Replace FRR Zebra SRv6 Manager with YANG Backend

Now I’m playing with FRRouting and contributing with tiny PRs(my PRs have merged twice). Today I’ll introduce my recent PoC that achieves replacement SRv6 Manager with YANG backend. This PoC is related with FRR Northbound API so I’m interesting in it. This PoC is incomplete but it’s maybe helpful if you want to know about the FRR’s mechanism of Northbound API. You can see the PoC in here .

In FRR, Zebra daemon manages SRv6 resources intensively. And routing daemons(bgpd/isisd/etc) requests the resources to Zebra. Zebra provides the resource’s ownerships to them. It’s just a “Server - Client” model as you know. there is a component called SRv6 Manager in zebra. it’s responsible for managing SRv6 resources and the requests to them. I recommend you to read this topotest that demonstrates VPNv6 with BGP SRv6 for understanding SRv6 Manager.

In FRR you can use vtysh to configure dynamically. I think vtysh is the most popular way to interact FRR daemons. vtysh provides the experiences that are almost the same as Cisco’s interactive CLI.

On the other hand, FRR provides another way called Northbound gRPC. It enables you to configure FRR daemons via gRPC communications. When FRR users want to interact with FRR daemons they only need to implement gRPC clients. You can see the detailed documentation in here and here .

FRR achieves the Northbound gRPC with following method.

  • modeling FRR configuration/operational-data by using YANG
  • attaching Northbound API callbacks into the data model

Each routing daemon doesn’t need to implement each API response handler. Requests from clients contain an XPath and corresponding callback is called automatically.

There are some merits in centralizing configurations/operational-data into YANG definitions. First, network operators only need to refer YANG files if operators want to know about the data models and their structures. network operators are familiar with YANG. And YANG doesn’t depend on any programming languages and implementations. Second, We can refer network vendor’s YANG models in here. You only need to refer them if you want to unify the operations among vendor’s appliances and yours.

As described above, there is a big motivation to replace FRR components with YANG backend. In fact FRR community set the roadmap to replace all daemons with YANG backend. Now the new management daemon called mgmtd is being proposed(I’ll describe it later). So I wanted to be familiar with FRR by implementing a YANG backend.

Now SRv6 Manager doesn’t support Northbound API. But SRv6 Manager is responsible for important things in small code-base. I had read overall of SRv6 Manager roughly so I didn’t think it’s difficult to replace for me(in fact I faced on some issues later). So I decided to implement SRv6 Manager YANG backend.

  • replace SRv6 components in practical ways
    • daemon side
    • SRv6 Manager vtysh commands
  • of course I keep SRv6 Manager correct
    • I’m careful of srv6_locator topotest

I considered the PoC of this task as three sub-tasks.

  • define YANG files
  • define and implement API callbacks that are corresponding to YANG data node
  • change the logic of SRv6 vtysh commands

First of all I defined YANG files. The YANG files are the foundation of daemon/vtysh’s implementation. So I had to design them carefully.

So I referred the YANG models of Cisco IOS-XR 7.5.1. I thought they’re already well-defined so they’re good references for me. There are some YANG files they’re related to SRv6, cfg and datatypes and oper. In FRR core logic I had to focus cfg and datatypes YANG.

And I saw segment-routing-srv6-cfg:srv6 container-block just augments segment-routing-ms-cfg:sr block. segment-routing-ms-cfg is the entire model of Segment-Rouring. For now there was no segment-routing manager in zebra(described later) but I didn’t care about that.

  • frr-srv6.yang … refers segment-routing-srv6-datatypes
  • frr-zebra-sr.yang … refers segment-routing-ms-cfg
    • that isn’t related to SRv6 Manager but it’s needed for consistency
  • frr-zebra-srv6.yang … refers segment-routing-srv6-cfg

Next I implemented Northbound API callbacks they’re corresponding to each YANG node. For instance, router isis AREA_TAG command creates a new isisd instance in isisd. this command also instantiates a new YANG list-node entry and stores it into isisd’s YANG data tree. At this time the corresponding create callback is called automatically. I’ll describe some kinds of Northbound API callbacks.

Callback Type Description
create Configuration at creation of a list-node entry/type empty/leaf-list entry
modify Configuration at modification of a leaf-node value
destroy Configuration at deletion of a list-node entry/leaf-list entry/etc
get_elem Operation at retrieving of a leaf/leaf-list entry/etc
lookup_entry Operation at searching of a list-node entry with given key
cli_show Operation prints a CLI command that is corresponding to a node
cli_show_end Operation if a node(container/list-node/etc) opens CLI command block this callback prints the block’s beginning

Next task is to implement these callbacks into each YANG model. I defined new struct frr_yang_module_info and passed it to the instantiation function of Zebra daemon.

