Gratuitous ARP's are usually used to allow the registration of a MAC
address into a Layer-2 switch's forwarding data base. In essence, the
gratuitous ARP lets the device tell the switch what physical port to use
to send a frame when the MAC address is encountered as the destination
MAC address in a frame. They are also frequently encountered when a
Layer-2 based virtual address changes host residency, for example
Virtual Router Redundancy Protocol, as opposed to Layer-3 based virtual
IP addresses which utilize a routing protocol such as OSPF to report
it's new location.
Harold Grovesteen
Alan Altmark wrote:
On Friday, 08/25/2006 at 12:48 MST, "Stricklin, Raymond J"
<[EMAIL PROTECTED]> wrote:
For the benefit of the rest of the list;
Alan's suggestion to enable faked frame headers solved the problem I was
having.
Unfortunately, a second problem surfaced immediately. The VSWITCH would
give me an error when the DHCP client received its lease:
HCPSWU2833E Error 'E00A'X adding IP address 192.54.6.16 for VSWITCH
SYSTEM VLINUX1.
HCPSWU2833E IP address is already in use on the LAN.
...snip...
We've been using IP VSWITCH without DHCP for about a year,
and why we never ran into this problem before, I couldn't exactly say.
Did you perhaps change the client from SLES9 to SLES10 in the process?
Somewhere along the way, Linux (dhclient?) changed to issue a gratuitous
ARP *before* it registers the IP address in the OSA. Of course, the
registration of an IP address in a Layer 3 OSA causes it to see if the IP
address is already used. WHAM! You hit the ARP cache entry created by
the "manual" grat ARP. Or Linux has always done that but timings have
changed sufficiently to finally surface the problem. (A gratuitous ARP is
what we call an unsolicited response to an ARP query.)
It doesn't make sense for Linux to send a grat ARP before it has finished
configuring the interface. In fact, I would think that would be a
function of interface configuration itself, left to the device driver
itself. Or maybe something's broken.
It works with Layer 2 because the OSA doesn't do any automatic grat ARPs.
<Chuckie>
I think the problem *could* be mitigated by the OSA itself. OSA could
allow an IP address to be registered if the grat ARP was received on a
data channel associated with the control channel over which the IP address
is being registered. Of course, the h/w guys would say that just
disabling the grat ARP (ARP=no) makes the problem go away and that's
sufficient. I'm also of the opinion that the *applications* should not
have to change to accomodate this odd behavior of the Layer 3 OSA. But,
then, my nickname is "Mr. Quixotehead". AND, the fix really does be in
Lniux itself.
</Chuckie>
Alan Altmark
z/VM Development
IBM Endicott
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