# Using LISP tunneling

LISP is a layer 3 tunneling mechanism, meaning that encapsulated packets do not carry Ethernet headers, and ARP requests shouldn’t be sent over the tunnel. Because of this, there are some additional steps required for setting up LISP tunnels in Open vSwitch, until support for L3 tunnels will improve.

This guide assumes tunneling between two VMs connected to OVS bridges on different hypervisors reachable over IPv4. Of course, more than one VM may be connected to any of the hypervisors, and a hypervisor may communicate with several different hypervisors over the same lisp tunneling interface. A LISP “map-cache” can be implemented using flows, see example at the bottom of this file.

There are several scenarios:

1. the VMs have IP addresses in the same subnet and the hypervisors are also in a single subnet (although one different from the VM’s);

2. the VMs have IP addresses in the same subnet but the hypervisors are separated by a router;

3. the VMs are in different subnets.

In cases 1) and 3) ARP resolution can work as normal: ARP traffic is configured not to go through the LISP tunnel. For case 1) ARP is able to reach the other VM, if both OVS instances default to MAC address learning. Case 3) requires the hypervisor be configured as the default router for the VMs.

In case 2) the VMs expect ARP replies from each other, but this is not possible over a layer 3 tunnel. One solution is to have static MAC address entries preconfigured on the VMs (e.g., `arp -f /etc/ethers` on startup on Unix based VMs), or have the hypervisor do proxy ARP. In this scenario, the eth0 interfaces need not be added to the br0 bridge in the examples below.

On the receiving side, the packet arrives without the original MAC header. The LISP tunneling code attaches a header with hard-coded source and destination MAC address `02:00:00:00:00:00`. This address has all bits set to 0, except the locally administered bit, in order to avoid potential collisions with existing allocations. In order for packets to reach their intended destination, the destination MAC address needs to be rewritten. This can be done using the flow table.

See below for an example setup, and the associated flow rules to enable LISP tunneling.

    Diagram

           +---+                               +---+
           |VM1|                               |VM2|
           +---+                               +---+
             |                                   |
        +--[tap0]--+                       +--[tap0]---+
        |          |                       |           |
    [lisp0] OVS1 [eth0]-----------------[eth0] OVS2 [lisp0]
        |          |                       |           |
        +----------+                       +-----------+

On each hypervisor, interfaces tap0, eth0, and lisp0 are added to a single bridge instance, and become numbered 1, 2, and 3 respectively:

    $ ovs-vsctl add-br br0
    $ ovs-vsctl add-port br0 tap0
    $ ovs-vsctl add-port br0 eth0
    $ ovs-vsctl add-port br0 lisp0 \
      -- set Interface lisp0 type=lisp options:remote_ip=flow options:key=flow

The last command sets up flow based tunneling on the lisp0 interface. From the LISP point of view, this is like having the Tunnel Router map cache implemented as flow rules.

Flows on br0 should be configured as follows:

    priority=3,dl_dst=02:00:00:00:00:00,action=mod_dl_dst:<VMx_MAC>,output:1
    priority=2,in_port=1,dl_type=0x0806,action=NORMAL
    priority=1,in_port=1,dl_type=0x0800,vlan_tci=0,nw_src=<EID_prefix>,action=set_field:<OVSx_IP>->tun_dst,output:3
    priority=0,action=NORMAL

The third rule is like a map cache entry: the `<EID_prefix>` specified by the `nw_src` match field is mapped to the RLOC `<OVSx_IP>`, which is set as the tunnel destination for this particular flow.

Optionally, if you want to use Instance ID in a flow, you can add `set_tunnel:<IID>` to the action list.