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Q & A Archive

Q & A

The least significant bit of the most-significant byte

by Scott Morris

Question:

November 5, 2002

Oh Great Jedi Master,

The sacred scrolls of former Jedi masters and standards institutes all state that the first bit of a 48-bit hardware address is the unicast/multicast bit (zero if by land, one if by sea), and that the second bit is the globally unique/locally re-writable bit -- 0 if globally unique, 1 if IBM re-wrote it for local use. However, every MC frame I ever sniffed (e.g. BPDU) had the first two bits set to 01, which, according to the sacred scrolls, should not be MC but UC-re-written. Am I missing something here? ,

Yours, a humble and unworthy Padawan,

--Flash Weldon
CCSI

Answer:

Padawan,

Sometimes the sacred scrolls of former Jedi are mired in the fog of Dagobah. There is much meaning in them, yes. But not all is clear upon first glance.

You are correct about the bit patterns. The least significant bit of the most-significant byte (don't say that 10 times fast) is also known as the I/G bit. This bit signifies an Individual or Group address. Both multicast addresses and broadcast addresses are considered a group address.

So, let's look at a Bridge Protocol Data Unit (BPDU). Switches (and bridges) send these out as part of the 802.1D Spanning Tree Protocol to prevent loops in a bridged network. BPDUs are sent out every port of every switch every two seconds, and, as you say, are only used on common LAN links (e.g. LAN segments between two switches). BPDUs are sent to a destination address of 01-80-c2-00-00-00.

This brings up a very interesting question as you put it. If two switches are talking to each other, why are they not using unicast addresses? Think about a unicast conversation. For us to converse, we assume that we know who we are. In other words, I would have to know your address in order to talk to you. In which case, my port's MAC would be the source address in the Layer 2 frame, and your port's MAC would be the destination address in the Layer 2 frame. But how would I know that? Or how would I know you were a switch or other device participating in spanning tree?

The point is that I wouldn't know, nor is it necessary for me to figure it out. Sending information (BPDU frames) to a specific multicast address that we all know about is the simplest method to be sure that we pay attention to the information without needing to know about who is on the other end (if anyone at all).

Cisco's Cisco Discovery Protocol (CDP) is exactly the same way. Using 01-00-0C-CC-CC-CC as the destination MAC address, it will flood information out to all devices. The frame will only be processed by devices that know what that multicast means. As an example of this with CDP, if there's a non-Cisco (say it isn't so!) switch in between two Cisco devices, the two Cisco devices will still see each other. This is because a non-Cisco switch will see the multicast address, have no idea what it is, and do what any switch would do with unknown frames: Flood it out to all ports.

The second least-significant bit is used to signify LAA or Locally Assigned Addresses. 0 means the burned-in address is used, 1 means the MAC has been locally assigned. Token Ring (IBM) uses this method a lot.

So those old Jedi scrolls really are accurate in what they say. Sometimes, though, it's hard to figure out why things are applied sometimes. ,

--Scott

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Scott Morris, quadruple CCIE, JNCIE and all-around uber-geek, can often be seen traveling around the world consulting and delivering CCIE training. He recently accepted a new Senior CCIE Instructor position with Internetwork Expert! For more information on him check out http://www.uber-geek.net or for CCIE training check out http://www.internetworkexpert.com. You can contact Scott via editor@tcpmag.com. You can contact Scott about "The least significant bit of the most-significant byte" at editor@tcpmag.com.

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