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Cisco MDS 9000 Family NX-OS Interfaces Configuration Guide
OL-29284-01, Release 6.x
Chapter 3 Configuring Fibre Channel Interfaces
Information About Fibre Channel Interfaces
Ports configured in dedicated rate 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. 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.
Unutilized bandwidth from the dedicated ports is shared among only the shared ports in a port group as
per the ratio of the configured operating speed. A port cannot be brought up unless the reserved
bandwidth is quaranteed for the shared ports (see
Table 3-9). For dedicated ports, configured bandwidth
is taken into consideration while calculating available bandwidth for the port group. This behavior can
be changed using bandwidth fairness by using the rate-mode bandwidth-fairness module number
command.
For example, consider a 48-port 8-Gbps module. This module has 6 ports per port group with 12.8 Gbps
bandwidth. Ports 3 to 6 are configured at 4 Gbps. If the first port is configured at 8 Gbps dedicated rate
mode, and the second port is configured at 4-Gbps dedicated rate mode, then no other ports can be
configured at 4 Gbps or 8 Gbps because the left over bandwidth of 0.8 Gbps (12.8-(8+4)) cannot meet
the required 0.96 Gbps for the remaining four ports. A minimum of 0.24 Gbps reserved bandwidth is
required for the for the rest of the four ports. However, if the two ports (for example, 5 and 6) are taken
out of service (which is not same as shutdown), required reserved bandwidth for the two ports (3 and 4)
is 0.48 and port 2 can be configured at 4 Gbps in dedicated rate mode. This behavior can be overridden
by the bandwidth fairness command in which case reserved bandwidth is not enforced. Once the port is
up, ports 3 and 4 can share the unutilized bandwidth from ports 1 and 2.
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 for the shared mode port, 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.
Oversubscription Ratio Restrictions
The 48-port and 24-port 4-Gbps, and all 8-Gbps Fibre Channel switching modules support
oversubscription on switches with shared rate mode configurations. By default, all 48-port and 24-port
4-Gbps, and 8-Gbps Fibre Channel switching modules have restrictions on oversubscription ratios
enabled. As of Cisco SAN-OS Release 3.1(1) and NX-OS Release 4.1(1), you can disable restrictions
on oversubscription ratios.
Table 3-9 describes the bandwidth allocation for oversubscribed interfaces configured in shared mode
on the 4-Gbps and 8-Gbps modules.
