lib, doc: add recursive support configuration for nexthop-group

In sharpd, configuring a nexthop-group with an IP nexthop that is not
directly connected does create an inactive NHG context in zebra:

> ubuntu2204(config)# interface loop1
> ubuntu2204(config-if)# ip address 192.0.2.1/24
> ubuntu2204(config-if)# exi
> ubuntu2204(config)# ip route 10.200.0.0/24 192.0.2.100
> ubuntu2204(config)# nexthop-group ABCD
> ubuntu2204(config-nh-group)# nexthop 10.200.0.62
> 2024/01/17 16:52:44 SHARP: [JWRCN-N9K90] Installed nhg 181818168
> ubuntu2204(config-nh-group)# do show nexthop-group rib 181818168
> ID: 181818168 (sharp)
>      RefCnt: 1
>      Uptime: 00:00:04
>      VRF: default
>      Depends: (841)
>            via 10.200.0.62 (vrf default) inactive, weight 1

Add the 'allow-recursion' vty command under nexthop-group configuration.
When set, the nexthop-group ABCD is added or updated, and will update
the nexthop resolution as expected.

> ubuntu2204(config)# interface loop1
> ubuntu2204(config-if)# ip address 192.0.2.1/24
> ubuntu2204(config-if)# exi
> ubuntu2204(config)# ip route 10.200.0.0/24 192.0.2.100
> ubuntu2204(config)# nexthop-group ABCD
> ubuntu2204(config-nh-group)# allow-recursion
> ubuntu2204(config-nh-group)# nexthop 10.200.0.62
> 2024/01/17 16:57:44 SHARP: [JWRCN-N9K90] Installed nhg 181818168
> ubuntu2204(config-nh-group)# do show nexthop-group rib 181818168
> ID: 181818168 (sharp)
>      RefCnt: 1
>      Uptime: 00:00:04
>      VRF: default
>      Valid, Installed
>      Depends: (842)
>         via 10.200.0.62 (vrf default) (recursive), weight 1
>            via 192.0.2.100, loop1 (vrf default), weight 1

The allow-recursion flag is disabled by default, as it is today with
other control plane daemons.

Signed-off-by: Philippe Guibert <[email protected]>

doc/user/nexthop_groups.rst

@@ -27,3 +27,9 @@ listing of ECMP nexthops used to forward packets.
    will be automatically re-assigned.  This cli command must be the first
    command entered currently.  Additionally this command only works with linux 5.19
    kernels or newer.
+
+.. clicmd:: allow-recursion
+
+   By default, a nexthop group is only marked as active when its nexthop is
+   directly connected. The ``allow-recursion`` option allows zebra to resolve the
+   nexthop using other types of routes.

lib/nexthop_group.c

@@ -650,6 +650,28 @@ DEFPY(nexthop_group_backup, nexthop_group_backup_cmd,
 	return CMD_SUCCESS;
 }
 
+DEFPY(nexthop_group_allow_recursion,
+      nexthop_group_allow_recursion_cmd,
+      "[no] allow-recursion",
+      NO_STR
+      "Allow recursion when the nexthop is not directly connected\n")
+{
+	VTY_DECLVAR_CONTEXT(nexthop_group_cmd, nhgc);
+
+	if (!!no == !CHECK_FLAG(nhgc->nhg.flags, NEXTHOP_GROUP_ALLOW_RECURSION))
+		return CMD_SUCCESS;
+
+	if (no)
+		UNSET_FLAG(nhgc->nhg.flags, NEXTHOP_GROUP_ALLOW_RECURSION);
+	else
+		SET_FLAG(nhgc->nhg.flags, NEXTHOP_GROUP_ALLOW_RECURSION);
+
+	if (nhg_hooks.modify)
+		nhg_hooks.modify(nhgc);
+
+	return CMD_SUCCESS;
+}
+
 DEFPY(no_nexthop_group_backup, no_nexthop_group_backup_cmd,
       "no backup-group [WORD$name]",
       NO_STR
@@ -1173,6 +1195,9 @@ static int nexthop_group_write(struct vty *vty)
 
 		vty_out(vty, "nexthop-group %s\n", nhgc->name);
 
+		if (CHECK_FLAG(nhgc->nhg.flags, NEXTHOP_GROUP_ALLOW_RECURSION))
+			vty_out(vty, " allow-recursion\n");
+
 		if (nhgc->nhg.nhgr.buckets)
 			vty_out(vty,
 				" resilient buckets %u idle-timer %u unbalanced-timer %u\n",
@@ -1376,6 +1401,7 @@ void nexthop_group_init(void (*new)(const char *name),
 
 	install_element(NH_GROUP_NODE, &nexthop_group_resilience_cmd);
 	install_element(NH_GROUP_NODE, &no_nexthop_group_resilience_cmd);
+	install_element(NH_GROUP_NODE, &nexthop_group_allow_recursion_cmd);
 
 	memset(&nhg_hooks, 0, sizeof(nhg_hooks));