Subject: Re: Bridge or

From: Norman Finn <[email-masked]>
Date: Thu, 05 Dec 2002 10:59:59 -0800

This is an old thread (Nov. 20), but let me take a crack at
your questions.  These questions will be examined very carefully
by IEEE P802.1, and I'm sure, will be answered at some point.

Cheng-Yin Lee wrote:
> 
> May I know if these two questions have been clarified yet:
> 1) Is VPLS emulating LAN or bridges?
> 2) what's the difference between emulating Ethernet and LAN?

> 1) Is VPLS emulating LAN or bridges?

Neither.  Also, both.  The problem is that the differences
between a LAN and a bridge (other than timing, of course) are
really a collection of differences in how certain L2 protocols
are handled.  A customer may want to make a choice on a
per-protocol basis, instead of an all-or-nothing "bridge or
LAN" choice.  Furthermore, there are some new possibilities
which are not applicable to either a bridge or a LAN, at
present.

                      Physical
                      LAN                   Provider
    protocol          segment    Bridge     Service

802.3x pause frames   xparent    engage or  (a)
                                 suppress
.3 Link Aggregation   xparent    engage or  (b)
                                 suppress
802.1X Port Access    xparent    engage or  (b)
                                 suppress
802.1D/W/S BPDUs      xparent    engage     (c)
802.1D GMRP           xparent    engage or  (d)
                                 xparent
802.1Q GVRP           xparent    engage or  (d)
                                 xparent
L3 data               xparent    xparent    xparent

 (a) The provider must not pass 802.3x pause frames through
     the service.  The timing assumed by 802.3x cannot be met
     by an L2-over-L3 service.
 (b) The customer may wish to run these protocols, especially
     802.1X, either 1) between the customer and the provider;
     2) end-to-end; or 3) both!  (This requires extensions to
     802.1X.)  Furthermore, the 802.1X authentication may be
     desirable on what is, otherwise, a transparent "LAN"-like
     service.
 (c) A provider "LAN-like" service may transport a customer's
     BPDUs end-to-end.  However, it is unlikely that the provider
     will handshake with a customer's BPDUs, and thus concatenate
     the Provider's spanning tree with all of the customers'
     spanning trees.  So, the "bridge" option is not available.
 (d) Similar to (c), but some use may well be found for these
     protocols in controlling the delivery of a customer's
     multicast MAC addresses and/or VLANs.  Easier to suppress
     them, however, in a "bridge-like" service.

There are many more protocols to consider, of course, but the
answers are similar.

> 2) what's the difference between emulating Ethernet and LAN?

Not clear to me what you mean.  In general, "LAN" includes
802.5 Token Ring, FDDI, 802.11 wireless, etc. etc.  Nobody
cares much about anything except Ethernet, any more.

If you meant the difference between emulating an Ethernet and
emulating a VLAN, the difference is in the eyes of the beholder.
To a router, they're the same thing.  To a bridge, a VLAN has
no independent existence; it is just an extra filter applied to
frames that are bridged from Ethernet to Ethernet.  Therefore,
some number of emulated VLANs are grouped together and treated
as a single emulated Ethernet, which has no existence outside
the bridge.

I hope this rather long-winded explanation helps.

-- Norm

---

From: "Cheng-Yin Lee" <[email-masked]>
Date: 5 Dec 2002 23:45:29 -0500

Norm,
Thanks for your clarification.

> by IEEE P802.1, and I'm sure, will be answered at some point.
[deleted]
> The problem is that the differences
> are handled.  
I agree with you on this. 

> cares much about anything except Ethernet, any more.
I meant emulating Ethernet and (Ethernet) LAN, there seems to be some
disagreement earlier on this issue, but I think there is agreement now
it is the same thing.

thanks
cheng-yin