1472 Bytes is a full sized packet when one is pinging from the command line... That is why he is seeing 1468 as the maximum (4 extra bytes vs my 1472). Here is from one of my boxes (notice the number in brackets? That is the total packet size including headers)... [root@core01-db root]# ping 206.126.22.33 -s 1472 PING 206.126.22.33 (206.126.22.33) 1472(1500) bytes of data. John :) ----- -----Urspr?ngliche Nachricht----- Von: Ard van Breemen [mailto:ard@xxxxxxxxx] Gesendet: Monday, November 15, 2004 1:24 PM An: Linux 802.1Q VLAN Cc: vlan@xxxxxxxxxxxx Betreff: Re: [VLAN] e100 VLAN and MTU issues (fwd) On Mon, Nov 15, 2004 at 01:32:43PM -0500, Iain McBride wrote: > While everything I can find online regarding the e100 driver in 2.6 > kernels not needing a patch to avoid MTU issues with VLAN tagging, I've > been unsuccessful in getting VLAN-tagged connections to function properly. > > VLAN router: 2x e100 NICs, 2.6.8.1 Linux kernel, VLANs config'd > > Switch: Cisco 2924, trunk enabled on the VLAN router's port, end > machine's port assigned to proper VLAN > > End machine: e100 NIC, 2.6.8.1 Linux kernel, normal interface config > > When I take another machine and put its switch port into a VLAN, I > experience the strange behaviour typical of MTU issues unless I configure > the end machine with an MTU of around 1468 or smaller. Large file This is plain weird. If you had 802.1Q problems the MTU problem should dissappears with MTU=1496 (the tag is only 4 bytes). Any lower indicates problems that are not 4 byte tag related. > transfers time out, packets silently dropped can't ping with 1500 byte > packet payload, etc. > > If I remove the VLAN trunking and bring up the router interface in the > normal fashion, everything is fine. > > Any hints on further troubleshooting? Jup: tcpdump the interface (-s 1500) and start pinging with a size that makes it almost fragment (so with 6 different sizes actually). The tcpdump should be done on the machine with the 802.1Q tag, and one without the 802.1Q tag. And please make sure you are using the e100 driver and not the eepro100. --
