16. L2 Forwarding Sample Application (in Real and Virtualized Environments)

The L2 Forwarding sample application is a simple example of packet processing using the Data Plane Development Kit (DPDK) which also takes advantage of Single Root I/O Virtualization (SR-IOV) features in a virtualized environment.


Please note that previously a separate L2 Forwarding in Virtualized Environments sample application was used, however, in later DPDK versions these sample applications have been merged.

16.1. Overview

The L2 Forwarding sample application, which can operate in real and virtualized environments, performs L2 forwarding for each packet that is received on an RX_PORT. The destination port is the adjacent port from the enabled portmask, that is, if the first four ports are enabled (portmask 0xf), ports 1 and 2 forward into each other, and ports 3 and 4 forward into each other. Also, if MAC addresses updating is enabled, the MAC addresses are affected as follows:

  • The source MAC address is replaced by the TX_PORT MAC address
  • The destination MAC address is replaced by 02:00:00:00:00:TX_PORT_ID

This application can be used to benchmark performance using a traffic-generator, as shown in the Fig. 16.1, or in a virtualized environment as shown in Fig. 16.2.


Fig. 16.1 Performance Benchmark Setup (Basic Environment)


Fig. 16.2 Performance Benchmark Setup (Virtualized Environment)

This application may be used for basic VM to VM communication as shown in Fig. 16.3, when MAC addresses updating is disabled.


Fig. 16.3 Virtual Machine to Virtual Machine communication.

The L2 Forwarding application can also be used as a starting point for developing a new application based on the DPDK.

16.1.1. Virtual Function Setup Instructions

This application can use the virtual function available in the system and therefore can be used in a virtual machine without passing through the whole Network Device into a guest machine in a virtualized scenario. The virtual functions can be enabled in the host machine or the hypervisor with the respective physical function driver.

For example, in a Linux* host machine, it is possible to enable a virtual function using the following command:

modprobe ixgbe max_vfs=2,2

This command enables two Virtual Functions on each of Physical Function of the NIC, with two physical ports in the PCI configuration space. It is important to note that enabled Virtual Function 0 and 2 would belong to Physical Function 0 and Virtual Function 1 and 3 would belong to Physical Function 1, in this case enabling a total of four Virtual Functions.

16.2. Compiling the Application

To compile the sample application see Compiling the Sample Applications.

The application is located in the l2fwd sub-directory.

16.3. Running the Application

The application requires a number of command line options:

./<build_dir>/examples/dpdk-l2fwd [EAL options] -- -p PORTMASK
                               [-q NQ]
                               [--portmap="(port, port)[,(port, port)]"]


  • p PORTMASK: A hexadecimal bitmask of the ports to configure
  • P: Optional, set all ports to promiscuous mode so that packets are accepted regardless of the MAC destination address. Without this option, only packets with the MAC destination address set to the Ethernet address of the port are accepted.
  • q NQ: A number of queues (=ports) per lcore (default is 1)
  • –[no-]mac-updating: Enable or disable MAC addresses updating (enabled by default)
  • –portmap=”(port,port)[,(port,port)]”: Determines forwarding ports mapping.

To run the application in linux environment with 4 lcores, 16 ports and 8 RX queues per lcore and MAC address updating enabled, issue the command:

$ ./<build_dir>/examples/dpdk-l2fwd -l 0-3 -n 4 -- -q 8 -p ffff

To run the application in linux environment with 4 lcores, 4 ports, 8 RX queues per lcore, to forward RX traffic of ports 0 & 1 on ports 2 & 3 respectively and vice versa, issue the command:

$ ./<build_dir>/examples/dpdk-l2fwd -l 0-3 -n 4 -- -q 8 -p f --portmap="(0,2)(1,3)"

Refer to the DPDK Getting Started Guide for general information on running applications and the Environment Abstraction Layer (EAL) options.

16.4. Explanation

The following sections provide some explanation of the code.

16.4.1. Command Line Arguments

The L2 Forwarding sample application takes specific parameters, in addition to Environment Abstraction Layer (EAL) arguments. The preferred way to parse parameters is to use the getopt() function, since it is part of a well-defined and portable library.

The parsing of arguments is done in the l2fwd_parse_args() function. The method of argument parsing is not described here. Refer to the glibc getopt(3) man page for details.

