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Re: STP BPDUs [7:46839] posted 06/18/2002
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""Brian Backer""  wrote in message
> Priscilla,
> Weird... all the documentation I have seen says that a BPDU is a
> broadcast just like CDP and HSRP...

My friend, I believe you are mistaken.

"CDP sends packets on LANs using the multicast address 0100.0CCC.CCCC"
"Each device configured for CDP sends periodic messages, known as
advertisements, to a multicast address."

"HSRP works by the exchange of multicast messages..."

"Routers that are participating in an HSRP group communicate to each other
via a multicast User Datagram Protocol (UDP)-based hello packet"

"The Destination MAC address uses the well-known STP multicast address of
- Cisco LAN Switching, Clark and Hamilton, page 184.

> b
> -----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
> address
> on Token Ring, reverse the bits, one byte at a time to get
> 80:01:43:00:00:00.
> 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
> Protocol
> >BPDU frames, the address is 0x800143000000.
> That is bit-reversed. You must have found a document that covers Token
> Ring.
> >  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
> sent
> 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
> Token
> Ring multicast address.) On Ethernet, PVST uses the standard destination
> I
> think, unless you use PVST+ which tunnels PVST BPDUs through an 802.1Q
> Mono
> Spanning Tree implementatoin, thus allowing each VLAN to maintain its
> own
> 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,
> so
> 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
> device
> knows to look at the address. A NIC driver software supports an
> application
> telling the NIC which particular multicasts to take in.
> Broadcast means every device in the broadcast domain. The first bit and
> all
> 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
> of
> 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.
> Priscilla
> >Thanks for your help.
> ________________________
> Priscilla Oppenheimer

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