/* packet-bpdu.c

 * Routines for BPDU (Spanning Tree Protocol) disassembly

 *

 * Copyright 1999 Christophe Tronche <ch.tronche@computer.org>

 *

 * Wireshark - Network traffic analyzer

 * By Gerald Combs <gerald@wireshark.org>

 * Copyright 1998 Gerald Combs

 *

 * SPDX-License-Identifier: GPL-2.0-or-later

 */

#include "config.h"

#include <epan/packet.h>

#include <epan/etypes.h>

#include <epan/addr_resolv.h>

#include <epan/prefs.h>

#include <epan/expert.h>

#include <epan/tfs.h>

#include "packet-llc.h"

#include "packet-chdlc.h"

#include "packet-cisco-pid.h"

/* Offsets of fields within a BPDU */

#define BPDU_IDENTIFIER                           0

#define BPDU_VERSION_IDENTIFIER                   2

#define BPDU_TYPE                                 3

#define BPDU_FLAGS                                4

#define BPDU_ROOT_IDENTIFIER                      5

#define BPDU_ROOT_PATH_COST                      13

#define BPDU_BRIDGE_IDENTIFIER                   17

#define BPDU_PORT_IDENTIFIER                     25

#define BPDU_MESSAGE_AGE                         27

#define BPDU_MAX_AGE                             29

#define BPDU_HELLO_TIME                          31

#define BPDU_FORWARD_DELAY                       33

#define BPDU_VERSION_1_LENGTH                    35

#define BPDU_VERSION_3_LENGTH                    36

#define BPDU_MST_CONFIG_FORMAT_SELECTOR          38

#define BPDU_MST_CONFIG_NAME                     39

#define BPDU_MST_CONFIG_REVISION_LEVEL           71

#define BPDU_MST_CONFIG_DIGEST                   73

#define BPDU_CIST_INTERNAL_ROOT_PATH_COST        89

#define BPDU_CIST_BRIDGE_IDENTIFIER              93

#define BPDU_CIST_REMAINING_HOPS                101

#define BPDU_MSTI                               102

#define BPDU_PVST_TLV                            36

#define MSTI_FLAGS                                0

#define MSTI_REGIONAL_ROOT                        1

#define MSTI_INTERNAL_ROOT_PATH_COST              9

#define MSTI_BRIDGE_IDENTIFIER_PRIORITY          13

#define MSTI_PORT_IDENTIFIER_PRIORITY            14

#define MSTI_REMAINING_HOPS                      15

#define CONF_BPDU_SIZE                  35

#define TC_BPDU_SIZE                     4

#define MST_BPDU_SIZE                   38

#define VERSION_3_STATIC_LENGTH         64

#define MSTI_MESSAGE_SIZE               16

/* Values for the Alternative MSTI format */

#define ALT_BPDU_CIST_BRIDGE_IDENTIFIER         89

#define ALT_BPDU_CIST_INTERNAL_ROOT_PATH_COST   97

#define ALT_MSTI_MSTID                           0

#define ALT_MSTI_FLAGS                           2

#define ALT_MSTI_REGIONAL_ROOT                   3

#define ALT_MSTI_INTERNAL_ROOT_PATH_COST        11

#define ALT_MSTI_BRIDGE_IDENTIFIER              15

#define ALT_MSTI_PORT_IDENTIFIER                23

#define ALT_MSTI_REMAINING_HOPS                 25

#define ALT_MSTI_MESSAGE_SIZE                   26

/* Flag bits */

#define BPDU_FLAGS_TCACK                0x80

#define BPDU_FLAGS_AGREEMENT            0x40

#define BPDU_FLAGS_FORWARDING           0x20

#define BPDU_FLAGS_LEARNING             0x10

#define BPDU_FLAGS_PORT_ROLE_MASK       0x0C

#define BPDU_FLAGS_PORT_ROLE_SHIFT      2

#define BPDU_FLAGS_PROPOSAL             0x02

#define BPDU_FLAGS_TC                   0x01

void proto_register_bpdu(void);

void proto_reg_handoff_bpdu(void);

static int proto_bpdu;

static int hf_bpdu_proto_id;

static int hf_bpdu_version_id;

static int hf_bpdu_type;

static int hf_bpdu_flags;

static int hf_bpdu_flags_tcack;

static int hf_bpdu_flags_agreement;

static int hf_bpdu_flags_forwarding;

static int hf_bpdu_flags_learning;

static int hf_bpdu_flags_port_role;

static int hf_bpdu_flags_proposal;

static int hf_bpdu_flags_tc;

static int hf_bpdu_root_prio;

static int hf_bpdu_root_sys_id_ext;

static int hf_bpdu_root_mac;

static int hf_bpdu_root_cost;

static int hf_bpdu_bridge_prio;

static int hf_bpdu_bridge_sys_id_ext;

static int hf_bpdu_bridge_mac;

static int hf_bpdu_port_id;

static int hf_bpdu_msg_age;

static int hf_bpdu_max_age;

static int hf_bpdu_hello_time;

static int hf_bpdu_forward_delay;

static int hf_bpdu_version_1_length;

static int hf_bpdu_version_3_length;

static int hf_bpdu_mst_config_format_selector;

static int hf_bpdu_mst_config_name;

static int hf_bpdu_mst_config_revision_level;

static int hf_bpdu_mst_config_digest;

static int hf_bpdu_cist_internal_root_path_cost;

static int hf_bpdu_cist_bridge_prio;

static int hf_bpdu_cist_bridge_sys_id_ext;

static int hf_bpdu_cist_bridge_mac;

static int hf_bpdu_cist_remaining_hops;

static int hf_bpdu_msti_flags;

static int hf_bpdu_msti_id;

static int hf_bpdu_msti_id_FFF;

static int hf_bpdu_mst_priority;

static int hf_bpdu_msti_regional_root_id;

static int hf_bpdu_msti_regional_root_mac;

static int hf_bpdu_msti_internal_root_path_cost;

static int hf_bpdu_msti_bridge_identifier_priority;

static int hf_bpdu_msti_port_identifier_priority;

static int hf_bpdu_msti_port_id;

static int hf_bpdu_msti_bridge_id;

