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RE: jumbo frames
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,...