From mainframe_s390:
Please teach me.
I want to comfirm VSWITCH's MTU size.
SLES9's eth0 connected to the VSWITCH is MTU 9000.
But VSWITCH's MTU size isn't diplayed on screen by using
"Q VSWITCH DETAILS" command.
VSWITCH don't have MTU??
I want to use eth0 for MTU size 9000.
The QDIO virtual NIC simulation presents an MTU size of
8992 to the device driver running in the virtual host.
This is the largest MTU supported by a real OSA card.
In practice, VM will not reject virtual NIC datagrams
due to MTU size, but since your VSWITCH connects to a
physical network, you need to configure the virtual
hosts (the users with virtual NIC coupled to this VSWITCH)
to match the MTU restrictions that exist in the physical
network.
It would not be impossible for a VSWITCH to display the
MTU size supported by the physical OSA card (when it is
active). This would indicate whether the OSA is limited
to 1492 or 8992. However, this could be misleading. If
this OSA is connected to a switch that supports 8992, but
the next hop is limited to 1492, you could lose frames
(or cause network congestion as frames are repackaged)
trying to reach other hosts on the same physical network.
Instead of asking what MTU size is available on the immediate
physical connection, you should learn what MTU is supported
across the physical network. All hosts on the physical
network should use the same MTU. Virtual hosts coupled to
your VSWITCH are now part of the physical network so they
should follow that same rule.
Even on a Guest LAN (which is isolated) you may have a router
that passes frames between simulated and physical networks.
In that case, it is more efficient to adopt the physical MTU
limit for virtual hosts coupled to the QDIO Guest LAN.
Regards,
Dennis Musselwhite ([EMAIL PROTECTED])
z/VM Virtual Networking
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