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Chapter 33
Configuring FCIP
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 .
Maximum Retransmissions
You can specify the maximum number of times a packet is retransmitted before TCP decides to close the
connection. To configure maximum retransmissions, follow these steps:
Command
Step 1
switch(config-profile)# tcp
max-retransmissions 6
switch(config-profile)# no tcp
max-retransmissions 6
Path MTUs
Path MTU (PMTU) is the minimum MTU on the IP network between the two endpoints of the FCIP link.
PMTU discovery is a mechanism by which TCP learns of the PMTU dynamically and adjusts the
maximum TCP segment accordingly (RFC 1191).
By default, PMTU discovery is enabled on all switches with a timeout of 3600 seconds. If TCP reduces
the size of the maximum segment because of PMTU change, the reset-timeout specifies the time after
which TCP tries the original MTU.
To configure PMTU, follow these steps:
Command
Step 1
switch(config-profile)# no tcp pmtu-enable
switch(config-profile)# tcp pmtu-enable
switch(config-profile)# tcp pmtu-enable
reset-timeout 90
switch(config-profile)# no tcp pmtu-enable
reset-timeout 600
Selective Acknowledgments
TCP may experience poor performance when multiple packets are lost within one window. With the
limited information available from cumulative acknowledgments, a TCP sender can only learn about a
single lost packet per round trip. A selective acknowledgment (SACK) mechanism helps overcome the
limitations of multiple lost packets during a TCP transmission.
The receiving TCP sends back SACK advertisements to the sender. The sender can then retransmit only
the missing data segments. By default, SACK is enabled on Cisco MDS 9000 Family switches.
To configure SACK, follow these steps:
Command
Step 1
switch(config-profile)# no tcp sack-enable
switch(config-profile)# tcp sack-enable
Window Management
The optimal TCP window size is automatically calculated using the maximum bandwidth parameter, the
minimum available bandwidth parameter, and the dynamically measured round trip time (RTT).
OL-6973-03, Cisco MDS SAN-OS Release 2.x
Purpose
Specifies the maximum number of retransmissions (6). The
range is from 1 to 8 retransmissions.
Reverts to the default of 4 retransmissions.
Purpose
Disables PMTU discovery.
Enables (default) PMTU discovery with the default
value of 3600 seconds.
Specifies the PMTU reset timeout to 90 seconds.
The range is 60 to 3600 seconds.
Leaves PMTU discovery enabled but reverts the
timeout to the default of 3600 seconds.
Purpose
Disables SACK.
Enables SACK (default).
Cisco MDS 9000 Family Configuration Guide
Advanced FCIP Profile Configuration
33-9
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