Re: HMC to Rack communcations

Possibly significant situational fact...

From albt-g0-0-903.corp.dekko.com (10.7.0.233) icmp_seq=13 Frag needed and

DF set (mtu = 1476)

<fromibm>
Hello Rob,

As per the update from the product engineering team, after looking at the
tcp dump they were able to see that there are multiple duplicate acks that
are occurring on port 30000 and port 30001 (the fsp connection port and
the vterm connection port) of the managed system.

This could be due to MTU mismatch.

MTU Mistatch: This issue is typically only seen when managing servers over
a wide area network. Depending on the version of HMC and power firmware,
the error may surface as incomplete state, a no connection or a connection
cycling
between no connection/connected/connecting. The issue may be the TCP
do-not-fragment (DNF) flag. HMC Version 6 and later sets the dnf bit.

You need to ensure that network MTU is large enough or that the network
passes ICMP destination unreachable as defined in RFC 1191 to allow a
smaller MTU to be negotiated. The HMC ping command can be used to test for
this condition.

Use the -M switch to set do not fragment and -s to increase the size. See
if it will pass the 1412 MTU. For example:ping -M do -s 1412

Check switch settings and switch traffic, the HMC shows up and running at
1000/full, ensure the port(s) going to the fsp's is set to auto since the
fsp's run at auto.
</fromibm>

<toibm>
Is there a way to tell HMC to allow the fragment?

It's possibly MTU. And maybe sporadic?

hscroot@HMC02:~> ping -M do -s 1450 rack1
PING rack1.corp.dekko.com (10.17.6.99) 1450(1478) bytes of data.
1458 bytes from rack1.dekko-1 (10.17.6.99): icmp_seq=1 ttl=56 time=6.19 ms
1458 bytes from rack1.corp.dekko.com (10.17.6.99): icmp_seq=2 ttl=56
time=6.18 ms
...
1458 bytes from rack1.corp.dekko.com (10.17.6.99): icmp_seq=11 ttl=56
time=6.18 ms
1458 bytes from fspsn21443bv.corp.dekko.com (10.17.6.99): icmp_seq=12
ttl=56 time=6.17 ms

From albt-g0-0-903.corp.dekko.com (10.7.0.233) icmp_seq=13 Frag needed and

DF set (mtu = 1476)
ping: local error: Message too long, mtu=1476
ping: local error: Message too long, mtu=1476
...
ping: local error: Message too long, mtu=1476
ping: local error: Message too long, mtu=1476
^C
--- rack1.corp.dekko.com ping statistics ---
23 packets transmitted, 12 received, +11 errors, 47% packet loss, time
22016ms
rtt min/avg/max/mdev = 6.120/6.177/6.207/0.067 ms
...
...
...
</toibm>

Rob Berendt