On Monday, 03/28/2011 at 05:35 EDT, Paul Garment <paul.garm...@barclays.com> wrote: > Thanks Alan, > > You were correct in your advice. There was a MTU size mismatch between > what I had set for the VM systems and what was set in the network. I > matched up the sizing and surprise, surprise it all works fine!
The symptom "works for small things, fails on large things" is almost always tied to MTU. Consider turning on path MTU discovery (PMTUD) by adding "PATHMTU" to AssortedParms. It enables you to set a large MTU on your local LAN while automatically slimming down packets that go to subnets with a smaller MTU. It's not a panacea, btw; it requires the intermediate hops to respond to "don't fragment" (DF) packets in a certain way (RFC 1191), and not all hosts do, or their responses are not received (typically due to misconfigured firewalls). But with a modern infrastructure built with PMTUD in mind, it's an effective way to optimize local LAN communications without creating a fragmentation problem elsewhere in the network. As a bonus, it automatically corrects a misconfigured MTU size. Alan Altmark z/VM and Linux on System z Consultant IBM System Lab Services and Training ibm.com/systems/services/labservices office: 607.429.3323 mobile; 607.321.7556 alan_altm...@us.ibm.com IBM Endicott
