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Re: [Xen-users] [SOLVED] domU PXE boot with bridge

  • To: xen-users@xxxxxxxxxxxxx
  • From: Alexandre Kouznetsov <alk@xxxxxxxxxx>
  • Date: Mon, 13 Aug 2012 12:27:11 -0500
  • Delivery-date: Mon, 13 Aug 2012 17:28:42 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>


El 13/08/12 11:49, Raj Mathur (ààà ààààà) escribiÃ:
1. Presumably the bridge forward delay is there for a purpose.  Can
reducing that value result in other problems?
Check the thread I have referenced (2012-04-03 08:42 -0500), this question has been raised there.

Generally speaking: _should_ be no problem.
At least I have not encountered any (yet), and nobody has raised his hand to tell he had some. It has mainly to do with STP, which becomes relevant only when there is a ethernet loop risk. In any case, I disable STP on my VM bridges and pay specific attention not to put more than one physical interface in the same bridge, hope that eliminates possibility of looping.

In a attempt to prevent surprises, I do not reduce the delay to 0, it is set to something closer to the defaults instead. 5 seems to be just small enough to allow gPXE to do it's job.

2. Is it possible to get the current value of the forward delay from a
bridge?  If we can do that, we can script a reduced delay when PXE
booting and installing a VM, then go back to the default delay after the
VM install is over.
Not sure if it's a good idea. 15 (the usual default) seems to be as good as 5. Personally, I prefer to leave this kind of things alone: set once, make it persistent, leave it static unless there is a good reason to change it dynamically.

brctl seems not to offer a _nice_ interface to get the forward_delay value. If you wish to parse a multi-colon output, try "brctl showstp <bridge>". Also, you could check under /sys/devices/virtual/net/*.


Alexandre Kouznetsov

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