EAL arguments are parsed first, then application-specific arguments. This is done at the beginning of the main() function:

ret = rte_eal_init(argc, argv);
if (ret < 0)
	rte_exit(EXIT_FAILURE, "Invalid EAL arguments\n");
argc -= ret;
argv += ret;

force_quit = false;
signal(SIGINT, signal_handler);
signal(SIGTERM, signal_handler);

/* parse application arguments (after the EAL ones) */
ret = l2fwd_parse_args(argc, argv);
if (ret < 0)
	rte_exit(EXIT_FAILURE, "Invalid L2FWD arguments\n");

16.4.2. Mbuf Pool Initialization

Once the arguments are parsed, the mbuf pool is created. The mbuf pool contains a set of mbuf objects that will be used by the driver and the application to store network packet data:

l2fwd_pktmbuf_pool = rte_pktmbuf_pool_create("mbuf_pool", nb_mbufs,
if (l2fwd_pktmbuf_pool == NULL)
	rte_exit(EXIT_FAILURE, "Cannot init mbuf pool\n");

The rte_mempool is a generic structure used to handle pools of objects. In this case, it is necessary to create a pool that will be used by the driver. The number of allocated pkt mbufs is NB_MBUF, with a data room size of RTE_MBUF_DEFAULT_BUF_SIZE each. A per-lcore cache of 32 mbufs is kept. The memory is allocated in NUMA socket 0, but it is possible to extend this code to allocate one mbuf pool per socket.

The rte_pktmbuf_pool_create() function uses the default mbuf pool and mbuf initializers, respectively rte_pktmbuf_pool_init() and rte_pktmbuf_init(). An advanced application may want to use the mempool API to create the mbuf pool with more control.

16.4.3. Driver Initialization

The main part of the code in the main() function relates to the initialization of the driver. To fully understand this code, it is recommended to study the chapters that related to the Poll Mode Driver in the DPDK Programmer’s Guide - Rel 1.4 EAR and the DPDK API Reference.

/* reset l2fwd_dst_ports */
for (portid = 0; portid < RTE_MAX_ETHPORTS; portid++)
	l2fwd_dst_ports[portid] = 0;
last_port = 0;

/* populate destination port details */
if (port_pair_params != NULL) {
	uint16_t idx, p;

	for (idx = 0; idx < (nb_port_pair_params << 1); idx++) {
		p = idx & 1;
		portid = port_pair_params[idx >> 1].port[p];
		l2fwd_dst_ports[portid] =
			port_pair_params[idx >> 1].port[p ^ 1];
} else {
		/* skip ports that are not enabled */
		if ((l2fwd_enabled_port_mask & (1 << portid)) == 0)

		if (nb_ports_in_mask % 2) {
			l2fwd_dst_ports[portid] = last_port;
			l2fwd_dst_ports[last_port] = portid;
		} else {
			last_port = portid;

	if (nb_ports_in_mask % 2) {
		printf("Notice: odd number of ports in portmask.\n");
		l2fwd_dst_ports[last_port] = last_port;

The next step is to configure the RX and TX queues. For each port, there is only one RX queue (only one lcore is able to poll a given port). The number of TX queues depends on the number of available lcores. The rte_eth_dev_configure() function is used to configure the number of queues for a port:

ret = rte_eth_dev_configure(portid, 1, 1, &local_port_conf);
if (ret < 0)
	rte_exit(EXIT_FAILURE, "Cannot configure device: err=%d, port=%u\n",
		  ret, portid);

16.4.4. RX Queue Initialization

The application uses one lcore to poll one or several ports, depending on the -q option, which specifies the number of queues per lcore.

For example, if the user specifies -q 4, the application is able to poll four ports with one lcore. If there are 16 ports on the target (and if the portmask argument is -p ffff ), the application will need four lcores to poll all the ports.

ret = rte_eth_rx_queue_setup(portid, 0, nb_rxd,
if (ret < 0)
	rte_exit(EXIT_FAILURE, "rte_eth_rx_queue_setup:err=%d, port=%u\n",
		  ret, portid);

The list of queues that must be polled for a given lcore is stored in a private structure called struct lcore_queue_conf.

struct __rte_cache_aligned lcore_queue_conf {
	unsigned n_rx_port;
	unsigned rx_port_list[MAX_RX_QUEUE_PER_LCORE];
struct lcore_queue_conf lcore_queue_conf[RTE_MAX_LCORE];

The values n_rx_port and rx_port_list[] are used in the main packet processing loop (see Receive, Process and Transmit Packets).