static int hf_bpdu_msti_bridge_id_priority;

static int hf_bpdu_msti_bridge_id_mac;

static int hf_bpdu_msti_remaining_hops;

static int hf_bpdu_version_4_length;

static int hf_bpdu_spt_config_format_selector;

static int hf_bpdu_spt_config_name;

static int hf_bpdu_spt_config_revision_level;

static int hf_bpdu_spt_config_digest;

static int hf_bpdu_flags_agree_num;

static int hf_bpdu_flags_dagree_num;

static int hf_bpdu_flags_agree_valid;

static int hf_bpdu_flags_restricted_role;

static int hf_bpdu_spt_agreement_digest;

static int hf_bpdu_agreement_digest_format_id;

static int hf_bpdu_agreement_digest_format_capabilities;

static int hf_bpdu_agreement_digest_convention_id;

static int hf_bpdu_agreement_digest_convention_capabilities;

static int hf_bpdu_agreement_digest_edge_count;

static int hf_bpdu_pvst_tlvtype;

static int hf_bpdu_pvst_tlvlength;

static int hf_bpdu_pvst_tlvvalue;

static int hf_bpdu_pvst_tlv_origvlan;

static int ett_bpdu;

static int ett_bpdu_flags;

static int ett_root_id;

static int ett_bridge_id;

static int ett_mstp;

static int ett_msti;

static int ett_cist_bridge_id;

static int ett_spt;

static int ett_aux_mcid;

static int ett_agreement;

static int ett_bpdu_pvst_tlv;

static expert_field ei_pvst_tlv_length_invalid;

static expert_field ei_pvst_tlv_origvlan_missing;

static expert_field ei_pvst_tlv_truncated;

static expert_field ei_pvst_tlv_unknown;

static expert_field ei_bpdu_type;

static expert_field ei_bpdu_version_support;

static bool bpdu_use_system_id_extensions = true;

static dissector_handle_t gvrp_handle;

static dissector_handle_t gmrp_handle;

static dissector_handle_t bpdu_handle;

static dissector_handle_t bpdu_cisco_handle;

static const value_string protocol_id_vals[] = {

  { 0, "Spanning Tree Protocol" },

  { 0, NULL }

};

#define BPDU_PVST_TLV_ORIGVLAN  0               /* Originating VLAN TLV in Cisco (R)PVST+ BPDUs */

static const value_string bpdu_pvst_tlv_vals[] = {

  { BPDU_PVST_TLV_ORIGVLAN,             "Originating VLAN" },

  { 0,                                  NULL }

};

#define BPDU_TYPE_CONF                  0x00    /* STP Configuration BPDU */

#define BPDU_TYPE_RST                   0x02    /* RST BPDU (or MST) */

#define BPDU_TYPE_TOPOLOGY_CHANGE       0x80    /* STP TCN (Topology change notify) BPDU */

static const value_string bpdu_type_vals[] = {

  { BPDU_TYPE_CONF,            "Configuration" },

  { BPDU_TYPE_RST,             "Rapid/Multiple Spanning Tree" },

  { BPDU_TYPE_TOPOLOGY_CHANGE, "Topology Change Notification" },

  { 0,                         NULL }

};

#define PROTO_VERSION_STP       0

#define PROTO_VERSION_RSTP      2

#define PROTO_VERSION_MSTP      3

#define PROTO_VERSION_SPB       4

#define MSTI_FORMAT_UNKNOWN     0

#define MSTI_FORMAT_IEEE_8021S  1

#define MSTI_FORMAT_ALTERNATIVE 2

static const value_string version_id_vals[] = {

  { PROTO_VERSION_STP,  "Spanning Tree" },

  { PROTO_VERSION_RSTP, "Rapid Spanning Tree" },

  { PROTO_VERSION_MSTP, "Multiple Spanning Tree" },

  { PROTO_VERSION_SPB,  "Shortest Path Tree" },

  { 0,                  NULL}

};

static const value_string role_vals[] = {

  { 1, "Alternate or Backup" },

  { 2, "Root" },

  { 3, "Designated" },

  { 0, NULL }

};

static void

dissect_bpdu_pvst_tlv(packet_info *pinfo, proto_tree *tree, tvbuff_t *tvb) {

  bool pvst_tlv_origvlan_present = false;

  uint16_t tlv_type, tlv_length;

  int offset = BPDU_PVST_TLV;

  proto_item * ti = NULL;

  proto_item * tlv_length_item = NULL;

  proto_tree * tlv_tree = NULL;

  if (tvb_reported_length_remaining(tvb, offset) < 4) /* TLV Type and Length fields occupy 4 bytes in total */

    expert_add_info(pinfo, tree, &ei_pvst_tlv_truncated);

  while (tvb_reported_length_remaining(tvb, offset) >= 4) { /* TLV Type and Length fields occupy 4 bytes in total */

    tlv_type = tvb_get_ntohs(tvb, offset);

    tlv_length = tvb_get_ntohs(tvb, offset + 2);

    tlv_tree = proto_tree_add_subtree(tree, tvb, offset, 4 + tlv_length,

                        ett_bpdu_pvst_tlv, &ti,

                        val_to_str(pinfo->pool, tlv_type, bpdu_pvst_tlv_vals, "Unknown TLV type: 0x%04x"));

    proto_tree_add_item(tlv_tree, hf_bpdu_pvst_tlvtype, tvb, offset, 2, ENC_BIG_ENDIAN);

    tlv_length_item = proto_tree_add_item(tlv_tree, hf_bpdu_pvst_tlvlength,

                        tvb, offset + 2, 2, ENC_BIG_ENDIAN);

    if (tvb_reported_length_remaining(tvb, offset + 4) < tlv_length) {

      expert_add_info(pinfo, tlv_length_item, &ei_pvst_tlv_truncated);

      break;

    }

    offset += 4;

    switch (tlv_type) {

      case BPDU_PVST_TLV_ORIGVLAN:

        if (tlv_length == 2) { /* Originating VLAN ID must be 2 bytes long */

          proto_item_append_text(ti, " (PVID): %u", tvb_get_ntohs(tvb, offset));

          proto_tree_add_item(tlv_tree, hf_bpdu_pvst_tlv_origvlan, tvb, offset, tlv_length, ENC_BIG_ENDIAN);

          pvst_tlv_origvlan_present = true;

