Haynes, Mark wrote: > > Hi Folks, > > Can anyone tell me, or point me to a link that does, what > happens on a > Catalyst Switch when an MTU problem occurs ? > I know that when a router is involved, if a packet is larger > than the MTU of > the interface it needs to traverse, the router would examine > the 'Don't > Fragment' (DF) bit within the packet. If the DF bit is set to 1 > then the > packet will be dropped and an ICMP packet will be sent back to > sending > device informing it of the fact. If, however, the DF bit is set > to 0 then > the packet will be fragmented to conform to the MTU of the > interface, and > will then be forwarded for re-assembly by the recieving device. > What I would like to know is, what happens if this problem > occurs within > a Switch ?
A layer 2 switch doesn't look past the layer 2 header so wouldn't know about the DF bit in the IP header. If a frame were too large to send out an interface, the switch would simply drop it. It wouldn't send an ICMP message. A frame arriving that was too large to forward would be an odd occurance, since switches usually connect like networks, but it could happen. Gigabit Ethernet sometimes uses jumbo frames, for example, although I think there is some controversy about whether it should. Another example is the infamous Ethernet/Token Ring bridges of lore, which had numerous problems including this very one. Now, there were vendors who had bridges that did fragmentation, but they were sort of considered hacks. The concept of a bridge looking at Layer 3 info gave many purists the heebie-geebies (technical term). If you have Radia Perlman's book, Interconnections, I believe she has a few choice words to say on this topic. If you're trying to troubleshoot an actual problem, let us know some more details. If it's just theory, than the info I provided should help. _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com > > > thanks, > Mark Haynes > > > ******************************************************************************************** > " This message contains information that may be privileged or > confidential and > is the property of the Cap Gemini Ernst & Young Group. It is > intended only for > the person to whom it is addressed. If you are not the intended > recipient, you > are not authorized to read, print, retain, copy, disseminate, > distribute, or use > this message or any part thereof. If you receive this message > in error, please > notify the sender immediately and delete all copies of this > message ". > ******************************************************************************************** > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58903&t=58894 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
