diff --git a/bin/mn_run.py b/bin/mn_run.py
index ffa49e1..ae92013 100755
--- a/bin/mn_run.py
+++ b/bin/mn_run.py
@@ -5,6 +5,7 @@
 '''
 
 from optparse import OptionParser
+import os.path
 import time
 
 try:
@@ -100,6 +101,8 @@ class MininetRunner(object):
         add_dict_option(opts, HOSTS, HOST_DEF, 'host')
         add_dict_option(opts, CONTROLLERS, CONTROLLER_DEF, 'controller')
 
+        opts.add_option('--custom', type = 'string', default = None,
+                        help = 'read custom mininet from current dir')
         opts.add_option('--test', type = 'choice', choices = TESTS,
                         default = TESTS[0],
                         help = '[' + ' '.join(TESTS) + ']')
@@ -137,6 +140,10 @@ class MininetRunner(object):
             raise Exception('multipath topos require multipath-capable '
                             'controller.')
 
+        if self.options.custom:
+            if not os.path.isfile(self.options.custom):
+                raise Exception('could not find custom file: %s' % custom)
+
     def begin(self):
         '''Create and run mininet.'''
 
@@ -156,10 +163,20 @@ class MininetRunner(object):
         xterms = self.options.xterms
         mac = self.options.mac
         arp = self.options.arp
-        mn = Mininet(topo, switch, host, controller, controller_params,
-                     in_namespace = in_namespace,
-                     xterms = xterms, auto_set_macs = mac,
-                     auto_static_arp = arp)
+        mn = None
+        if not self.options.custom:
+            mn = Mininet(topo, switch, host, controller, controller_params,
+                         in_namespace = in_namespace,
+                         xterms = xterms, auto_set_macs = mac,
+                         auto_static_arp = arp)
+        else:
+            globals_ = {}
+            locals_ = {}
+            execfile(self.options.custom, globals_, locals_)
+            if 'mn' not in locals_:
+                raise Exception('could not find mn var in custom file')
+            else:
+                mn = locals_['mn']
 
         test = self.options.test
         if test != 'build':
diff --git a/custom/README b/custom/README
new file mode 100644
index 0000000..45b06e2
--- /dev/null
+++ b/custom/README
@@ -0,0 +1,9 @@
+This directory should hold configuration files for custom mininets.
+
+See custom_example.py, which loads the default minimal topology.  The advantage of defining a mininet in a separate file is that you then use the --custom option in mn_run.py to run the CLI or specific tests with it.
+
+Each custom file must define a Mininet object for an 'mn' variable.
+
+To start up a mininet with this custom topology, do:
+  sudo mn_run.py --custom custom_example.py
+
diff --git a/custom/custom_example.py b/custom/custom_example.py
new file mode 100644
index 0000000..9670890
--- /dev/null
+++ b/custom/custom_example.py
@@ -0,0 +1,25 @@
+'''Example of custom topo
+
+@author Brandon Heller (brandonh@stanford.edu)
+
+'''
+
+from mininet.topo import SingleSwitchTopo
+from mininet.net import Mininet
+from mininet.node import KernelSwitch, Host, Controller, ControllerParams
+
+topo = SingleSwitchTopo(k = 2) # build topology object
+switch = KernelSwitch
+host = Host
+controller = Controller
+controller_params = ControllerParams(0x0a000000, 8) # 10.0.0.0/8
+in_namespace = False
+xterms = False
+mac = True
+arp = True
+
+mn = Mininet(topo, switch, host, controller, controller_params,
+             in_namespace = in_namespace,
+             xterms = xterms, auto_set_macs = mac,
+             auto_static_arp = arp)
+