        }

        else

          expert_add_info(pinfo, tlv_length_item, &ei_pvst_tlv_length_invalid);

        break;

      default:

        proto_tree_add_item(tlv_tree, hf_bpdu_pvst_tlvvalue, tvb, offset, tlv_length, ENC_NA);

        expert_add_info(pinfo, tlv_tree, &ei_pvst_tlv_unknown);

        break;

    }

    offset += tlv_length;

  }

  if (pvst_tlv_origvlan_present == false) /* If a (R)PVST+ BPDU lacks the Originating VLAN TLV, it is malformed */

    expert_add_info(pinfo, tree, &ei_pvst_tlv_origvlan_missing);

}

static void

dissect_bpdu(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, bool is_bpdu_pvst)

{

  uint16_t protocol_identifier;

  uint8_t protocol_version_identifier;

  uint8_t bpdu_type;

  uint8_t flags;

  uint16_t root_identifier_bridge_priority;

  uint16_t root_identifier_system_id_extension = 0;

  const char *root_identifier_mac_str;

  uint32_t root_path_cost;

  uint16_t bridge_identifier_bridge_priority;

  uint16_t bridge_identifier_system_id_extension = 0;

  const char *bridge_identifier_mac_str;

  uint16_t port_identifier;

  double message_age;

  double max_age;

  double hello_time;

  double forward_delay;

  uint8_t version_1_length;

  uint16_t version_3_length;

  uint16_t version_4_length = 0;

  uint16_t bpdu_version_4_length = 0;

  uint8_t config_format_selector;

  uint16_t cist_bridge_identifier_bridge_priority;

  uint16_t cist_bridge_identifier_system_id_extension = 0;

  const char *cist_bridge_identifier_mac_str;

  uint32_t msti_regional_root_mstid, msti_regional_root_priority;

  const char *msti_regional_root_mac_str;

  uint16_t msti_bridge_identifier_priority, msti_port_identifier_priority;

  int   total_msti_length, offset, msti_format;

  int   msti_length_remaining;

  int spt_offset = 0;

  int MCID_LEN = 51;

  uint8_t spt_agree_data = 0;

  proto_tree *bpdu_tree;

  proto_tree *mstp_tree, *msti_tree, *spt_tree = NULL, *aux_mcid_tree = NULL, *agreement_tree = NULL;

  proto_item *bpdu_item;

  proto_item *agreement_item, *proto_id_item, *type_item;

  proto_tree *root_id_tree;

  proto_tree *bridge_id_tree;

  proto_tree *cist_bridge_id_tree;

  static int * const bpdu_flags[] = {

    &hf_bpdu_flags_tcack,

    &hf_bpdu_flags_tc,

    NULL

  };

  static int * const rst_flags[] = {

    &hf_bpdu_flags_tcack,

    &hf_bpdu_flags_agreement,

    &hf_bpdu_flags_forwarding,

    &hf_bpdu_flags_learning,

    &hf_bpdu_flags_port_role,

    &hf_bpdu_flags_proposal,

    &hf_bpdu_flags_tc,

    NULL

  };

  /* GARP application frames require special interpretation of the

     destination address field; otherwise, they will be mistaken as

     BPDU frames.

     Fortunately, they can be recognized by checking the first 6 octets

     of the destination address, which are in the range from

     01-80-C2-00-00-20 to 01-80-C2-00-00-2F.

     Yes - we *do* need to check the destination address type;

     on Linux cooked captures, there *is* no destination address,

     so it's AT_NONE. */

  if (pinfo->dl_dst.type == AT_ETHER) {

    const uint8_t *dstaddr;

    dstaddr = (const uint8_t *)pinfo->dl_dst.data;

    if(dstaddr[0] == 0x01 && dstaddr[1] == 0x80 &&

       dstaddr[2] == 0xC2 && dstaddr[3] == 0x00 &&

       dstaddr[4] == 0x00 && ((dstaddr[5] == 0x0D) || ((dstaddr[5] & 0xF0) == 0x20))) {

      switch (dstaddr[5]) {

      case 0x20:

        /* for GMRP */

        call_dissector(gmrp_handle, tvb, pinfo, tree);

        return;

      case 0x21:

      case 0x0D:

        /* for GVRP */

        call_dissector(gvrp_handle, tvb, pinfo, tree);

        return;

      }

      col_set_str(pinfo->cinfo, COL_PROTOCOL, "GARP");

        /* Generic Attribute Registration Protocol */

      col_add_fstr(pinfo->cinfo, COL_INFO,

                   "Unknown GARP application (0x%02X)",

                   dstaddr[5]);

      return;

    }

  }

  col_set_str(pinfo->cinfo, COL_PROTOCOL, "STP"); /* Spanning Tree Protocol */

  col_clear(pinfo->cinfo, COL_INFO);

  bpdu_type = tvb_get_uint8(tvb, BPDU_TYPE);

  protocol_version_identifier = tvb_get_uint8(tvb, BPDU_VERSION_IDENTIFIER);

  switch (bpdu_type) {

  case BPDU_TYPE_CONF:

  case BPDU_TYPE_RST:

    flags = tvb_get_uint8(tvb, BPDU_FLAGS);

    root_identifier_bridge_priority = tvb_get_ntohs(tvb,BPDU_ROOT_IDENTIFIER);

    if (bpdu_use_system_id_extensions ) {

      root_identifier_system_id_extension = root_identifier_bridge_priority & 0x0fff;

      root_identifier_bridge_priority &= 0xf000;

    }

    root_identifier_mac_str = tvb_ether_to_str(pinfo->pool, tvb, BPDU_ROOT_IDENTIFIER + 2);

    root_path_cost = tvb_get_ntohl(tvb, BPDU_ROOT_PATH_COST);

    port_identifier = tvb_get_ntohs(tvb, BPDU_PORT_IDENTIFIER);

    break;

  default:

