Like Linux, take SPEED=0 to mean use autonegotiation on an interface.

clientside/tmcc/freebsd/liblocsetup.pm

 #!/usr/bin/perl -wT
 #
-# Copyright (c) 2000-2015 University of Utah and the Flux Group.
+# Copyright (c) 2000-2016 University of Utah and the Flux Group.
 # 
 # {{{EMULAB-LICENSE
 # 
@@ -107,8 +107,10 @@ my $IFALIAS     = "$IFCONFIGBIN %s alias %s netmask %s";
 my $IFC_1000MBS = "media 1000baseTX";
 my $IFC_100MBS  = "media 100baseTX";
 my $IFC_10MBS   = "media 10baseT/UTP";
+my $IFC_AUTO	= "";
 my $IFC_FDUPLEX = "mediaopt full-duplex";
 my $IFC_HDUPLEX = "mediaopt half-duplex";
+my $IFC_ADUPLEX = "";
 my $MKDIR	= "/bin/mkdir";
 my $GATED	= "/usr/local/sbin/gated";
 my $ROUTE	= "/sbin/route";
@@ -177,7 +179,7 @@ sub os_ifconfig_line($$$$$$$$;$$%)
 	    }
 	    if ($speed == 10000) {
 		# 10G is 10G, take it or leave it
-		$media = "";
+		$media = $IFC_AUTO;
 	    }
 	    elsif ($speed == 1000) {
 		$media = $IFC_1000MBS;
@@ -188,17 +190,23 @@ sub os_ifconfig_line($$$$$$$$;$$%)
 	    elsif ($speed == 10) {
 		$media = $IFC_10MBS;
 	    }
+	    elsif ($speed == 0) {
+		$media = $IFC_AUTO;
+	    }
 	    else {
-		warn("*** Bad Speed $speed in ifconfig, default to 100Mbps\n");
-		$speed = 100;
-		$media = $IFC_100MBS;
+		warn("*** Bad Speed $speed in ifconfig, default to autoconfig\n");
+		$speed = 0;
+		$media = $IFC_AUTO;
 	    }
 	}
 
+	if ($media eq $IFC_AUTO) {
+	    $mediaopt = $IFC_ADUPLEX;
+	} else {
 	    if ($duplex eq "full") {
 		if ($speed == 10000) {
 		    # 10G is always full duplex, no need to set
-		$mediaopt = "";
+		    $mediaopt = $IFC_ADUPLEX;
 		} else {
 		    $mediaopt = $IFC_FDUPLEX;
 		}
@@ -212,6 +220,7 @@ sub os_ifconfig_line($$$$$$$$;$$%)
 		$mediaopt = $IFC_FDUPLEX;
 	    }
 	}
+    }
 
     $uplines = "";