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Author Topic: Dumbo would like to know  (Read 1291 times)


  • Kitizen
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  • Posts: 3326
Dumbo would like to know
« on: December 09, 2018, 09:18:12 PM »

Sorry, if this is misplaced, or has been explained but missed. But Dumbo saw a reference to "baby jumbo firmware" and would like to know, what is special about it; its +/-.


  • Kitizen
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  • Posts: 1012
Re: Dumbo would like to know
« Reply #1 on: December 09, 2018, 10:14:21 PM »



  • Senior Kitizen
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  • Posts: 10795
  • Retd s/w dev; A&A; 3x7km ADSL2 lines; Firebrick
Re: Dumbo would like to know
« Reply #2 on: December 09, 2018, 10:25:37 PM »

Baby jumbo means sending packets that are slightly larger than the usual maximum limit of 1500 bytes (size of IP). People who are a pain in the neck like me and perfectionist about certain small things.

People who use a two-box modem plus router setup and use PPPoE on the DSL line, not just in your own cable between modem and router, which all PPPoE modems do, have come up against an MTU/MRU bottleneck (ie limit on the maximum size of a packet) because PPP+PPPoE adds another 8 bytes to the size of each IP packet sent across the modem-to-router cable. The maximum size IP packet allowed traditionally in a LAN is 1500 bytes. When you add the 8 extra bytes for PPP+PPPoEoE on the modem-to-router cable this means that the size of an IP packet wrapped in PPPoEoE can not exceed 1492 bytes. This has been the traditional IP size limit for all PPPoE modems.

However if a system out there in the internet absolutely insists on sending 1500 bytes to you then there could, in theory at least, be trouble, if your kit can not handle anything larger than 1492 bytes. This was discussed in another thread very recently in fact.

If you would really like to get full functioning back and be able to send/receive full sized 1500 byte packets, then there is still hope.

Some kindly routers, some capable modems and well-behaved Ethernet cables are willing to forgive you for sending slightly oversized 1508 byte IP+PPP+PPPoE packets wrapped around a 1500 byte IP packet. The Ethernet frames on the link between modem and router that contain payload larger than 1500 bytes are called ‘baby jumbo’ frames.

To get things to work your service provider also needs to support oversized such packets; the DSLAM needs to be ok with handling full sized packets on the DSL link too.

Jumbo frames are those with a payload larger than 1500 bytes, often as much as ~9000 bytes, very occasionally even more. Those whose payload is only slightly larger than 1500 bytes are called ‘baby jumbo’.

A while ago, our Kitizen Johnson built custom firmware for ZyXEL modems which allows the use of full 1500+8=1508 byte IP+PPP+PPPoE packets, so that full size 1500 bytes IP packets can be sent and received with no problem and you are compatible with everything on the internet, just like those lucky people with one-box systems.

So it’s all a + and no -. I am running Johnson’s latest firmware, which has other advantages too, on my ZyXEL VMG 1312-B10A modems.