[rescue] The Practical Guide to FDDI

[Phil Brutsche]

Chris Hedemark wrote:
> On Thursday, March 27, 2003, at 06:01 PM, jwbirdsa at picarefy.com wrote:
> 
>> what technical mistakes have I made?
> 
> 
> "Even though it has the same speed as Fast Ethernet (100Mbps), FDDI is 
> more efficient because it has a much larger MTU. Ethernet is limited to 
> a packet size of 1536 bytes."
> 
> Not so.  That's just the default setting.  Unless you have a Sun 
> ethernet card (at least the gigabit ones, anyway, I'm not sure about the 
> HME's) you can bump the MTU size effectively to 9000 bytes or so.  9000 
> bytes is a good size because NFS frames are 8K and the extra accounts 
> for the packet overhead.

Jumbo frames are specific to Gigabit ethernet, and it's also the only 
way to get anywhere near 100MB/sec (GigE runs at 350MHz; using the 
"normal" 1500 byte MTU you're limited to 20MB/sec).  It's part of the 
GigE specification.

If you set the MTU much higher (I don't think you can), it won't be 
ethernet anymore :)  The checksum used by ethernet breaks once you get 
past 11000 bytes or so.

10/100 cards (such as HMEs) can't do jumbo frames.  If you try to set 
the MTU past 1500 bytes (or whatever the value is these days) on 10/100 
cards you'll get "Invalid argument" or something similar.

> The small default MTU on ethernet is just there for backwards 
> compatibility.  And actually, it is up to 1540 bytes now (an extra 4 
> bytes was added at some point).

The extra bytes are for VLAN tagging, I think.


-- 

Phil Brutsche
phil at tux.obix.com