1500 bytes is the Ethernet frame without the header and CRC. It's the maximum amount of data the upper layer can stuff into an Ethernet frame. It's the payload in the Ethernet frame.
When books refer to the Ethernet MTU as 1518 bytes, they are including the following: destination address = 6 bytes source address = 6 bytes length or EtherType = 2 bytes payload = 1500 bytes (for example, a TCP/IP or whatever packet) CRC = 4 bytes You may see 1514 too. Sometimes, books don't count the CRC. You may also see the 8 byte preamble before the destination address included. Technically, the inter-frame gap after the frame could also be included. And then there's MPLS and VLAN tags that can get thrown into the mix too. At the most basic level, however, understanding where 1500 versus 1518 comes from is essential. Priscilla At 08:05 AM 3/27/02, Joshua Dughi wrote: >Hi, > >With regards to Ethernet's MTU size, I'd like to provide the fine-tuned >response, which >I have cut & pasted from Cisco document: > >The packet size, referred to as MTU is the greatest amount of data that can >be >transferred in one physical frame on the network. For Ethernet, >the MTU is 1518 bytes, >for PPPoE it is 1492, >while dial-up connections often use 576. The difference comes from the fact >that when >using larger packets the overhead is smaller, you have less routing >decisions, and >clients have less protocol processing and device interrupts. > >I haven't taken the time to find the explanation as to why 1500 bytes is so >often >referred to as the MTU size. Perhaps someone can nail down the distinction >between the >1500 byte figure, which is quoted so often, and the 1518 MTU byte #. > >Joshua Dughi >[EMAIL PROTECTED] > >John Green wrote: > > > reposting as i got just one response.... > > > > > MTU for .... > > > Ethernet is 1500 bytes > > > SLIP is 576 bytes > > > Frame Relay ??? > > > Token Ring ??? > > > > __________________________________________________ > > Do You Yahoo!? > > Yahoo! Movies - coverage of the 74th Academy Awards. > > http://movies.yahoo.com/ ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=39664&t=39301 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
