• BPF_MAP_TYPE_LPM_TRIE was introduced in kernel version 4.11

BPF_MAP_TYPE_LPM_TRIE provides a longest prefix match algorithm that can be used to match IP addresses to a stored set of prefixes. Internally, data is stored in an unbalanced trie of nodes that uses prefixlen,data pairs as its keys. The data is interpreted in network byte order, i.e. big endian, so data[0] stores the most significant byte.

LPM tries may be created with a maximum prefix length that is a multiple of 8, in the range from 8 to 2048. The key used for lookup and update operations is a struct bpf_lpm_trie_key, extended by max_prefixlen/8 bytes.

  • For IPv4 addresses the data length is 4 bytes

  • For IPv6 addresses the data length is 16 bytes

The value type stored in the LPM trie can be any user defined type.


When creating a map of type BPF_MAP_TYPE_LPM_TRIE you must set the BPF_F_NO_PREALLOC flag.


Kernel BPF


void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)

The longest prefix entry for a given data value can be found using the bpf_map_lookup_elem() helper. This helper returns a pointer to the value associated with the longest matching key, or NULL if no entry was found.

The key should have prefixlen set to max_prefixlen when performing longest prefix lookups. For example, when searching for the longest prefix match for an IPv4 address, prefixlen should be set to 32.


long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)

Prefix entries can be added or updated using the bpf_map_update_elem() helper. This helper replaces existing elements atomically.

bpf_map_update_elem() returns 0 on success, or negative error in case of failure.


The flags parameter must be one of BPF_ANY, BPF_NOEXIST or BPF_EXIST, but the value is ignored, giving BPF_ANY semantics.


long bpf_map_delete_elem(struct bpf_map *map, const void *key)

Prefix entries can be deleted using the bpf_map_delete_elem() helper. This helper will return 0 on success, or negative error in case of failure.


Access from userspace uses libbpf APIs with the same names as above, with the map identified by fd.


int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key)

A userspace program can iterate through the entries in an LPM trie using libbpf's bpf_map_get_next_key() function. The first key can be fetched by calling bpf_map_get_next_key() with cur_key set to NULL. Subsequent calls will fetch the next key that follows the current key. bpf_map_get_next_key() returns 0 on success, -ENOENT if cur_key is the last key in the trie, or negative error in case of failure.

bpf_map_get_next_key() will iterate through the LPM trie elements from leftmost leaf first. This means that iteration will return more specific keys before less specific ones.


Please see tools/testing/selftests/bpf/test_lpm_map.c for examples of LPM trie usage from userspace. The code snippets below demonstrate API usage.

Kernel BPF

The following BPF code snippet shows how to declare a new LPM trie for IPv4 address prefixes:

#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>

struct ipv4_lpm_key {
        __u32 prefixlen;
        __u32 data;

struct {
        __uint(type, BPF_MAP_TYPE_LPM_TRIE);
        __type(key, struct ipv4_lpm_key);
        __type(value, __u32);
        __uint(map_flags, BPF_F_NO_PREALLOC);
        __uint(max_entries, 255);
} ipv4_lpm_map SEC(".maps");

The following BPF code snippet shows how to lookup by IPv4 address:

void *lookup(__u32 ipaddr)
        struct ipv4_lpm_key key = {
                .prefixlen = 32,
                .data = ipaddr

        return bpf_map_lookup_elem(&ipv4_lpm_map, &key);


The following snippet shows how to insert an IPv4 prefix entry into an LPM trie:

int add_prefix_entry(int lpm_fd, __u32 addr, __u32 prefixlen, struct value *value)
        struct ipv4_lpm_key ipv4_key = {
                .prefixlen = prefixlen,
                .data = addr
        return bpf_map_update_elem(lpm_fd, &ipv4_key, value, BPF_ANY);

The following snippet shows a userspace program walking through the entries of an LPM trie:

#include <bpf/libbpf.h>
#include <bpf/bpf.h>

void iterate_lpm_trie(int map_fd)
        struct ipv4_lpm_key *cur_key = NULL;
        struct ipv4_lpm_key next_key;
        struct value value;
        int err;

        for (;;) {
                err = bpf_map_get_next_key(map_fd, cur_key, &next_key);
                if (err)

                bpf_map_lookup_elem(map_fd, &next_key, &value);

                /* Use key and value here */

                cur_key = &next_key;