rate of traffic at the maximum configured operating speed, and ports configured in shared mode share the available remaining bandwidth within the port group. Fair allocation of bandwidth among a group of ports is determined, in part, by the rate mode and speed configurations. For example, if the set ports in a module are configured with the same rate mode and speed (such as 4 Gbps of shared bandwidth), then all the ports should have fair allocation of bandwidth and eventually, similar throughput. When you enable bandwidth fairness, you should notice a reduction in any disparity that may otherwise exist in similar configurations. Bandwidth allocation among the shared mode ports is based on the operational speed of the ports. For example, if four ports operating at speeds 1 Gbps, 1 Gbps, 2 Gbps, and 4 Gbps share bandwidth of 8 Gbps, the ratio of allocation would be 1:1:2:4. Note If dedicated ports are not using all of their allocated bandwidth, the unused bandwidth is made available for use by all ports configured for shared bandwidth mode. Tip When migrating a host that supports up to 2-Gbps traffic (that is, not 4-Gbps with autosensing capabilities) to the 4-Gbps switching modules, use autosensing with a maximum bandwidth of 2-Gbps. Note If you configure an interface for autosensing speed with a maximum bandwidth of 2 Gbps and want to change to the default of 4 Gbps, ensure that there are enough shared resources available to support the configuration on the module. Out-of-Service Interfaces On supported modules and fabric switches, you might need to allocate all the shared resources for one or more interfaces to another interface in the port group or module. You can take interfaces out of service to release shared resources that are needed for dedicated bandwidth. When an interface is taken out of service, all shared resources are released and made available to the other interface in the port group or module. These shared resources include bandwidth, rate mode, BB_credits, and extended BB_credits. All shared resource configurations are returned to their default values when the interface is brought back into service. Corresponding resources must be made available in order for the port to be successfully returned to service. Caution If you need to bring an interface back into service, you might disrupt traffic if you need to release shared resources from other interfaces in the same port group. Buffer Credit Allocation This sections describe how buffer credits are allocated to switches and modules, and includes the following topics: • Buffer Pools • BB_Credit Buffers for Switching Modules, page 22-9 • BB_Credit Buffers for Fabric Switches, page 22-14 OL-16184-01, Cisco MDS SAN-OS Release 3.x Cisco MDS 9000 Family CLI Configuration Guide 22-7