mac-learning: Increase MAC learning timeout to 300 seconds.

When Cisco (and other?) routers are configured in high-availability modes,
they use two different MAC addresses.  The router uses MAC 1 only for ARP
replies.  The router uses MAC 2 for forwarding IP packets to end hosts.
When a MAC learning switch is attached to the router, therefore, it will
only learn the location of MAC 1 from ARP replies.  If the end host's ARP
cache refresh timer is longer than the switch's MAC learning timeout, then
packets to the router will be flooded from the MAC learning timeout until
the next ARP reply.

This commit fixes the problem by increasing the MAC learning timeout from
60 seconds to 300 seconds.  According to research by Sanjay Sane, this is
always sufficient, even with operating systems that use ARP timeouts
longer than 300 seconds (such as FreeBSD and Mac OS, which have 1200
seconds ARP timeouts)  because the routers that cause this problem send
unsolicited ARP replies every 180 seconds.

This issue arises in any situation where traffic between two hosts flows
only in one direction.  The explanation above describes only one special
case.

NICS-11.
Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/lib/mac-learning.h b/lib/mac-learning.h
index ab739545c975260715c9624dfd0b9ec4a506ed0b..1ae9ea5d9e8513dbd0aba3f7f6781068f90c0736 100644 (file)
--- a/lib/mac-learning.h
+++ b/lib/mac-learning.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,7 @@
 #define MAC_MAX 2048
 
 /* Time, in seconds, before expiring a mac_entry due to inactivity. */
-#define MAC_ENTRY_IDLE_TIME 60
+#define MAC_ENTRY_IDLE_TIME 300
 
 /* Time, in seconds, to lock an entry updated by a gratuitous ARP to avoid
  * relearning based on a reflection from a bond slave. */