From: gah@ugcs.caltech.edu   
      
   Rick Jones  wrote:   
   > rahul.dev.agg@gmail.com wrote:   
      
   (snip)   
   >> If yes, what would be the pmtu for the path between these nodes ?   
   >> Can node A find the mtu of node B on the same link ?   
      
   > The starting point is this, in pseudo IEEEspeak (which I've probably   
   > botched) with a smattering of IETF style:   
      
   >  All stations in the same broadcast domain MUST have the same MTU.   
      
   Is this still true if you have more than one IP net in the same   
   broadcast domain?   
      
   I haven't thought of this for some time, but it seems to me that   
   you should be able to run separate (sub)nets on the same ethernet,   
   with different MTU for the different subnets. You then need a router   
   that can route between them.   
      
   > Ethernet has no way to communicate frame size between peers.  If a   
   > frame larger than the station is prepared to receive arrives, that   
   > frame will be dropped.   
      
   > Path MTU is up at the IP layer and uses ICMP messages to communicate   
   > MTU between hosts.  The PathMTU logic is run when a router looks to   
   > forward the IP datagram - when it goes to send it.  That means it must   
   > have received it in the first place.  For IP to "receive" the datagram   
   > it must first be received at the layer below it - in this case   
   > Ethernet.   
      
   Which gets interesting with different interfaces on different   
   types of nets, or different datagram limits on the same type   
   of network. (The latter being the case for ethernet with and   
   without jumbo frames, the former maybe for eithernet-fddi or   
   ethernet-token ring.)   
      
   > However, if the interface on which it was going to receive the   
   > datagram has an MTU/framesize smaller than the size of the datagram   
   > sent, the datagram won't be "received" by the IP layer so it cannot be   
   > resent, so the Path MTU logic cannot trigger.   
      
   Yes, but there are routers that can route larger packets than   
   they can receive if addressed to them.   
      
   > Thus the reason why all stations (hosts, systems, what you will) in   
   > the broadcast domain (everything joined at layer 2 eg ethernet) MUST   
   > have the same MTU.   
      
   (snip)   
      
   -- glen   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)