16.4.5. TX Queue Initialization

Each lcore should be able to transmit on any port. For every port, a single TX queue is initialized.

txq_conf = dev_info.default_txconf;
txq_conf.offloads = local_port_conf.txmode.offloads;
ret = rte_eth_tx_queue_setup(portid, 0, nb_txd,
if (ret < 0)
	rte_exit(EXIT_FAILURE, "rte_eth_tx_queue_setup:err=%d, port=%u\n",
		ret, portid);

16.4.6. Receive, Process and Transmit Packets

In the l2fwd_main_loop() function, the main task is to read ingress packets from the RX queues. This is done using the following code:

for (i = 0; i < qconf->n_rx_port; i++) {

	portid = qconf->rx_port_list[i];
	nb_rx = rte_eth_rx_burst(portid, 0,
				 pkts_burst, MAX_PKT_BURST);

	if (unlikely(nb_rx == 0))

	port_statistics[portid].rx += nb_rx;

	for (j = 0; j < nb_rx; j++) {
		m = pkts_burst[j];
		rte_prefetch0(rte_pktmbuf_mtod(m, void *));
		l2fwd_simple_forward(m, portid);

Packets are read in a burst of size MAX_PKT_BURST. The rte_eth_rx_burst() function writes the mbuf pointers in a local table and returns the number of available mbufs in the table.

Then, each mbuf in the table is processed by the l2fwd_simple_forward() function. The processing is very simple: process the TX port from the RX port, then replace the source and destination MAC addresses if MAC addresses updating is enabled.


In the following code, one line for getting the output port requires some explanation.

During the initialization process, a static array of destination ports (l2fwd_dst_ports[]) is filled such that for each source port, a destination port is assigned that is either the next or previous enabled port from the portmask. Naturally, the number of ports in the portmask must be even, otherwise, the application exits.

static void
l2fwd_simple_forward(struct rte_mbuf *m, unsigned portid)
	unsigned dst_port;
	int sent;
	struct rte_eth_dev_tx_buffer *buffer;

	dst_port = l2fwd_dst_ports[portid];

	if (mac_updating)
		l2fwd_mac_updating(m, dst_port);

	buffer = tx_buffer[dst_port];
	sent = rte_eth_tx_buffer(dst_port, 0, buffer, m);
	if (sent)
		port_statistics[dst_port].tx += sent;

Then, the packet is sent using the l2fwd_send_packet (m, dst_port) function. For this test application, the processing is exactly the same for all packets arriving on the same RX port. Therefore, it would have been possible to call the l2fwd_send_burst() function directly from the main loop to send all the received packets on the same TX port, using the burst-oriented send function, which is more efficient.

However, in real-life applications (such as, L3 routing), packet N is not necessarily forwarded on the same port as packet N-1. The application is implemented to illustrate that, so the same approach can be reused in a more complex application.

The l2fwd_send_packet() function stores the packet in a per-lcore and per-txport table. If the table is full, the whole packets table is transmitted using the l2fwd_send_burst() function:

static int
l2fwd_send_packet(struct rte_mbuf *m, uint16_t port)
	unsigned lcore_id, len;
	struct lcore_queue_conf *qconf;

	lcore_id = rte_lcore_id();

	qconf = &lcore_queue_conf[lcore_id];
	len = qconf->pkt_buf[port].len;
	qconf->pkt_buf[port].buffer[len] = m;

	/* enough pkts to be sent */
	if (unlikely(len == MAX_PKT_BURST)) {
		l2fwd_send_burst(qconf, MAX_PKT_BURST, port);
		len = 0;

	qconf->pkt_buf[port].len = len;
	return 0;

To ensure that no packets remain in the tables, each lcore does a draining of TX queue in its main loop. This technique introduces some latency when there are not many packets to send, however it improves performance:

cur_tsc = rte_rdtsc();

 * TX burst queue drain
diff_tsc = cur_tsc - prev_tsc;
if (unlikely(diff_tsc > drain_tsc)) {

	for (i = 0; i < qconf->n_rx_port; i++) {

		portid = l2fwd_dst_ports[qconf->rx_port_list[i]];
		buffer = tx_buffer[portid];

		sent = rte_eth_tx_buffer_flush(portid, 0, buffer);
		if (sent)
			port_statistics[portid].tx += sent;


	/* if timer is enabled */
	if (timer_period > 0) {

		/* advance the timer */
		timer_tsc += diff_tsc;

		/* if timer has reached its timeout */
		if (unlikely(timer_tsc >= timer_period)) {

			/* do this only on main core */
			if (lcore_id == rte_get_main_lcore()) {
				/* reset the timer */
				timer_tsc = 0;

	prev_tsc = cur_tsc;