[ovs-dev] [PATCH] vswitchd: Explain why mirroring to a VLAN can cause network problems.

Ben Pfaff blp at nicira.com
Thu Sep 3 00:16:24 UTC 2009

Bug #1963.
 vswitchd/ovs-vswitchd.conf.5.in |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/vswitchd/ovs-vswitchd.conf.5.in b/vswitchd/ovs-vswitchd.conf.5.in
index d613419..8d15f57 100644
--- a/vswitchd/ovs-vswitchd.conf.5.in
+++ b/vswitchd/ovs-vswitchd.conf.5.in
@@ -267,6 +267,21 @@ a trunk port, the frame's VLAN tag will be set to \fIvid\fR, replacing
 any existing tag; when it is sent out an implicit VLAN port, the frame
 will not be tagged.  This type of mirroring may be referred to as
+Mirroring to a VLAN can disrupt a network that contains unmanaged
+switches.  Consider an unmanaged physical switch with two ports: port
+1, connected to an end host, and port 2, connected to an Open vSwitch
+configured to mirror received packets into VLAN 123 on port 2.
+Suppose that the end host sends a packet on port 1 that the physical
+switch forwards to port 2.  The Open vSwitch forwards this packet to
+its destination and then reflects it back on port 2 in VLAN 123.  This
+reflected packet causes the physical switch to learn that the end
+host's MAC address is on port 2.  Afterward, the physical switch will
+direct packets destined for the end host to the Open vSwitch on port
+2, instead of to the end host on port 1, disrupting connectivity.  If
+mirroring to a VLAN must be used in this scenario, then the physical
+switch must be replaced by one that learns Ethernet addresses on a
+per-VLAN basis.
 .ST "Example"
 The following \fBovs\-vswitchd\fR configuration copies all frames received
 on \fBeth1\fR or \fBeth2\fR to \fBeth3\fR.

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