BPDU Frame Formatting

I got around to looking at a BPDU and decided to compare it to an Ethernet frame and I noticed some differences and had some questions I couldn't find answers for.

Does a BPDU frame have a preamble?
If a BPDU doesn't have a preamble why does an Ethernet frame need one if a BPDU doesn't?
Does a BPDU frame have a CRC?
Is a BPDU frame followed by an interframe gap?
Is the frame control field the same as the start delimiter?
For per vlan stp which field is used to identify which vlan a bpdu is meant for? (llc?)




imgur.com/uIRsn

Hey,

BPDU Packets are encapsulated in Ethernet Frames! Basically you showed it on your graphic already, so you might already know about.

Does a BPDU frame have a preamble?
-Yes, since its part of the Ethernet frame

Does a BPDU frame have a CRC?
-Yes, since its part of the Ethernet frame

Is a BPDU frame followed by an interframe gap?
-Yes, since its part of the Ethernet frame

Is the frame control field the same as the start delimiter?
Wasnt sure about this myself so i looked it up in the RFC...

In the RFC it says:
On 802.4, 802.5, and FDDI LANs, there are a few octets preceding the Destination MAC Address, one of which is protected by the FCS. The MAC Type of the frame determines the contents of the Frame Control field. A pad octet is present to provide 32-bit packet alignment.

So the Frame Control field is only used when you are having Token-Ring(802.5), Token-Bus(802.4) or FDDI as Medium Access.

This is what it contains:
Frame Control
The Frame Control field indicates the frame type and contains the following:
• Frame type bit—Used to indicate whether this is a MAC or LLC frame.
• Reserved bit—Reserved for future use.
• Control bits—Used to indicate whether the frame is to be processed by the normal buffer or the high-priority
buffer.

For per vlan stp which field is used to identify which vlan a bpdu is meant for? (llc?)
-Pretty much the same stuff. For example on Ethernet it is just the 4Bytes added after Layer 2 header

hope it helped you and thanks for the question, didnt know the part about frame control yet.

-chrnxR

Hello chrnxR,

Thank you very much for the reply. I'm still a bit unsure about how this all fits together so I drew up a few diagrams to work from. At any rate you stated that:

BPDU Packets are encapsulated in Ethernet Frames!

Which to me means take a BPDU packet and stick EVERYTHING into the PAYLOAD field of an ethernet frame. This makes sense and I'm happy to go along with this but I want to be sure that is the case rather then:

"BPDU FRAMES are Ethernet Frames"

or

"BPDU FRAMES are encapsulated in Ethernet Frames"






Finally if everything isn't just simply tossed into the payload field which order does everything fall into? Does the Frame control replace the start of feild delimiter or does it just sit next to it. Does the LLC feild replace the type feild or does it just sit in the payload or is it placed directly after it.

Hey again,

first id like to differ between Frames and Packets..
For example an Ethernet Frame contains Mac header, IP/IPX header, tcp/udp header, ..., FCS
Packets are encapsulated in the payload of the Frame. So Frames contain Packets.

So BDPU Packets are encapsulated in Ethernet Frames. Or "can be encapsulated in Ethernet Frames" since if ur using Token-Ring for example its a bit different.

Now to have a look at the graphics...
(Im referring to the graphic in my post)

Graphic 1 and 2 both show the same thing basically. Thats most likely what irritated you a bit. In number 1 the Ethernet Frame is shown more detailed from preamble to Interframe gap. Even the optionally Vlan tag field is shown.
In number 2 its again an Ethernet Frame. But this graphic was not made to show all detailed information. It simply shows you the order of the fields, but specifically for BDPU. The Preamble and FCS and all the stuff that is missing just follows the same order like the standard ethernet frame. The graphic just tells you in which order the frame control field and the llc field follows up in the frame.
By the way, thats why the picures name is "IEEE 802.1d STP BPDU Frame Format" Its already a complete frame with an encapsulated BDPU packet.
Number 3 shows a BDPU packet (not a frame) as it is encapsulated in the payload of the ethernet frame.

hope i could clear it up a bit

-chrnxR





sources:
docstore.mik.ua/univercd/cc/td/doc/produ...an/trsrb2/frames.pdf
en.wikipedia.org/wiki/Ethernet_frame