Adjust Thread BBR neighbor/destination cache size (ARMmbed#1793)

Allow Thread BBR to have more entries in neighbor and destination
cache.

source/6LoWPAN/Thread/thread_bbr_api.c

@@ -40,6 +40,7 @@
 #include "common_functions.h"
 #include "thread_border_router_api.h"
 #include "thread_bbr_api.h"
+#include "net_ipv6_api.h"
 #include "Common_Protocols/ipv6_constants.h"
 #include "DHCPv6_Server/DHCPv6_server_service.h"
 #include "thread_management_if.h"
@@ -1099,7 +1100,11 @@ int thread_bbr_start(int8_t interface_id, int8_t backbone_interface_id)
     // By default multicast forwarding is not enabled as it causes multicast loops
     multicast_fwd_set_forwarding(this->interface_id, false);
 
+    // Adjust BBR neighbor and destination cache size
+    arm_nwk_ipv6_max_cache_entries(THREAD_BBR_IPV6_DESTINATION_CACHE_SIZE);
+
     thread_extension_bbr_init(interface_id,backbone_interface_id);
+
     return 0;
 #else
     return -1;

source/6LoWPAN/Thread/thread_config.h

@@ -326,6 +326,12 @@
  */
 #define THREAD_BBR_ROUTER_ID_REQUEST_STATUS THREAD_COAP_STATUS_TLV_HAVE_CHILD_ID_REQUEST
 
+/*
+ * Number of destination and neighbor cache entries assuming 250 thread devices (worst case) connecting to cloud service.
+ * Six entries reserved for backbone devices.
+ */
+#define THREAD_BBR_IPV6_DESTINATION_CACHE_SIZE  256
+
 /*
  * Timeout to solicit address from DHCP if previous request fails.
  */