At 16:47 31/05/01, Lane Patterson wrote:

I have been tracking jumbo frame support trends for a while, and am reasonably disappointed by lack of standards and vendor willingness to support jumbos (yes there are very REAL h/w design considerations, so until operators demand jumbo support and folks test it in realistic environments, it's not going to happen).

I believe most GigE switch vendors currently support ~9180 IP MTU over GigE interfaces. IEEE 802 committee has repeatedly and deliberately declined to make that an official standard however. A number of the host vendors (e.g. Sun) appear to be listening to customer requests for support of that IP MTU size also.

Unfortunately, many of the folks most adamant about maintaining 4470 in their core are therefore sticking with POS everywhere, so their requirement is not making it to the ether vendors.

POS could be configured at various MTUs, right ? 4470 is just a historical choice equal to FDDI, right ? Folks could engineer their POS links to use ~9180, I think, if they wanted to do so.

There are different reasons to use several different sizing parameters:

"Mini-jumbo": say 1518, 1540, etc. the idea here is that you can handle stacked tunnels and LAN encapsulations, such as stacked headers of 802.1Q, MPLS, IP/GRE tunnels, etc. while still preserving "1500 for the edge"

Applicability: 802.1Q, VPN, MPLS, and other encap-based or tunnel-based applications

I believe at least 802.1q support is quite commonplace these days.

"Mid-jumbo": say 4470: the idea is to make sure a backbone can preserve its MTU across both ethernet, ATM, and POS links within its diameter, and conceivably between networks via IX's that support jumbos. This in fact may be critical for folks running large ISIS implementations that need to ration # of LSPs:

This number derives from FDDI MTU, which was formerly commonly used as the basis for many an exchange point.

http://www.ietf.org/internet-drafts/draft-kaplan-isis-ext-eth-02.txt

I believe IEEE 802 committee officially opposes the above draft, though I could be misinformed.

"Real jumbo": not standardized,

Kindly see RFC-1626, which is where this number and rationale came from. The correct number, btw, is 9180 bytes as an IP MTU.

but somewhere between 8100-9100B,...