/* stripped */
const struct frr_yang_module_info frr_zebra_srv6_info = {
  .name = "frr-zebra-srv6",
  .nodes = {
      .xpath = "/frr-zebra-sr:sr/frr-zebra-srv6:srv6",
      .cbs = {
        .cli_show = cli_show_segment_routing_srv6,
        .cli_show_end = cli_show_segment_routing_srv6_end,
      .priority = NB_DFLT_PRIORITY - 1,
      .xpath = "/frr-zebra-sr:sr/frr-zebra-srv6:srv6/locators",
      .cbs = {
        .cli_show = cli_show_srv6_locators,
        .cli_show_end = cli_show_srv6_locators_end,
      .xpath = "/frr-zebra-sr:sr/frr-zebra-srv6:srv6/locators/locators",
      .cbs = {
        .cli_show = cli_show_srv6_locators_locators,
        .cli_show_end = cli_show_srv6_locators_locators_end,
      .xpath = "/frr-zebra-sr:sr/frr-zebra-srv6:srv6/locators/locators/locator",
      .cbs = {
        .cli_show = cli_show_srv6_locator,
        .cli_show_end = cli_show_srv6_locator_end,
        .create = nb_lib_srv6_locator_create,
        .destroy = nb_lib_srv6_locator_destroy,
    /* stripped */

I also defined struct frr_yang_module_info frr_zebra_sr_info same as frr_zebra_srv6_info. So finally I just needed to implement specified callbacks as above one by one. For instance I’ll show youw nb_lib_srv6_locator_create. that is a typical create callback. that is called when a new SRv6 locator is created.

First of all, In FRR we should manage the both of YANG data tree and C data-structures. In this case nb_lib_srv6_locator_create calls zebra_srv6_locator_add() to update the master variable of SRv6 Manager.

// stripped

 * XPath: /frr-zebra-sr:sr/frr-zebra-srv6:srv6/locators/locators/locator
int nb_lib_srv6_locator_create(struct nb_cb_create_args *args)
  struct srv6_locator *loc;
  struct srv6_locator_chunk *chunk;
  const char *loc_name;

  if (args->event != NB_EV_APPLY) return NB_OK;

  loc_name = yang_dnode_get_string(args->dnode, "./name");
  loc = zebra_srv6_locator_lookup(loc_name);
  if (!loc) {
    /* SRv6 manager pre-allocates one chunk for zclients */
    loc = srv6_locator_alloc(loc_name);
    chunk = srv6_locator_chunk_alloc();
    chunk->proto = NO_PROTO;
    listnode_add(loc->chunks, chunk);

  nb_running_set_entry(args->dnode, loc);

  return NB_OK;

// stripped

Next example is nb_lib_srv6_locator_prefix_modify. that is called when leaf prefix is modified.

// stripped

 * XPath: /frr-zebra-sr:sr/frr-zebra-srv6:srv6/locators/locators/locator/prefix/prefix
int nb_lib_srv6_locator_prefix_modify(struct nb_cb_modify_args *args) {
  struct srv6_locator *locator;

  if (args->event != NB_EV_APPLY) return NB_OK;

  locator = nb_running_get_entry(args->dnode, NULL, true);
  yang_dnode_get_prefix(&locator->prefix, args->dnode, NULL);

  return NB_OK;

// stripped

Last I passed struct frr_yang_module_info into FRR_DAEMON_INFO in zebra/main.c.

// stripped

static const struct frr_yang_module_info *const zebra_yang_modules[] = {
  &frr_srv6_info, // added
  &frr_zebra_sr_info, // added
  &frr_zebra_srv6_info, // added

// stripped

Next I implemented vtysh commands side. Previously SRv6 Manager vtysh commands directly called daemon’s functions. So replace them with just calling Northbound API. First I’ll show srv6_locator_cmd that transitions to SRV6_LOC_NODE.

// stripped

                "locator LOC_NAME$name",
  char xpath[XPATH_MAXLEN];
  int rv;

  snprintf(xpath, sizeof(xpath),
           "/locators/locators/locator[name='%s']", name);

  nb_cli_enqueue_change(vty, xpath, NB_OP_CREATE, NULL);

  rv = nb_cli_apply_changes(vty, xpath);
  if (rv == CMD_SUCCESS)

  return rv;

// stripped

I’ll also show an example of a cli_show callback.

// stripped

void cli_show_srv6_locator(struct vty *vty, const struct lyd_node *dnode,
      bool show_defaults)
  const char *loc_name = NULL;

  loc_name = yang_dnode_get_string(dnode, "./name");
  vty_out(vty, "   locator %s\n", loc_name);

// stripped

finally all configurations can be printed by cli_show callbacks. zebra_sr_config() function is called when show-running command is called. So I replaced it with cli_show callbacks. FRR provides nb_cli_show_dnode_cmds() , that recursively calls cli_show callback from a given dnode.

// stripped

static int zebra_sr_config(struct vty *vty)
  int write_count = 0;
  struct lyd_node *dnode;

  if (zebra_srv6_is_enable()) {
    dnode = yang_dnode_get(running_config->dnode, 

    if (dnode) {
      nb_cli_show_dnode_cmds(vty, dnode, false);

  return write_count;

// stripped

So I’ve finished to pass srv6_locator topotest with YANG backend. But there are some issues yet.

For now this PoC only supports configurational callbacks. If user calls operational callbacks, it doesn’t work correctly. this indicates the show yang operational-data command will fail.

To achieve it, I should know how to use get_keys/get_next/get_elem/lookup_entry callback. those API don’t have any comprehensive docs, so I’ll have to read the existing implementations.

Now a new YANG-based management daemon called mgmtd is proposed to FRR. this daemon will support following features. I think the daemon will improve our development/operational experiences.

  • Frontend Interface for all API clients
    • running/candidate/startup/etc databases
  • Backend Interface for all FRR daemons
    • each daemon only needs to interact with mgmtd and can handle any config/data
    • this enables FRR daemons to unify implementations among them
  • Candidate Config Commit Rollbacks/History

Today I described my PoC and FRR current status. If you want to know more informations, please check the FRR community(GitHub and Slack)!