    /* Squelch GCC complaints. */

    flags = 0;

    root_identifier_bridge_priority = 0;

    root_identifier_mac_str = NULL;

    root_path_cost = 0;

    port_identifier = 0;

    break;

  }

  switch (bpdu_type) {

  case BPDU_TYPE_CONF:

    if (bpdu_use_system_id_extensions ) {

      col_add_fstr(pinfo->cinfo, COL_INFO,

                   "Conf. %sRoot = %d/%d/%s  Cost = %d  Port = 0x%04x",

                   flags & 0x1 ? "TC + " : "",

                   root_identifier_bridge_priority,

                   root_identifier_system_id_extension,

                   root_identifier_mac_str,

                   root_path_cost, port_identifier);

    } else {

      col_add_fstr(pinfo->cinfo, COL_INFO,

                   "Conf. %sRoot = %d/%s  Cost = %d  Port = 0x%04x",

                   flags & 0x1 ? "TC + " : "",

                   root_identifier_bridge_priority, root_identifier_mac_str,

                   root_path_cost, port_identifier);

    }

    break;

  case BPDU_TYPE_TOPOLOGY_CHANGE:

    col_set_str(pinfo->cinfo, COL_INFO, "Topology Change Notification");

    break;

  case BPDU_TYPE_RST:

    if (bpdu_use_system_id_extensions ) {

      col_add_fstr(pinfo->cinfo, COL_INFO,

                   "%sT. %sRoot = %d/%d/%s  Cost = %d  Port = 0x%04x",

                     protocol_version_identifier == 3 ? "MS":

                     (protocol_version_identifier == 4 ? "SP":"RS"),

                     flags & 0x1 ? "TC + " : "",

                     root_identifier_bridge_priority,

                     root_identifier_system_id_extension,

                     root_identifier_mac_str,

                     root_path_cost, port_identifier);

    } else {

      col_add_fstr(pinfo->cinfo, COL_INFO,

                   "%sT. %sRoot = %d/%s  Cost = %d  Port = 0x%04x",

                   protocol_version_identifier == 3 ? "MS":

                   (protocol_version_identifier == 4 ? "SP":"RS"),

                   flags & 0x1 ? "TC + " : "",

                   root_identifier_bridge_priority, root_identifier_mac_str,

                   root_path_cost, port_identifier);

    }

    break;

  default:

    col_add_fstr(pinfo->cinfo, COL_INFO, "Unknown BPDU type (%u)", bpdu_type);

    break;

  }

    bpdu_item = proto_tree_add_protocol_format(tree, proto_bpdu, tvb, 0, -1,

                                               "Spanning Tree Protocol");

    bpdu_tree = proto_item_add_subtree(bpdu_item, ett_bpdu);

    protocol_identifier = tvb_get_ntohs(tvb, BPDU_IDENTIFIER);

    proto_tree_add_uint(bpdu_tree, hf_bpdu_proto_id, tvb, BPDU_IDENTIFIER, 2,

                        protocol_identifier);

    proto_id_item = proto_tree_add_uint(bpdu_tree, hf_bpdu_version_id, tvb,

                        BPDU_VERSION_IDENTIFIER, 1,

                        protocol_version_identifier);

    switch (protocol_version_identifier) {

    case 0:

      break;

    case 2:

    case 3:

    case 4:

      break;

    default:

      expert_add_info(pinfo, proto_id_item, &ei_bpdu_version_support);

      break;

    }

    type_item = proto_tree_add_uint(bpdu_tree, hf_bpdu_type, tvb, BPDU_TYPE, 1, bpdu_type);

    if (bpdu_type == BPDU_TYPE_TOPOLOGY_CHANGE) {

      set_actual_length(tvb, TC_BPDU_SIZE);

      return;

    }

    if (bpdu_type != BPDU_TYPE_CONF && bpdu_type != BPDU_TYPE_RST) {

      /* Unknown BPDU type - just display the rest of the PDU as data */

      expert_add_info(pinfo, type_item, &ei_bpdu_type);

      return;

    }

    bridge_identifier_bridge_priority = tvb_get_ntohs(tvb, BPDU_BRIDGE_IDENTIFIER);

    if (bpdu_use_system_id_extensions ) {

      bridge_identifier_system_id_extension = bridge_identifier_bridge_priority & 0x0fff;

      bridge_identifier_bridge_priority &= 0xf000;

    }

    bridge_identifier_mac_str = tvb_ether_to_str(pinfo->pool, tvb, BPDU_BRIDGE_IDENTIFIER + 2);

    if (bpdu_type == BPDU_TYPE_RST) {

       proto_tree_add_bitmask_value_with_flags(bpdu_tree, tvb, BPDU_FLAGS, hf_bpdu_flags, ett_bpdu_flags, rst_flags, flags, BMT_NO_FALSE|BMT_NO_TFS);

    } else {

       proto_tree_add_bitmask_value_with_flags(bpdu_tree, tvb, BPDU_FLAGS, hf_bpdu_flags, ett_bpdu_flags, bpdu_flags, flags, BMT_NO_FALSE|BMT_NO_TFS);

    }

    /* add Identifier with format based on preference value

     * bpdu_use_system_id_extensions

     * */

    if (bpdu_use_system_id_extensions) {

      root_id_tree = proto_tree_add_subtree_format(bpdu_tree, tvb,

                                         BPDU_ROOT_IDENTIFIER, 8,

                                         ett_root_id, NULL,

                                         "Root Identifier: %d / %d / %s",

                                         root_identifier_bridge_priority,

                                         root_identifier_system_id_extension,

                                         root_identifier_mac_str);

      proto_tree_add_uint(root_id_tree, hf_bpdu_root_prio, tvb,

                          BPDU_ROOT_IDENTIFIER , 1,

                          root_identifier_bridge_priority);

      proto_tree_add_uint(root_id_tree, hf_bpdu_root_sys_id_ext, tvb,

                          BPDU_ROOT_IDENTIFIER , 2,

                          root_identifier_system_id_extension);

      proto_tree_add_item(root_id_tree, hf_bpdu_root_mac, tvb,

                          BPDU_ROOT_IDENTIFIER + 2, 6, ENC_NA);

    } else {

      root_id_tree = proto_tree_add_subtree_format(bpdu_tree, tvb,

                                         BPDU_ROOT_IDENTIFIER, 8,

                                         ett_root_id, NULL,

                                         "Root Identifier: %d / %s",

                                         root_identifier_bridge_priority,

                                         root_identifier_mac_str);

      proto_tree_add_uint(root_id_tree, hf_bpdu_root_prio, tvb,

                          BPDU_ROOT_IDENTIFIER , 2,

                          root_identifier_bridge_priority);

      proto_tree_add_item(root_id_tree, hf_bpdu_root_mac, tvb,

                          BPDU_ROOT_IDENTIFIER + 2, 6, ENC_NA);

    }

    /* end of Identifier formatting */

    proto_tree_add_uint(bpdu_tree, hf_bpdu_root_cost, tvb,

                        BPDU_ROOT_PATH_COST, 4, root_path_cost);

    /* add Identifier with format based on preference value

     * bpdu_use_system_id_extensions

     * */

    if (bpdu_use_system_id_extensions) {

      bridge_id_tree = proto_tree_add_subtree_format(bpdu_tree, tvb,

                                           BPDU_BRIDGE_IDENTIFIER, 8,

                                           ett_bridge_id, NULL,

                                           "Bridge Identifier: %d / %d / %s",

                                           bridge_identifier_bridge_priority,

                                           bridge_identifier_system_id_extension,

                                           bridge_identifier_mac_str);

      proto_tree_add_uint(bridge_id_tree, hf_bpdu_bridge_prio, tvb,

                          BPDU_BRIDGE_IDENTIFIER , 1,

                          bridge_identifier_bridge_priority);

      proto_tree_add_uint(bridge_id_tree, hf_bpdu_bridge_sys_id_ext, tvb,

                          BPDU_BRIDGE_IDENTIFIER , 2,

                          bridge_identifier_system_id_extension);

      proto_tree_add_item(bridge_id_tree, hf_bpdu_bridge_mac, tvb,

                          BPDU_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

    } else {

      bridge_id_tree = proto_tree_add_subtree_format(bpdu_tree, tvb,

                                           BPDU_BRIDGE_IDENTIFIER, 8,

                                           ett_bridge_id, NULL,

                                           "Bridge Identifier: %d / %s",

                                           bridge_identifier_bridge_priority,

                                           bridge_identifier_mac_str);

      proto_tree_add_uint(bridge_id_tree, hf_bpdu_bridge_prio, tvb,

                          BPDU_BRIDGE_IDENTIFIER , 2,

                          bridge_identifier_bridge_priority);

      proto_tree_add_item(bridge_id_tree, hf_bpdu_bridge_mac, tvb,

                          BPDU_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

    }

    /* end of Identifier formatting */

    proto_tree_add_uint(bpdu_tree, hf_bpdu_port_id, tvb,

                        BPDU_PORT_IDENTIFIER, 2, port_identifier);

    message_age = tvb_get_ntohs(tvb, BPDU_MESSAGE_AGE) / 256.0;

    proto_tree_add_double(bpdu_tree, hf_bpdu_msg_age, tvb, BPDU_MESSAGE_AGE, 2,

                          message_age);

    max_age = tvb_get_ntohs(tvb, BPDU_MAX_AGE) / 256.0;

    proto_tree_add_double(bpdu_tree, hf_bpdu_max_age, tvb, BPDU_MAX_AGE, 2,

                          max_age);

    hello_time = tvb_get_ntohs(tvb, BPDU_HELLO_TIME) / 256.0;

    proto_tree_add_double(bpdu_tree, hf_bpdu_hello_time, tvb,

                          BPDU_HELLO_TIME, 2, hello_time);

    forward_delay = tvb_get_ntohs(tvb, BPDU_FORWARD_DELAY) / 256.0;

    proto_tree_add_double(bpdu_tree, hf_bpdu_forward_delay, tvb,

                          BPDU_FORWARD_DELAY, 2, forward_delay);

    if (bpdu_type == BPDU_TYPE_CONF) {

      if (is_bpdu_pvst)

        dissect_bpdu_pvst_tlv(pinfo, bpdu_tree, tvb);

      /* Nothing more in this BPDU */

      set_actual_length(tvb, CONF_BPDU_SIZE);

      return;

    }

    /* RST or MST BPDU */

    version_1_length = tvb_get_uint8(tvb, BPDU_VERSION_1_LENGTH);

    proto_tree_add_uint(bpdu_tree, hf_bpdu_version_1_length, tvb,

                        BPDU_VERSION_1_LENGTH, 1, version_1_length);

    /* Is this an MST BPDU? */

    if (protocol_version_identifier >= 3 && version_1_length == 0 &&

        tvb_reported_length(tvb) >= 102) {

      /*

       * OK, it passes the "Protocol Identifier is 0000 0000

       * 0000 0000", "Protocol Version Identifier is 3 or

       * greater", "BPDU Type is 0000 0010", "contains 102 or

       * more octets", and "a Version 1 Length of 0" tests.

       */

      version_3_length = tvb_get_ntohs(tvb, BPDU_VERSION_3_LENGTH);

      proto_tree_add_uint(bpdu_tree, hf_bpdu_version_3_length, tvb,

                          BPDU_VERSION_3_LENGTH, 2, version_3_length);

      /*

       * Check the Version 3 Length, and see whether it's a

       * multiple of the MSTI Configuration Message length. Also

       * check the config_format_selector because some MST BPDU's

       * have BPDU_VERSION_3_LENGTH set to 0 and use the

       * field BPDU_MST_CONFIG_FORMAT_SELECTOR as a length-field

       * for the MSTI data.

       */

      config_format_selector = tvb_get_uint8(tvb, BPDU_MST_CONFIG_FORMAT_SELECTOR);

      if (version_3_length != 0) {

        msti_format = MSTI_FORMAT_IEEE_8021S;

        if (version_3_length >= VERSION_3_STATIC_LENGTH) {

          total_msti_length = version_3_length - VERSION_3_STATIC_LENGTH;

        } else {

          /*

           * XXX - there appears to be an ambiguity in the 802.3Q-2003

           * standard and at least some of the 802.3s drafts.

           *

           * The "Version 3 Length" field is defined to be "the number of

           * octets taken by the parameters that follow in the BPDU", but

           * it's spoken of as "representing an integral number, from 0 to

           * 64 inclusive, of MSTI Configuration Messages".

           *

           * According to mail from a member of the stds-802-1@ieee.org list,

           * the latter of those is just saying that the length must not have

           * a value that implies that there's a partial MSTI message in the

           * packet; it's still in units of octets, not messages.

           *

           * However, it appears that Cisco's C3550 software (C3550-I5Q3L2-M,

           * Version 12.1(12c)EA1) might be sending out lengths in units of

           * messages.

           *

           * This length can't be the number of octets taken by the parameters

           * that follow in the BPDU, because it's less than the fixed-length

           * portion of those parameters, so we assume the length is a count of

           * messages.

           */

          total_msti_length = version_3_length * MSTI_MESSAGE_SIZE;

        }

      } else {

        if (tvb_reported_length(tvb) == (unsigned)config_format_selector + MST_BPDU_SIZE + 1 ) {

          msti_format = MSTI_FORMAT_ALTERNATIVE;

          total_msti_length = config_format_selector - VERSION_3_STATIC_LENGTH;

        } else {

          /*

           * XXX - Unknown MSTI format, since version_3_length is 0

           * lets assume there are no msti instances in the packet.

           */

          msti_format = MSTI_FORMAT_UNKNOWN;

          total_msti_length = 0;

        }

      }

      if (protocol_version_identifier == 3) {

        set_actual_length(tvb, BPDU_MSTI + total_msti_length);

      }

      mstp_tree = proto_tree_add_subtree(bpdu_tree, tvb, BPDU_VERSION_3_LENGTH,

                                      -1, ett_mstp, NULL, "MST Extension");

      proto_tree_add_item(mstp_tree, hf_bpdu_mst_config_format_selector, tvb,

                          BPDU_MST_CONFIG_FORMAT_SELECTOR, 1, ENC_BIG_ENDIAN);

      proto_tree_add_item(mstp_tree, hf_bpdu_mst_config_name, tvb,

                          BPDU_MST_CONFIG_NAME, 32, ENC_ASCII|ENC_NA);

      proto_tree_add_item(mstp_tree, hf_bpdu_mst_config_revision_level, tvb,

                          BPDU_MST_CONFIG_REVISION_LEVEL, 2, ENC_BIG_ENDIAN);

      proto_tree_add_item(mstp_tree, hf_bpdu_mst_config_digest, tvb,

                          BPDU_MST_CONFIG_DIGEST, 16, ENC_NA);

      switch(msti_format) {

      case MSTI_FORMAT_IEEE_8021S:

        proto_tree_add_item(mstp_tree, hf_bpdu_cist_internal_root_path_cost, tvb,

                            BPDU_CIST_INTERNAL_ROOT_PATH_COST, 4, ENC_BIG_ENDIAN);

        cist_bridge_identifier_bridge_priority = tvb_get_ntohs(tvb,BPDU_CIST_BRIDGE_IDENTIFIER);

        cist_bridge_identifier_mac_str = tvb_ether_to_str(pinfo->pool, tvb, BPDU_CIST_BRIDGE_IDENTIFIER + 2);

        /* add Identifier with format based on preference value

         * bpdu_use_system_id_extensions

         * */

        if (bpdu_use_system_id_extensions ) {

          cist_bridge_identifier_system_id_extension = cist_bridge_identifier_bridge_priority & 0x0fff;

          cist_bridge_identifier_bridge_priority &= 0xf000;

          cist_bridge_id_tree = proto_tree_add_subtree_format(mstp_tree, tvb,

                                                    BPDU_CIST_BRIDGE_IDENTIFIER, 8,

                                                    ett_cist_bridge_id, NULL,

                                                    "CIST Bridge Identifier: %d / %d / %s",

                                                    cist_bridge_identifier_bridge_priority,

                                                    cist_bridge_identifier_system_id_extension,

                                                    cist_bridge_identifier_mac_str);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_prio, tvb,

                              BPDU_CIST_BRIDGE_IDENTIFIER , 1,

                              cist_bridge_identifier_bridge_priority);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_sys_id_ext, tvb,

                              BPDU_CIST_BRIDGE_IDENTIFIER , 2,

                              cist_bridge_identifier_system_id_extension);

          proto_tree_add_item(cist_bridge_id_tree, hf_bpdu_cist_bridge_mac, tvb,

                              BPDU_CIST_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

        } else {

          cist_bridge_id_tree = proto_tree_add_subtree_format(mstp_tree, tvb,

                                                    BPDU_CIST_BRIDGE_IDENTIFIER, 8,

                                                    ett_cist_bridge_id, NULL,

                                                    "CIST Bridge Identifier: %d / %s",

                                                    cist_bridge_identifier_bridge_priority,

                                                    cist_bridge_identifier_mac_str);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_prio, tvb,

                              BPDU_CIST_BRIDGE_IDENTIFIER , 2,

                              cist_bridge_identifier_bridge_priority);

          proto_tree_add_item(cist_bridge_id_tree, hf_bpdu_cist_bridge_mac, tvb,

                              BPDU_CIST_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

        }

        /* end of Identifier formatting */

        break;

      case MSTI_FORMAT_ALTERNATIVE:

        cist_bridge_identifier_bridge_priority = tvb_get_ntohs(tvb,ALT_BPDU_CIST_BRIDGE_IDENTIFIER);

        cist_bridge_identifier_mac_str = tvb_ether_to_str(pinfo->pool, tvb, ALT_BPDU_CIST_BRIDGE_IDENTIFIER + 2);

        /* add Identifier with format based on preference value

         * bpdu_use_system_id_extensions

         * */

        if (bpdu_use_system_id_extensions ) {

          cist_bridge_identifier_system_id_extension = cist_bridge_identifier_bridge_priority & 0x0fff;

          cist_bridge_identifier_bridge_priority &= 0xf000;

          cist_bridge_id_tree = proto_tree_add_subtree_format(mstp_tree, tvb,

                                                    ALT_BPDU_CIST_BRIDGE_IDENTIFIER, 8,

                                                    ett_cist_bridge_id, NULL,

                                                    "CIST Bridge Identifier: %d / %d / %s",

                                                    cist_bridge_identifier_bridge_priority,

                                                    cist_bridge_identifier_system_id_extension,

                                                    cist_bridge_identifier_mac_str);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_prio, tvb,

                              ALT_BPDU_CIST_BRIDGE_IDENTIFIER , 1,

                              cist_bridge_identifier_bridge_priority);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_sys_id_ext, tvb,

                              ALT_BPDU_CIST_BRIDGE_IDENTIFIER , 2,

                              cist_bridge_identifier_system_id_extension);

          proto_tree_add_item(cist_bridge_id_tree, hf_bpdu_cist_bridge_mac, tvb,

                              ALT_BPDU_CIST_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

        } else {

          cist_bridge_id_tree = proto_tree_add_subtree_format(mstp_tree, tvb,

                                                    ALT_BPDU_CIST_BRIDGE_IDENTIFIER, 8,

                                                    ett_cist_bridge_id, NULL,

                                                    "CIST Bridge Identifier: %d / %s",

                                                    cist_bridge_identifier_bridge_priority,

                                                    cist_bridge_identifier_mac_str);

          proto_tree_add_uint(cist_bridge_id_tree, hf_bpdu_cist_bridge_prio, tvb,

                              ALT_BPDU_CIST_BRIDGE_IDENTIFIER , 2,

                              cist_bridge_identifier_bridge_priority);

          proto_tree_add_item(cist_bridge_id_tree, hf_bpdu_cist_bridge_mac, tvb,

                              ALT_BPDU_CIST_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

        }

        /* end of Identifier formatting */

        proto_tree_add_item(mstp_tree, hf_bpdu_cist_internal_root_path_cost, tvb,

                            ALT_BPDU_CIST_INTERNAL_ROOT_PATH_COST, 4, ENC_BIG_ENDIAN);

        break;

      }

      proto_tree_add_item(mstp_tree, hf_bpdu_cist_remaining_hops, tvb,

                          BPDU_CIST_REMAINING_HOPS, 1, ENC_BIG_ENDIAN);

      /* MSTI messages */

      offset = BPDU_MSTI;

      msti_length_remaining = total_msti_length;

      while (msti_length_remaining > 0) {

        switch(msti_format) {

        case MSTI_FORMAT_IEEE_8021S:

          msti_regional_root_mstid = tvb_get_uint8(tvb,  offset+ MSTI_REGIONAL_ROOT);

          msti_regional_root_priority = (msti_regional_root_mstid &0xf0) << 8;

          msti_regional_root_mstid = ((msti_regional_root_mstid & 0x0f) << 8) +

                                     tvb_get_uint8(tvb,  offset+ MSTI_REGIONAL_ROOT+1);

          msti_regional_root_mac_str = tvb_ether_to_str(pinfo->pool, tvb, offset + MSTI_REGIONAL_ROOT + 2);

          msti_tree = proto_tree_add_subtree_format(mstp_tree, tvb, offset, 16, ett_msti, NULL,

                                          "MSTID %d, Regional Root Identifier %d / %s",

                                          msti_regional_root_mstid,

                                          msti_regional_root_priority,

                                          msti_regional_root_mac_str);

          /* flags */

          proto_tree_add_bitmask_with_flags(msti_tree, tvb, offset+MSTI_FLAGS, hf_bpdu_msti_flags, ett_bpdu_flags, rst_flags, ENC_NA, BMT_NO_FALSE|BMT_NO_TFS);

          /* pri, MSTID, Regional root */

          proto_tree_add_item(msti_tree, hf_bpdu_mst_priority, tvb, offset + MSTI_REGIONAL_ROOT, 1, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_id_FFF, tvb, offset + MSTI_REGIONAL_ROOT, 2, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_regional_root_mac, tvb,

                                            offset + MSTI_REGIONAL_ROOT + 2, 6, ENC_NA);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_internal_root_path_cost, tvb,

                              offset+MSTI_INTERNAL_ROOT_PATH_COST, 4, ENC_BIG_ENDIAN);

          msti_bridge_identifier_priority = tvb_get_uint8(tvb, offset+MSTI_BRIDGE_IDENTIFIER_PRIORITY) >> 4;

          msti_port_identifier_priority = tvb_get_uint8(tvb, offset+MSTI_PORT_IDENTIFIER_PRIORITY) >> 4;

          proto_tree_add_uint(msti_tree, hf_bpdu_msti_bridge_identifier_priority, tvb,

                              offset+MSTI_BRIDGE_IDENTIFIER_PRIORITY, 1,

                              msti_bridge_identifier_priority);

          proto_tree_add_uint(msti_tree, hf_bpdu_msti_port_identifier_priority, tvb,

                              offset+MSTI_PORT_IDENTIFIER_PRIORITY, 1,

                              msti_port_identifier_priority);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_remaining_hops, tvb,

                              offset + MSTI_REMAINING_HOPS, 1, ENC_BIG_ENDIAN);

          msti_length_remaining -= MSTI_MESSAGE_SIZE;

          offset += MSTI_MESSAGE_SIZE;

          break;

        case MSTI_FORMAT_ALTERNATIVE:

          msti_regional_root_mstid = tvb_get_uint8(tvb,  offset+ ALT_MSTI_REGIONAL_ROOT);

          msti_regional_root_priority = (msti_regional_root_mstid &0xf0) << 8;

          msti_regional_root_mstid = ((msti_regional_root_mstid & 0x0f) << 8) +

                                     tvb_get_uint8(tvb,  offset+ ALT_MSTI_REGIONAL_ROOT+1);

          msti_regional_root_mac_str = tvb_ether_to_str(pinfo->pool, tvb, offset+ ALT_MSTI_REGIONAL_ROOT + 2);

          msti_tree = proto_tree_add_subtree_format(mstp_tree, tvb, offset, 16, ett_msti, NULL,

                                          "MSTID %d, Regional Root Identifier %d / %s",

                                          msti_regional_root_mstid,

                                          msti_regional_root_priority,

                                          msti_regional_root_mac_str);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_id, tvb, offset+ALT_MSTI_MSTID, 2, ENC_BIG_ENDIAN);

          /* flags */

          proto_tree_add_bitmask_with_flags(msti_tree, tvb, offset+ALT_MSTI_FLAGS, hf_bpdu_msti_flags, ett_bpdu_flags, rst_flags, ENC_NA, BMT_NO_FALSE|BMT_NO_TFS);

          /* pri, MSTID, Regional root */

          proto_tree_add_item(msti_tree, hf_bpdu_mst_priority, tvb, offset + ALT_MSTI_REGIONAL_ROOT, 1, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_regional_root_id, tvb, offset + ALT_MSTI_REGIONAL_ROOT, 2, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_regional_root_mac, tvb,

                                            offset + ALT_MSTI_REGIONAL_ROOT + 2, 6, ENC_NA);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_internal_root_path_cost, tvb,

                              offset+ALT_MSTI_INTERNAL_ROOT_PATH_COST, 4, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_bridge_id, tvb, offset + ALT_MSTI_BRIDGE_IDENTIFIER, 2, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_bridge_id_priority, tvb, offset + ALT_MSTI_BRIDGE_IDENTIFIER, 2, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_bridge_id_mac, tvb,

                                            offset + ALT_MSTI_BRIDGE_IDENTIFIER + 2, 6, ENC_NA);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_port_id, tvb, offset+ALT_MSTI_PORT_IDENTIFIER, 2, ENC_BIG_ENDIAN);

          proto_tree_add_item(msti_tree, hf_bpdu_msti_remaining_hops, tvb,

                              offset + ALT_MSTI_REMAINING_HOPS, 1, ENC_BIG_ENDIAN);

          msti_length_remaining -= ALT_MSTI_MESSAGE_SIZE;

          offset += ALT_MSTI_MESSAGE_SIZE;

          break;

        }

      }

      if (protocol_version_identifier >= 4 && version_1_length == 0

          && tvb_reported_length(tvb) >= 106) {

        /*

         * OK, it passes the "Protocol Identifier is 0000 0000

         * 0000 0000", "Protocol Version Identifier is 4 or

         * greater", "BPDU Type is 0000 0010", "contains 106 or

         * more octets", and "a Version 1 Length of 0" tests.

         */

        bpdu_version_4_length = BPDU_MSTI + total_msti_length;

        version_4_length = tvb_get_ntohs(tvb, bpdu_version_4_length);

        proto_tree_add_uint(bpdu_tree, hf_bpdu_version_4_length, tvb,

                            bpdu_version_4_length, 2, version_4_length);

        /* version 4 length is 55 or more.

         */

        if (version_4_length >= 53) {

          static int * const agreements[] = {

              &hf_bpdu_flags_agree_num,

              &hf_bpdu_flags_dagree_num,

              &hf_bpdu_flags_agree_valid,

              &hf_bpdu_flags_restricted_role,

              NULL

          };

          spt_tree = proto_tree_add_subtree(bpdu_tree, tvb, bpdu_version_4_length, -1,

              ett_spt, NULL, "SPT Extension");

          spt_offset = (bpdu_version_4_length + 2);

          /* Aux MCID: */

          aux_mcid_tree = proto_tree_add_subtree(spt_tree, tvb, spt_offset,

                                              MCID_LEN, ett_aux_mcid, NULL, "MCID Data");

          proto_tree_add_item(aux_mcid_tree,

                              hf_bpdu_spt_config_format_selector, tvb, spt_offset, 1,

                              ENC_BIG_ENDIAN);

          proto_tree_add_item(aux_mcid_tree, hf_bpdu_spt_config_name, tvb,

                              spt_offset + 1, 32, ENC_ASCII | ENC_NA);

          proto_tree_add_item(aux_mcid_tree,

                              hf_bpdu_spt_config_revision_level, tvb, spt_offset + 33,

                              2, ENC_BIG_ENDIAN);

          proto_tree_add_item(aux_mcid_tree, hf_bpdu_spt_config_digest,

                              tvb, spt_offset + 35, 16, ENC_NA);

          spt_offset += MCID_LEN;

          /* Agreement Data */

          agreement_tree = proto_tree_add_subtree(spt_tree, tvb, spt_offset,

                                               -1, ett_agreement, &agreement_item, "Agreement Data");

          spt_agree_data = tvb_get_uint8(tvb, spt_offset);

          proto_item_append_text(agreement_item, " (AN: %d", (spt_agree_data & 0x03));

          proto_tree_add_bitmask_list_value(agreement_tree, tvb, spt_offset, 1, agreements, spt_agree_data);

          proto_item_append_text(agreement_item, ", DAN: %d)", ((spt_agree_data & 0x0C) >> 2));

          spt_offset += 2;

          proto_tree_add_item(agreement_tree, hf_bpdu_agreement_digest_format_id, tvb, spt_offset, 1, ENC_NA);

          proto_tree_add_item(agreement_tree, hf_bpdu_agreement_digest_format_capabilities, tvb, spt_offset, 1, ENC_NA);

          spt_offset += 1;

          proto_tree_add_item(agreement_tree, hf_bpdu_agreement_digest_convention_id, tvb, spt_offset, 1, ENC_NA);

          proto_tree_add_item(agreement_tree, hf_bpdu_agreement_digest_convention_capabilities, tvb, spt_offset, 1, ENC_NA);

          spt_offset += 1;

          proto_tree_add_item(agreement_tree, hf_bpdu_agreement_digest_edge_count, tvb, spt_offset, 2, ENC_BIG_ENDIAN);

          spt_offset += 10;

          proto_tree_add_item(agreement_tree, hf_bpdu_spt_agreement_digest,

                              tvb, spt_offset, 20, ENC_NA);

          /*spt_offset += 20;*/

          if (protocol_version_identifier == 4) {

            set_actual_length(tvb, (bpdu_version_4_length + version_4_length + 2));

          }

        }

      }

    } else { /* It is RSTP BPDU and may still be Rapid PVST+ */

      if (is_bpdu_pvst)

        dissect_bpdu_pvst_tlv(pinfo, bpdu_tree, tvb);