> I am a little confused about how the MTU size
> configured on an interface affects the transmission of
> packets through that interface. My question is does it
> affects packets received on the interface or packets
> transmitted out of the interface?
Great question. It definitely affects packets transmitted OUT of the
interface - if the packet is larger than the interface ip mtu, the router
fragments the packet using the configured interface MTU value. A
demonstration of this is in GRE tunnel applications, which are by default
1478 bytes (as opposed to 1500), so that a 1500 byte packet gets fragmented
when traversing the tunnel.
On an INBOUND packet, the MTU is ignored.
Just to verify this, I ran a bunch of debugs that show outbound
fragmentation, but inbound the packets are not fragmented, just forwarded to
the next interface.
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