[nsp] AToM

[Maik Bachmann]

On Tue, Dec 23, 2003 at 12:24:59PM +0100, Lars Erik Gullerud wrote:
> On Tue, 2003-12-23 at 11:42, Sergey V. Artjushkin wrote:
> > > > But I had a problem with big packets between two PC that was connected
> > > > throught trunk port to my cisco devices.
> > > 
> > > Keep in mind that MPLS normally requires at least 8 bytes extra, and
> > > EoMPLS has further encapsulation overhead.
> > 
> > I know about additional bytes because of VLAN and MPLS tags.
> > Let's count the maximum size of the packet.
> > One PC sending 1500 bytes to other PC.
> 
> Actually 1518. 14 Ethernet header, 1500 payload, 4 FCS.
> 
> > When the packet arrives at the switch, switch adds 4 bytes VLAN tag.
> 
> Right. 1522 now.
> 
> > When packet arrives at the PE router (6509/7401) it adds 8 bytes (2x4 bytes)
> > MPLS label tags. So total packet size is 1500 + 4 + 8 = 1512 bytes.
> > Right? 
> 
> No. You now have 1522 + 12 bytes MPLS - 4 bytes FCS. 1530 now.
> 
> > All input/output ports of the switches was configured with MTU=1540 bytes.
> > But big packets didn't reach remote PE. 
> 
> If 1540 is in fact the physical MTU, you are short. The 1530 byte packet
> is in itself a payload that is encapsulated in a local ethernet frame,
> meaning another header+FCS will be added, i.e. another 18 bytes. The
> actual ethernet frame you are transmitting over the wire is now 1548
> bytes.

two times eternet framing ? Is this visible in e.g. ethereal or something
similar ?

---Maik

> 
> If however 1540 is actually IOS-style MTU (e.g. you are referring to the
> actual Cisco switches here, no other switches involved), which only
> counts the payload, then 1540 should have been sufficient.
> 
> /leg
> 
> 
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