Bug 1273327 : added troubleshooting for virtual networking

admin_guide/sdn_troubleshooting.adoc

@@ -495,6 +495,58 @@ normal operation, so do not be surprised to see entries there.
 
 Check external firewalls, if any, allow traffic to the target address (this is site-dependent, and beyond the purview of this guide).
 
+[[debugging-virtual-networking]]
+== Debugging Virtual Networking
+
+[[builds-on-a-virtual-network-are-failing]]
+=== Builds on a Virtual Network are Failing
+
+If you are installing {product-title} using a virtual network (for example,
+OpenStack), and a build is failing, the maximum transmission unit (MTU) of the
+target node host might not be compatible with the MTU of the primary network
+interface (for example, `eth0`).
+
+For a build to complete successfully, the MTU of an SDN must be less than the
+eth0 network MTU in order to pass data to between node hosts.
+
+. Check the MTU of your network by running the `ip addr` command:
++
+====
+----
+# ip addr
+---
+2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
+    link/ether fa:16:3e:56:4c:11 brd ff:ff:ff:ff:ff:ff
+    inet 172.16.0.0/24 brd 172.16.0.0 scope global dynamic eth0
+       valid_lft 168sec preferred_lft 168sec
+    inet6 fe80::f816:3eff:fe56:4c11/64 scope link 
+       valid_lft forever preferred_lft forever
+---
+----
+====
++
+The MTU of the above network is 1500.
+
+. The MTU in your node configuration must be lower than the network value. Check
+the `*mtu*` in the node configuration of the targeted node host:
++
+====
+----
+# cat /etc/origin/node/node-config.yaml
+...
+networkConfig:
+   mtu: 1450
+   networkPluginName: company/openshift-ovs-subnet
+...
+----
+====
++
+In the above node configuration file, the `*mtu*` value is lower than the
+network MTU, so no configuration is needed. If the `*mtu*` value was higher,
+edit the file and lower the value to at least 50 units fewer than the MTU of the
+primary network interface, then restart the node service. This would allow
+larger packets of data to pass between nodes.
+
 == Debugging Pod Egress
 
 If you are trying to access an external service from a pod, e.g.: