Subnetsarelocal's impact for HACMP
ITEM: RS4000003038
**************> QUESTION level 1 --> level 2 SPECIALIST <**************
Hello, I have a pressing ploblem about HACMP.
When using HACMP, it is recomended that network option
"subnetsarelocal"
is set to 0, but if I set the option to 1, does HACMP/6000
V3.1.1 work with no problem?
If it does not work correctly, what problems are expected for the
system?
My customer now uses the RS/6000 SPas a production
system with HACMP. In the daily job, the SP receives
several learge text files from a remote host by using ftp ascii
transfer mode.
Now, since to set subnetsarelocal to 0 by rc.cluster degrades file
transer performance, sometimes the remote host sends packets
containing only a byte of the text file. If the byte is just 0x0d,
ftp removes it and try to write null data to the destination file,
so file transfer stops with fail.
This is a bug of the AIX V3.2 ftp, but no ptf is available.
The SP system must avoid this problem immediately. And to set
subnetsarelocal to 1 gives good results for the system. But I
don't have a chance to test the HACMP, because it is not allowed
to stop the production SP.
So, I'd like to have your good advice. I appriciate your spending
time answering my question, and I look forward to reply from you
soon.
**************> ANSWER level 2 --> level 1 SPECIALIST <**************
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PMR E0710,998,760 was created on 96/10/30 at 13:45:06.
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Received by Austin ITSC and assigned to AIXOPSYS.
Your question has been received and assigned to a specialist. Please
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**************> QUESTION level 1 --> level 2 SPECIALIST <**************
====> THIS TEXT HAS BEEN ENTERED BY IBM IN JAPAN
Hello specialist.
I'd like to make sure the reason why rc.cluster changes subnetsarelocal
from 1 to 0, though AIX's default value is 1.
And is HACMP officially supported running with "subnetsarelocal=1" ?
My customer is very worried about this ploblem, so please mentionme
the answer of these two questions clearly.
Best regards.
**************> ANSWER level 2 --> level 1 SPECIALIST <**************
====> THIS TEXT HAS BEEN ENTERED BY IBM IN USA
PMR E0710,998,760 was updated on 96/10/31 at 12:53:59.
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====> RESP: AIXOPSYS AT WTSCPOK ================= DATE:961030
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Response:
There should not be any complications with setting the subnetsarelocal
parameter equal to "1".
However, if it is desired to stay within the requirements of HACMP,
subnetsarelocal can be left equal to 0, and the "tcp_mssdflt" parameter
can be increased.
When subnetsarelocal is set equal to 0, the "tcp_mssdflt" is used
as the packet size. Increasing this parameter will increase the
packet sized used in the connections, and will avoid the AIX v3.2
ftp problem.
To set "tcp_mssdflt", use the "no" command as follows:
no -o tcp_mssdlft=#
Where # is equal to the number of bytes in each packet sent. Typical
values are higher than the AIX default of 512. For example, if the
connection used was ethernet, "tcp_mssdflt" would be set to 1500.
Thank you for using AIX Technical Support.
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====> ASGN: BUNLOEUR AT WTSCPOK ================= DATE:961031
TIME:1336
Received by Austin ITSC and assigned to AIXOPSYS.
Your question has been received, and assigned to a specialist. Please
wait for a reply. Thank you.
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====> RESP: AIXOPSYS AT WTSCPOK ================= DATE:961104
TIME:1517
Response to Question 1.
The subnetsarelocal network option is set by HACMP through the
rc.cluster script and it is set to 0 to prevent packets from getting
routed outside the local area network when using a subnet on that
local area network.
Here is the excerpt from the rc.cluster script:
####################################################################
# Set Network Option subnetsarelocal to 0
# Otherwise, unpredictable network errors may occur
####################################################################
/etc/no -o subnetsarelocal=0
Here is the information on this network option from info explorer:
subnetsarelocal determines if a packet address is on the local
network. This option is used by the in_localaddress subroutine. The
default value of 1 specifies that addresses that match the local
network mask are local. If the value is 0, only addresses matching
the local subnetwork are local.
The subnetsarelocal network option is used to adjust the MSS value.
In an HACMP/6000 environment, where you have two adapters
on the same RS/6000 configured on the same physical
network, this value also effects the routing of the packets.
matter is addressed in the "Setting Network Options" paragraph on Page
5-2 of the HACMP 3.1 Installation Guide. Here is the relevant quote:
"HACMP/6000 uses the subnet feature of IP addressing to force IP
datagrams to be sent across the designated network interface. To
enable this feature, the subnetsarelocal option of the /etc/no command
must be set to 0. If subnetsarelocal is set to 1, datagrams may not
go to their intended destination. As a result, adapter swapping
cannot work."
Response to Question 2:
From the above paragraph, subnetsarelocal MUST be set to 0. If it is
set to 1 then you will have a HACMP environment that is not supported.
I hope this answers your questions regarding HACMP. If you have any
further questions, or if I have not sufficiently answered this one,
please feel free to contact us again.
Thank you for using AIX technical support
WWQA: ITEM: RS4000003038 ITEM: RS4000003038
Dated: 12/1996 Category: AIX6000
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