Brocade Network Advisor SAN User Manual v11.1x (53-1002167-01, May 2011)

Document Title 637
xx-xxxxxxx-01
Adaptive Rate Limiting
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• Circuits 0 and 1 are created with a metric of 0. Circuit 0 is created with a maximum
transmission rate of 1 Gbps, and Circuit 1 is created with a maximum transmission rate of 500
Mbps. Together, Circuits 0 and 1 provide an available bandwidth of 1.5 Gbps.
• Circuits 2 and 3 are created with a metric of 1. Both are created with a maximum transmission
rate of 1 Gbps, for a total of 2 Gbps. This bandwidth is held in reserve.
• If either circuit 0 or circuit 1 fails, traffic flows over the remaining circuit while the failed circuit
is being recovered. The available bandwidth is still considered to be 1.5 Gbps.
• If both circuit 0 and circuit 1 fail, there is a failover to circuits 2 and 3, and the available
bandwidth is updated as 2 Gbps.
• If a low metric circuit becomes available again, the high metric circuits go back to standby
status, and the available bandwidth is updated again. For example, if circuit 0 is recovered, the
available bandwidth is updated as 1 Gbps. If circuit 1 is also recovered, the available
bandwidth is updated as 1.5 Gbps.
Adaptive Rate Limiting
Adaptive Rate Limiting (ARL) is performed on FCIP tunnel connections to change the rate in which
the FCIP tunnel transmits data through the TCP connections. This feature is available only on the 8
Gbps extension switches and 8 Gbps Extension Blades. ARL uses information from the TCP
connections to determine and adjust the rate limit for the FCIP tunnel dynamically. This allows FCIP
connections to utilize the maximum available bandwidth while providing a minimum bandwidth
guarantee.
ARL applies a minimum and maximum traffic rate, and allows the traffic demand and WAN
connection quality to dynamically determine the rate. As traffic increases, the rate grows towards
the maximum rate, and if traffic subsides, the rate reduces towards the minimum. If traffic is
flowing error-free over the WAN, the rate grows towards the maximum rate. If TCP reports an
increase in retransmissions, the rate reduces towards the minimum.
FSPF link cost calculation when ARL is used
Fabric Shortest Path First (FSPF) is a link state path selection protocol that directs traffic along the
shortest path between the source and destination based upon the link cost. When ARL is used, The
link cost is equal to the sum of maximum traffic rates of all established, currently active low metric
circuits in the tunnel. The following formulas are used:
• If the bandwidth is greater than or equal to 2 Gbps, the link cost is 500.
• If the bandwidth is less than 2 Gbps, but greater than or equal to 1 Gbps, the link cost is
1000000 divided by the bandwidth.
• If the bandwidth is less than 1 Gbps, the link cost is 2000 minus the bandwidth
QoS SID/DID priorities over an FCIP trunk
QoS SID/DID traffic prioritization is a capability of Fabric OS Adaptive Networking licensed feature.
This feature allows you to prioritize FC traffic flows between hosts and targets.
Four internal TCP connections provide internal circuits for managing QoS SID/DID priorities over an
FCIP tunnel, as illustrated in Figure 285. The priorities are as follows: