0

There are two of port states in STP: Disabled and Blocking,and the a website explain their difference

Disabled : This port is shutdown,and it won't send or receive any frame

Blocking : This port won't send or receive any data frame,but it will still receive BPDU

Now,here is a problem,i thought BPDU is a data frame,when some switches decide who can be the root bridge,they will send this data frame,i mean BPDU,to each others to decide.But now according to the explanation above,it seems that BPDU and data frame are not the same ,because the port won't send or receive any data frame,but it will still receive BPDU when port is blocking.

Can anyone tell me the difference between data frame and BPDU in STP??thanks a lot

user16266657
  • 91
  • 3
  • Did any answer help you? if so, you should accept the answer so that the question does not keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer. – Ron Maupin Dec 23 '21 at 21:48

2 Answers2

3

BPDUs are sent to a special multicast address. The IEEE has reserved the 01:80:c2 OUI for link protocols, and frames with this special multicast OUI are not allowed to be forwarded from the interface on which they are received to another interface on 802.1D compliant bridges. A standards compliant bridge (switches are bridges) will recognize a BPDU by the special BPDU multicast destination address (01:80:C2:00:00:00).

Cisco PVST does per VLAN STP, so it can block some VLAN data frames while allowing data frames for other VLANs. Cisco uses a different special multicast address (01:00:0C:CC:CC:CD) for BPDUs to accomplish that.

There are more STP states than disabled and blocking. One could argue that disabled is an interface, not STP, state. STP has blocking, listening, learning and forwarding.

Ron Maupin
  • 98,218
  • 26
  • 115
  • 191
  • Isn’t saying “Cisco PVST does per VLAN STP” redundant, given that’s literally what PVST stands for? – Jesse P. Sep 04 '21 at 20:37
  • 1
    Expanding a bit: all data on all layers are processed as data units, which on L2 are called "frames". There is no difference in the structure of the STP Bridge Protocol Data Unit compared to any other L2 data units, there's header containing destination and sender MAC addresses and flags; payload; and CRC. The only thing telling the switch that a BPDU contains STP-related data is the specific recipient L2 address. Disabled is not an STP state at all in the sense that STP could place a port in that state - it's an administrative state manually set by the network admin. – Peregrino69 Sep 05 '21 at 06:53
  • So BPDU is part of the data frame,i mean data frame=BDPU+other data ?and when switch receive the data frame ,they will decide who can be root bridge,then transmit the data later? – user16266657 Sep 05 '21 at 23:30
  • https://networkengineering.stackexchange.com/q/49651/8499 – Ron Maupin Sep 05 '21 at 23:32
1

it seems to me like a confusion with terminology.

First, what is a frame? In OSI model each layer has a different name for a packet and at layer 2 this is called frame [*].

a better explanation from wikipedia:

In the seven-layer OSI model of computer networking, packet strictly refers to a protocol data unit at layer 3, the network layer.[citation needed] A data unit at layer 2, the data link layer, is a frame. In layer 4, the transport layer, the data units are segments and datagrams. Thus, in the example of TCP/IP communication over Ethernet, a TCP segment is carried in one or more IP packets, which are each carried in one or more Ethernet frames.

Second, what is a data frame? When talking about network protocols we usually distinguish between control packets and data packets (frames if we are talking about layer 2). Data packets carry user traffic. Control packets do not carry user traffic. They carry "stuff" that has to be exchanged between network devices in order for network to configure itself and work properly. For example in STP bridges exchange BPDUs, which are control packets, to compute the spanning tree.

BPDUs and data frames are 2 different things. BPDUs are control frames used by STP and data frames are Ethernet frames that carry user traffic.

[*] to be absolutely clear, the term packet refers to a concept of packet in packet switched networks.

Effie
  • 1,788
  • 3
  • 17
  • now i am confused. so, SDU is not a packet, SDU is what is passed from upper layer to lower. Then PDU is a packet (of a given protocol). So, data packets do encapsulate SDUs, because user traffic originally comes from application (layer). But Ethernet frame that carries BPDU does not encapsulate SDU, because STP is L2. what am i missing? – Effie Sep 08 '21 at 10:05
  • well, note paragraph is then wrong :X. thank you for your help! – Effie Sep 08 '21 at 10:18