Re: Access Concentrators and MTU.

Jean-Lou,

1. Real ethernet uses frames that can be a maximum of 1518 octets for untagged or 1522 octets for tagged frames. Each frame has a maximum payload of 1500 octets for Ethernet II or 1492 octets for IEEE 802.3. Note that PPPoE (RFC2516) is only defined for Ethernet II. PPPoE is not defined for gigabit-ethernet jumbo frames, but the 1492 limit is only there because of the 1500 octet payload limit of Ethernet II. Maybe an updated PPPoE RFC should redefine the PPPoE payload limit as the maximum ethernet payload size minus 8 octets.
2. Rfc1483/MPoA Bridged ethernet runs on top of ATM AAL5. Standard ATM AAL5 packets have a maximum payload of 65534 octets. A "fake" Rfc1483/MPoA ethernet frame is just some headers inside an AAL5 packet. When a "service gateway" _receives_ such a packet, they just remove the AAL5 trailer, the Rfc1483/MPoA headers and the ethernet header to get at the payload without checking the size of the payload (well usually). Mind you, the size of a packet _is_ checked before sending it over an interface. This only works if both the ATM and the Rfc1483/MPoA ethernet layers are terminated at the same time in the same box. And must be the case at both ends of the ATM connection.
3. If I understand correctly, you have a DSLAM with on the side to the provider's network a gigabit-ethernet connection. Gigabit-ethernet uses real ethernet frames. Normally a real ethernet frame can only have a payload of maximum 1500 octets. Gigabit-ethernet can use jumbo frames with larger payload sizes upto 9000 octets. I assume you will be using jumbo ethernet frames and that both the DSLAM and the service gateway support these frames.
4. So yes, you can have PPPoE packets with a PPPoE payload greater than 1492 octets just as with PPPoEoA. Provided that: + both the DSLAM and the service gateway support gigabit-ethernet jumbo frames, + the allowed MTU of the gigabit-ethernet is large enough, + the PPPoE layer, that is carried over gigabit-ethernet jumbo frames, is terminated at the same as the gigabit-ethernet layer is terminated (wel, actually, you could turn this PPPoJE into PPPoEoA)
5. Do conventional Service Gateways (such as Cisco 6400 and Redback SMS) support this? I don't know. Depends on their firmware, I guess. (Assuming they support jumbo ethernet.) Ask the vendor if they support PPP-over-JumboEthernet. Note that the Cisco 6400 does support Ethernet-over-PPP (BCP) and that it doesn't care what kind of PPP it is. It could be PPPoA, PPPoE, PPPoH, etc.. You must get the MTU/MRU configuration right though.

Tschüß,
Manuel

On Thursday 19 June 2003 17:28, jeanlou.dupont@marconi.com wrote:
> Thanks Manuel.
>
> Assuming I am building an "Ethernet DSLAM" (Digital Subscriber Line
> Access Multiplexer), I would like to be able to hand-off the customer
> Ethernet streams to a network side Gigabit Ethernet interface. This
> network interface would use PPPoE in order to logically separate the
> customer Ethernet streams.
>
> So, assuming I am doing this, the conventional Service Gateways (such
> as Cisco 6400 and Redback SMS) would accept Ethernet packets of 1500
> bytes encapsulated in PPPoE, thereby forming a frame of 1500+8 bytes?
>
> Many thanks.
> Jean-Lou Dupont.
> Marconi
> Montreal (QC) Canada
>
>

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