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RE: STP BPDUs [7:46839] posted 06/18/2002
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Weird... all the documentation I have seen says that a BPDU is a
broadcast just like CDP and HSRP...  I guess I need to take a trace...
Your explanation is too precise to question compared to my sources :)

-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Priscilla Oppenheimer
Sent: Monday, June 17, 2002 9:33 PM
To: cisco@xxxxxxxxxxxxxx
Subject: Re: STP BPDUs [7:46839]

BPDUs are sent to a multicast address that means "all bridges." In 
Ethernet, they are sent to 01:80:C2:00:00:00. For the destination
on Token Ring, reverse the bits, one byte at a time to get

At 08:39 PM 6/17/02, Tim Potier wrote:
>This might be a simple answer, but what type of message is a BPDU:
>Unicast, Multicast or Broadcast.  I have searched all over Cisco's
site, and
>dug through STP luck.

It's not an RFC. It's IEEE 802.1D. See here:

>  I have come up with the following:
>"The Destination Address field indicates the destination address as
>specified in the Bridge Group Address table. For IEEE Spanning-Tree
>BPDU frames, the address is 0x800143000000.

That is bit-reversed. You must have found a document that covers Token

>  For IBM Spanning-Tree Protocol
>BPDU frames, the address is 0xC00000000100.

IBM had their own version of STP. DEC also had a variety of STP. They
to the broadcast address (FF:FF:FF:FF:FF:FF).

>For Cisco Spanning-Tree Protocol
>BPDU frames, the address is 0x800778020200."

Cisco spanning tree? What is that?? Something to do with the Per VLAN 
Spanning Tree (PVST) on Token Ring?? (Once again, that looks like a
Ring multicast address.) On Ethernet, PVST uses the standard destination
think, unless you use PVST+ which tunnels PVST BPDUs through an 802.1Q
Spanning Tree implementatoin, thus allowing each VLAN to maintain its
spanning tree. Cisco uses the multicast address 01:00:0C:CC:CC:CD for
on Ethernet. Bit reverse that and you don't get 0x800778020200, however,
I don't know what that adddress is for.

>I guess it also depends on the definition of multi/broad/unicast.

It better not depend on that. There shouldn't be any argument on those 
definitions. ;-)

Multicast means a group address. The first bit of the destination MAC 
address (which is the first bit transmitted) is a one so that every
knows to look at the address. A NIC driver software supports an
telling the NIC which particular multicasts to take in.

Broadcast means every device in the broadcast domain. The first bit and
bits in the MAC destination address are ones. Every NIC in the broadcast

domain takes in the frame and interrupts the host CPU to see if the rest
the frame is interesting.

Unicast means a specific address. The first bit of the destination MAC 
address (which is the first bit transmitted) is a zero.



>Thanks for your help.

Priscilla Oppenheimer

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