The spanning-tree protocol sends Bridge Protocol Data Units (BPDUs) to a
known data-link-layer multicast address. The first bit transmitted of the
Ethernet or Token Ring (or whatever) address is a one, meaning it's a group
(multicast) address.
The Cisco switching books and classes seem to be teaching multicast in a
bizarre way that is leaving people with the impression that it's an IP thing.
Multicasting at the data-link layer has been in use for about 20 years. IP
multicasting, although always supported by IP RFCs, has just started to get
popular.
Hope that makes sense.
Priscilla
At 03:47 PM 1/25/01, you wrote:
>Actually I thought the MAC address used by STP was using a well known
>MAC but it wasn't actually a Multicast MAC?
>
>Darren
>
>Priscilla Oppenheimer wrote:
> >
> > At 03:49 PM 1/24/01, Jim Dixon wrote:
> > >Wouldn't that make it layer 2?
> > >A bridge then?
> > >Broadcast and don't care if I get a response?
> >
> > Bridges don't broadcast. They send multicasts for the spanning-tree
> > algorithm, but that's not relevant. They forward broadcasts (and other
> > frames), but that's not relevant either. We're talking about a single
> > subnet that has two devices. The two devices probably aren't bridges. What
> > would be the point of connecting two bridges together in one subnet that
> > allows only two devices?
> >
> > Whether I get a response or not depends on what upper layer I'm using. For
> > example, I could use ping to send a message from the first device
> > (10.0.0.0/31) to the second device (10.0.0.1/31) on a point-to-point link.
> > I should get a response (assuming the operating system would let me
> > configure these addresses to start with.)
> >
> > If it's two routers on the ends of this point-to-point link, they are
> > mostly just forwarding traffic on behalf of other stations. The IP
> > addresses of the routers themselves are irrelevant in that case. In fact, I
> > could use ip unnumbered, but then I couldn't ping the point-to-point
> > interfaces, which makes management a bit harder.
> >
> > >Since I can't tell if anyone
> > >is actually there, I just know that
> > >someone is sending me broadcasts from somewhere on this link.
> >
> > What does "somewhere on this link" mean? It's a point-to-point link.
> > There's only one other thing besides myself on the link.
> >
> > If I yell into a room that has only one person in it, "Hello, I'm trying to
> > reach everyone in this room," won't I get an answer? Does it matter that I
> > wasn't more precise? No.......
> >
> > >Correct me if my thinking is in
> > >need of more coffee.
> >
> > You need more coffee. (or maybe less??) &;-)
> >
> > Priscilla
> >
> > P.S. Please don't send messages directly to me. Please address them to the
> > group. Thanks.
> >
> > >Jim :)
> > >
> > >-----Original Message-----
> > >From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]]
> > >Sent: Wednesday, January 24, 2001 3:17 PM
> > >To: Jeff McCoy; [EMAIL PROTECTED]
> > >Subject: Re: slash 31 address
> > >
> > >
> > >Yes, but on a point-to-point link, so what if you use the network number
> > >and broadcast address to identify the two nodes? If one station sends to
> > >the broadcast address it's not a problem, there's only one other station
> > >anyway! I think /31 should be allowed on point-to-point links.
> > >
> > >Priscilla
> > >
> > >At 08:22 AM 1/24/01, Jeff McCoy wrote:
> > >
> > > >Michael...
> > > >/30 = 4 address (1st is network number, 2 & 3 host ip's, 4th is
> broadcast
> > > >address)
> > > >/31 = 2 address (1st is network number, 2nd is broadcast address)
> > > > no host ip's...this is not useful..
> > > >/32 = 1 address (1 host address) i use this for loopbacks
> > > >
> > > >
> > > >""Neil Schneider"" <[EMAIL PROTECTED]> wrote in message
> > > >94mool$d33$[EMAIL PROTECTED]">news:94mool$d33$[EMAIL PROTECTED]...
> > > > > AFAIK you can only go as far as /30.
> > > > >
> > > > > Neil Schneider
> > > > >
> > > > > <[EMAIL PROTECTED]> wrote in message
> > > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > > > > >
> > > > > >
> > > > > > Hello good guys of the group, can anyone with the knowledge of
> ip/31,
> > > >help
> > > > > me?
> > > > > > please.
> > > > > >
> > > > > > I need to know what it is use for, and how it works, pls.
> > > > > >
> > > > > > Thanks in advance,
> > > > > >
> > > > > > Michael Taiwo.
> > > > > >
> >
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
> >
> > _________________________________
> > FAQ, list archives, and subscription info:
> http://www.groupstudy.com/list/cisco.html
> > Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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