NAPI-279 overlay nic tag creation needs to consider min nic tag MTU

Reviewed by: Matt Smillie <[email protected]>

Author: rmustacc

lib/init.js

@@ -192,13 +192,24 @@ function loadInitialData(opts, callback) {
 
         vasync.pipeline({ funcs: [
             function initOverlayTag(_, cb) {
+                var mtu;
+
                 if (!config.overlay.enabled) {
                     return setImmediate(cb);
                 }
 
+                /*
+                 * The minimum nic tag MTU is 1500, as that's what most physical
+                 * networks end up supporting. Therefore, if the overlay's MTU
+                 * is say, 1400 (a common non-Jumbo frame configuration), we
+                 * need to still create the nic tag at the minimum. Other
+                 * software needs to always be explicit about what MTU it
+                 * desires for the network (which has a much lower minimum MTU).
+                 */
+                mtu = Math.max(constants.OVERLAY_MTU, constants.MTU_NICTAG_MIN);
                 initNicTag(opts.app, log, {
                     name: constants.OVERLAY_TAG,
-                    mtu: constants.OVERLAY_MTU
+                    mtu: mtu
                 }, function (tagErr, tag) {
 
                     if (tagErr) {