Configuring Generation 2 Switches and Modules
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
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
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.
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.
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.
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.
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.
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
OL-16184-01, Cisco MDS SAN-OS Release 3.x
BB_Credit Buffers for Switching Modules, page 14-9
BB_Credit Buffers for Fabric Switches, page 14-14
Cisco MDS 9000 Family CLI Configuration Guide
Buffer Credit Allocation