Chapter 11. Advanced Copy Services 601
In Figure 11-155 on page 600, the amount of time in microseconds that is required to transmit 
a packet across network links of varying bandwidth capacity is compared. The following 
packet sizes are used:
/SM59000064 bytes: The size of the common ping packet
/SM5900001500 bytes: The size of the standard TCP/IP packet
/SM5900002148 bytes: The size of an FC frame
Finally, your path maximum transmission unit (MTU) affects the delay that is incurred to get a 
packet from one location to another location. An MTU might cause fragmentation or be too 
large and cause too many retransmits when a packet is lost.
11.11.2  1720 error
The 1720 error (event ID 050020) is the other problem remote copy might encounter. The 
amount of bandwidth that is needed for system-to-system communications varies based on 
the number of nodes. It is important that it is not zero. When a partner on either side stops 
communication, a 1720 is displayed in your error log. According to the product documentation, 
there are no likely field-replaceable unit breakages or other causes. 
The source of this error is most often a fabric problem or a problem in the network path 
between your partners. When you receive this error, check your fabric configuration for zoning 
of more than one host bus adapter (HBA) port for each node per I/O Group if your fabric has 
more than 64 HBA ports zoned. The suggested zoning configuration for fabrics is one port for 
each node per I/O Group per fabric that is associated with the host.
For those fabrics with 64 or more host ports, this suggestion becomes a rule. Therefore, you 
see four paths to each volume discovered on the host because each host needs to have at 
least two FC ports from separate HBA cards, each in a separate fabric. On each fabric, each 
host FC port is zoned to two IBM SAN Volume Controller node ports where each node port 
comes from a different IBM SAN Volume Controller node. This configuration provides four 
paths per volume. More than four paths per volume are supported but not recommended.
Improper zoning can lead to SAN congestion, which can inhibit remote link communication 
intermittently. Checking the zero buffer credit timer with IBM Spectrum Control and comparing 
against your sample interval reveals potential SAN congestion. If a zero buffer credit timer is 
more than 2% of the total time of the sample interval, it might cause problems. 
Next, always ask your network provider to check the status of the link. If the link is acceptable, 
watch for repeats of this error. It is possible in a normal and functional network setup to have 
occasional 1720 errors, but multiple occurrences could indicate a larger problem. 
If you receive multiple 1720 errors, recheck your network connection and then check the 
system partnership information to verify its status and settings. Then, perform diagnostics for 
every piece of equipment in the path between the two IBM Storwize systems. It often helps to 
have a diagram that shows the path of your replication from both logical and physical 
configuration viewpoints.
Note: Unlike 1720 errors, 1920 errors are deliberately generated by the system because it 
evaluated that a relationship could impact the host’s response time. The system has no 
indication on if/when the relationship can be restarted. Therefore, the relationship cannot 
be restarted automatically and it needs to be done manually.