[nsp] AToM

[sthaug at nethelp.no]

> > 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.
> When the packet arrives at the switch, switch adds 4 bytes VLAN tag.
> 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, there is more. If you're using EoMPLS, you have a normal Ethernet
frame minus the CRC (1514 bytes without VLAN tag, 1518 bytes with VLAN
tag), then add 2x4 bytes MPLS label and 4 bytes EoMPLS control word, for
a total of 1530 bytes.

> All input/output ports of the switches was configured with MTU=1540 bytes.
> But big packets didn't reach remote PE. 

With 1540 bytes your configuration *should* have worked. I'm afraid I
have no idea why it didn't.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no