Re: VM TCP/IP Routing Question

[Miguel Delapaz]

Dennis,

When more than one path to a destination
exists, and they have the same "cost" (as defined by the routing
protocol), z/VM TCP/IP will round robin packets to that destination through
the available paths.  For static routing, this behavior is controlled
by the EQUALCOSTMULTIPATH parameter on the ASSORTEDPARMS statement.  For
dynamic routing (MPRoute), this behavior is always enabled.  z/VM
TCP/IP only does this on a per-packet basis (we are looking other options
for the future).  A "hack" you can use to get around this,
in your case, is to modify the COST0 parameter on one your OSPF_INTERFACE
statements.  By forcing the links to have different costs, MPRoute
will only use the path with the lowest total cost, and will no longer round-robin
the packets.  In the event of adapter failure, MPRoute would fail-over
to using the higher cost path.

Regards,
Miguel Delapaz
z/VM TCP/IP Development 


The IBM z/VM Operating System <IBMVM@LISTSERV.UARK.EDU>
wrote on 08/02/2006 10:26:34 AM:

> Hi,
> 
> Our network folks noticed that one of our zLinux systems is driving
a lot
> of packet retransmissions, caused mainly because packets are being
received
> out-of-sequence, outside the target workstation's response window
(I hope
> I'm getting the terminology correct here).
> 
> Going one step further, the network folks say this is happening because
the
> zLinux system (actually, this is probably VM TCP/IP, as I'll explain
in a
> moment) is transmitting packets sort of round-robin across both of
our OSA
> interfaces, rather than picking one interface and transmitting all
packets
> across that interface.
> 
> They've asked if we can tweak our configuration to send packets across
a
> single adapter in order to reduce retransmissions.
> 
> VM TCP/IP is involved because the zLinux system is connected to a
guest lan
> which is connected to VM's TCP/IP stack.  The zLinux system has
just one
> logical ethernet interface but VM TCPIP is connected to both OSA
> interfaces, via separate VSWITCH connections.  Each of the VSWITCH/OSAs
is
> attached to a separate IP subnet.
> 
> Does anyone know if there's a TCP/IP or MPROUTE configuration parameter
> which can affect this behavior?
> 
> Would this be more easily controlled using a zLinux TCP/IP stack?
 We've
> experimented with connecting a zLinux system to the VSWITCHs directly
and
> using Zebra/OSPF in zLinux but didn't feel our traffic volume justified
the
> increased management and automation requirements.  But, maybe
this would
> make it worthwhile?
> 
> Thanks in advance for any help you can provide.
> 
> Dennis Schaffer
> Mutual of Omaha