FAQ: Explain the two kinds of "promiscuous mode" and how to configure them.

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

diff --git a/FAQ b/FAQ
index 75d9007..a0eb1cd 100644 (file)
--- a/FAQ
+++ b/FAQ
@@ -250,6 +250,40 @@ A: The following commands configure br0 with eth0 and tap0 as trunk
 
        ovs-vsctl clear bridge br0 mirrors
 
+Q: Does Open vSwitch support configuring a port in promiscuous mode?
+
+A: Yes.  How you configure it depends on what you mean by "promiscuous
+   mode":
+
+      - Conventionally, "promiscuous mode" is a feature of a network
+        interface card.  Ordinarily, a NIC passes to the CPU only the
+        packets actually destined to its host machine.  It discards
+        the rest to avoid wasting memory and CPU cycles.  When
+        promiscuous mode is enabled, however, it passes every packet
+        to the CPU.  On an old-style shared-media or hub-based
+        network, this allows the host to spy on all packets on the
+        network.  But in the switched networks that are almost
+        everywhere these days, promiscuous mode doesn't have much
+        effect, because few packets not destined to a host are
+        delivered to the host's NIC.
+
+        This form of promiscuous mode is configured in the guest OS of
+        the VMs on your bridge, e.g. with "ifconfig".
+
+      - The VMware vSwitch uses a different definition of "promiscuous
+        mode".  When you configure promiscuous mode on a VMware vNIC,
+        the vSwitch sends a copy of every packet received by the
+        vSwitch to that vNIC.  That has a much bigger effect than just
+        enabling promiscuous mode in a guest OS.  Rather than getting
+        a few stray packets for which the switch does not yet know the
+        correct destination, the vNIC gets every packet.  The effect
+        is similar to replacing the vSwitch by a virtual hub.
+
+        This "promiscuous mode" is what switches normally call "port
+        mirroring" or "SPAN".  For information on how to configure
+        SPAN, see "How do I configure a port as a SPAN port, that is,
+        enable mirroring of all traffic to that port?"
+
 Q: How do I configure a VLAN as an RSPAN VLAN, that is, enable
    mirroring of all traffic to that VLAN?