librdmacm/README: Update readme file to describe two interface problem

Signed-off-by: Sean Hefty <sean.hefty@intel.com>

diff --git a/README b/README
index 28a28b7a2fc815ddb6950b74416a64011dde4ed8..4d104cde64e6dfb770cf7b377bfbb805b5fea058 100644 (file)
--- a/README
+++ b/README
@@ -1,7 +1,8 @@
 This README is for userspace RDMA cm library.
 
-Building
 
+Building
+========
 To make this directory, run:
 ./autogen.sh && ./configure && make && make install
 
@@ -10,8 +11,9 @@ time unless configure.in or Makefile.am changes.
 
 Libraries are installed by default at /usr/local/lib.
 
-Device files
 
+Device files
+============
 The userspace CMA uses a single device file regardless of the number
 of adapters or ports present.
 
@@ -27,3 +29,22 @@ can be used.  This will create the device node named
 or you can create it manually
 
   mknod /dev/infiniband/rdma_cm c 231 255
+
+
+Common issues
+=============
+
+Using multiple interfaces
+       The librdmacm does support multiple interfaces.  To make use
+       of multiple interfaces, however, you need to instruct linux
+       to only send ARP reples on the interface targetted in the ARP
+       request.  This can be done using a command similar to the
+       following:
+
+       sysctl -w net.ipv4.conf.all.arp_ignore=2
+
+       Without this change, it's possible for linux to resopnd to ARP
+       requests on a different interface (IP address) than the IP
+       address carried in the ARP request.  This causes the RDMA stack
+       to incorrectly map the remote IP address to the wrong RDMA
+       device.