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<TITLE>RE: [Openib-windows] IPoIB GUID to MAC conversion</TITLE>
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<P><FONT SIZE=2>> -----Original Message-----</FONT>
<BR><FONT SIZE=2>> From: Fab Tillier [<A HREF="mailto:ftillier@silverstorm.com">mailto:ftillier@silverstorm.com</A>] </FONT>
<BR><FONT SIZE=2>> Sent: Thursday, May 26, 2005 10:43 PM</FONT>
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<P><FONT SIZE=2>> > > > 4) Use the 48 bits to contain the path information. A non</FONT>
<BR><FONT SIZE=2>> > > IB routable</FONT>
<BR><FONT SIZE=2>> > > > approach will use the bits as follows: 16 LID, 4 </FONT>
<BR><FONT SIZE=2>> SL, 24 QPN,</FONT>
<BR><FONT SIZE=2>> > > > 2 static rate (might be not enough to hold all static</FONT>
<BR><FONT SIZE=2>> > > rate options)</FONT>
<BR><FONT SIZE=2>> > > > 2 reserved (local/global, unicast/mcast) - total 48 bits.</FONT>
<BR><FONT SIZE=2>> > ></FONT>
<BR><FONT SIZE=2>> > > This is interesting, as it would allow creating static </FONT>
<BR><FONT SIZE=2>> ARP entries. </FONT>
<BR><FONT SIZE=2>> > > Does it break the ability of IPoIB routers to route </FONT>
<BR><FONT SIZE=2>> traffic from IB </FONT>
<BR><FONT SIZE=2>> > > to Ethernet?</FONT>
<BR><FONT SIZE=2>> ></FONT>
<BR><FONT SIZE=2>> > No. This approach is IP routable (IPoIB to IPoEther). It is not IB </FONT>
<BR><FONT SIZE=2>> > routable because we won't be able to add a GID when sending packets.</FONT>
<BR><FONT SIZE=2>> </FONT>
<BR><FONT SIZE=2>> Actually, does it prevent IB global routing? These generated </FONT>
<BR><FONT SIZE=2>> MAC addresses are only used to interface with the OS. On the </FONT>
<BR><FONT SIZE=2>> wire, the MAC is always the 20-byte IPoIB MAC.</FONT>
<BR><FONT SIZE=2>> </FONT>
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<P><FONT SIZE=2>It is not IB routable because you lose the routing (GID) information.</FONT>
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<P><FONT SIZE=2>Imagine 2 nodes behind an IB router talking to you. And let's assume that</FONT>
<BR><FONT SIZE=2>they use same QP number (because the drivers there are identical and they</FONT>
<BR><FONT SIZE=2>just came out of reset). They will end up having the same MAC address in</FONT>
<BR><FONT SIZE=2>your code too: same QPN, same LID/SL (the IB router LID/SL). You just</FONT>
<BR><FONT SIZE=2>lost the fact that they are behind an IB router and that they need to use</FONT>
<BR><FONT SIZE=2>a GRH to be able to talk.</FONT>
<BR><FONT SIZE=2>When the OS hands you a packet for transmission, you can't really tell</FONT>
<BR><FONT SIZE=2>to whom to send it to... you need to be able to guess the GID.</FONT>
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