[ovs-discuss] gratuitous arp replies on behalf of virtual machine

Stefan Kooman stefan at bit.nl
Wed Mar 18 14:35:53 UTC 2015

Hi All,

I'm trying to achieve the following: I would like to send gratuitous arp
replies on a OpenvSwitch (tagged) virtual machine (VM) interface (vnet5) from
the host itself (Ubuntu 14.04, OVS 2.0.2) (to update upstream switch(es) their
mac-address-tables after a live-migration of the VM). Therefore I made a little
python script that utilizes Scapy, relevant part:

sendp(Ether(src=args.macaddress,dst="ff:ff:ff:ff:ff:ff") / ARP(op=2,
hwsrc=args.macaddress, psrc=args.ipaddress, hwdst="ff:ff:ff:ff:ff:ff",
pdst=""), iface=args.vnet, count=4, inter=0.2)

The gratuitous arp replies do come in one the Virtual Machine itself, but never
make it out of the bridge (uplink), bond (bond1) and physical interface(s)
(eth2, eth5): 

Bridge uplink
        Port uplink
            Interface uplink
                type: internal
        Port "bond1"
            trunks: [998, 999]
            Interface "eth5"
            Interface "eth2"
        Port "vnet5"
            tag: 999
            Interface "vnet5"

There are no specific flows active on the "in_port" of vnet5. I have tried an
openflow rule to allow a packet out if its ingress port (ovs-ofctl add-flow
uplink in_port=10,actions=in_port) but to no avail. So basically I can inject
packets into the vnet, but they will not make it out of the vnet. Does
OpenvSwitch filter this somewhere, or is it the linux kernel discarding these

Thanks for any hints on how to debug this issue.

Kind regards,

Stefan Kooman

P.s. Sending the gratuitous arp replies from the virtual machine itself works
like expected.

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