Examples

4ward ships with example P4 programs that demonstrate core concepts. Each one works with the CLI and the web playground.

For a guided walkthrough using these examples, see the tutorial.

passthrough

Source: examples/passthrough.p4

The simplest possible v1model program. Parses an Ethernet header, hardcodes every packet to port 1, emits it unchanged. No tables, no actions.

control MyIngress(inout headers_t hdr, inout metadata_t meta,
                  inout standard_metadata_t standard_metadata) {
    apply {
        standard_metadata.egress_spec = 1;
    }
}

Try it:

4ward run examples/passthrough.p4 - <<'EOF'
packet 0 FFFFFFFFFFFF 000000000001 0800
expect 1 FFFFFFFFFFFF 000000000001 0800
EOF

What to look for in the trace: a linear trace with no tables — just parser, ingress (no events), and output on port 1.

basic_table

Source: examples/basic_table.p4

An exact-match table keyed on etherType. Matching packets are forwarded to a specified port; non-matching packets are dropped.

table port_table {
    key = { hdr.ethernet.etherType : exact; }
    actions = { forward; drop; NoAction; }
    default_action = drop();
}

Try it:

4ward run examples/basic_table.p4 - <<'EOF'
add port_table hdr.ethernet.etherType:0x0800 forward(1)

packet 0 FFFFFFFFFFFF 000000000001 0800 DEADBEEF
expect 1 FFFFFFFFFFFF 000000000001 0800 DEADBEEF

packet 0 FFFFFFFFFFFF 000000000001 0806 DEADBEEF
EOF

What to look for in the trace: the first packet shows table port_table: hit → forward; the second shows table port_table: miss → drop followed by mark_to_drop().

mirror

Available in: web playground (select mirror from the example dropdown)

An IPv4 router with port mirroring. Parses Ethernet + IPv4, routes via LPM on destination IP, and clones every forwarded packet to a mirror session.

apply {
    if (hdr.ipv4.isValid()) {
        ipv4_lpm.apply();
        clone(CloneType.I2E, 32w100);  // mirror to session 100
    }
}

The egress pipeline uses instance_type to distinguish originals from clones and tags cloned packets with a distinctive source MAC.

What to look for in the trace: the trace tree forks at the clone point — one branch for the original packet (exits on the routed port) and one for the clone (exits on the mirror port with rewritten source MAC).

Concepts demonstrated:

• Parser state transitions (select on etherType)
• LPM table matching
• Packet cloning and trace tree forking
• Egress branching on instance_type

sai_middleblock

Available in: web playground (select sai_middleblock from the example dropdown)

A simplified version of the SAI P4 middleblock — Google's production P4 pipeline for network switches. Real table names, real control flow, with @p4runtime_translation on all ID types.

This example demonstrates:

• Multiple tables applied in sequence (VRF lookup, L3 routing, next-hop resolution, neighbor rewrite)
• String-typed port names ("Ethernet0", "Ethernet1")
• Type translation across the full stack (note: @p4runtime_translation on port types is aspirational for SAI P4; the middleblock example uses it on non-port ID types like VRF and nexthop)

This is an advanced example. Start with basic_table to understand the fundamentals, then explore sai_middleblock to see how a production pipeline works.