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RE: [RPRWG] RE: [IPORPR] payload length and padding and stuff



Dan -

If your question translates as "Can a Bridge be built that supports more than one type of MAC", then the answer is Yes. And indeed, there are Bridges that support plug-in blades for each port, and which can therefore potentially support a wide variety of frame sizes.

However, at least as far as the standard is concerned, there is no configurable per-Port parameter involved; if a Port supports an 802.X MAC, then the frame size supported on that Port is as stated in the standard for the 802.X MAC (i.e., there is no standard means to configurable the frame size independently of the MAC type).

So, if you are asking "Is it possible to build a Bridge that will forward jumbo frames?" then clearly, the answer is "Yes". However, that is a different question to asking "Is forwarding jumbo frames conformant behaviour for a Bridge?"; the interpretation of the 802.1D standard relative to that question is "No".

Regards,
Tony

At 10:34 09/12/2002 +0200, Romascanu, Dan (Dan) wrote:
Tony,
 
You are right, and I obviously stand corrected. My question would be - is the reference mac_service_data_unit supported by the LAN configurable per port? If the answer is yes, the implication would be that standard 802.1D bridges can be configured to support these MAC and PHY implementations that accommodate larger frame sizes ('jumbo frames') that some vendors built as proprietary extensions to the 802.3 MAC and PHYs.
 
Regards,
 
Dan
 
-----Original Message-----
From: Tony Jeffree [mailto:tony@xxxxxxxxxxxxx]
Sent: Thursday, December 05, 2002 11:36 AM
To: Romascanu, Dan (Dan)
Cc: Nader Vijeh; iporpr@xxxxxxxx; stds-802-17@xxxxxxxx
Subject: RE: [RPRWG] RE: [IPORPR] payload length and padding and stuff

Dan -

I think that you will find the 802.1D standard requires conformant Bridges to discard frames that exceed the max frame size for the destination LAN:

"7.7.1 Enforcing topology restriction
Each Port is selected as a potential transmission Port if, and only if
a) The Port on which the frame was received was in a forwarding state (8.4), and
b) The Port considered for transmission is in a forwarding state, and
c) The Port considered for transmission is not the same as the Port on which the frame was received,
and
d) The size of the mac_service_data_unit conveyed by the frame does not exceed the maximum size of
mac_service_data_unit supported by the LAN to which the Port considered for transmission is
attached.
For each Port not selected as a potential transmission Port, the frame shall be discarded."

Regards,
Tony





At 18:25 04/12/2002 +0200, Romascanu, Dan (Dan) wrote:

Nader,

There are actually two different layers here.

802.3 MACs and PHYs MUST support a max frame length of 1522. You may find implementations that are both standard compliant and support jumbo sizes as proprietary extensions.

I think that 802.1 bridges do not have such a limitation within the standard. As you mention, they can be configured to support larger frame sizes. Actually some of the non-Ethernet 802 technologies already support larger frame sizes.

Dan



> -----Original Message-----
> From: Nader Vijeh [mailto:nader@xxxxxxxxxxxxxx]
> Sent: Wednesday, December 04, 2002 6:10 PM
> To: Romascanu, Dan (Dan)
> Cc: iporpr@xxxxxxxx; stds-802-17@xxxxxxxx
> Subject: RE: [RPRWG] RE: [IPORPR] payload length and padding and stuff
>
>
> Dan,
>
> Thank you for the correction. Jumbo frames are not supported
> by the 802.3
> standard.
> There are Ethernet switches on the market that support jumbo
> frames in a
> proprietary manner. Of course these will not interoperate
> with standard
> compliant 802.1 bridges. As I pointed out in the rest of the
> message, even
> if jumbo frames are supported, max frame size "needs to be
> configurable to
> be lower, in order to comply with transparent bridging requirements".
>
> Nader
>
> -----Original Message-----
> From: Romascanu, Dan (Dan) [mailto:dromasca@xxxxxxxxx]
> Sent: Wednesday, December 04, 2002 12:45 AM
> To: Nader Vijeh; Necdet Uzun; Anoop Ghanwani
> Cc: Frank Kastenholz; iporpr@xxxxxxxx; stds-802-17@xxxxxxxx
> Subject: RE: [RPRWG] RE: [IPORPR] payload length and padding and stuff
>
>
> >
> > The max frame size may be extended beyond 802.3 1522 bytes
> > limit as there is
> > precedence in 802.3 community.
>
> Nader,
>
> I am not sure what you exactly mean. If you refer to what is
> popularly known
> as 'Jumbo frames', they are not supported by the IEEE 802.3
> standards.
>
> Dan
>
Regards,
Tony

Regards,
Tony