I'm trying to find a definitive answer on this one...
A lot of network engineers (myself included) when conducting loop testing on a /30 pe-ce wan connection (hdlc encapsulation) will ping from the pe the local interface of the pe, and given that the pe sees the loop and is up, up, the ping works a treat.
However, I'm wondering how this actually works...
Reason being, a show ip cef of the local interface ip, will show the "destination" as receive.
Now, if cef WAS being used, cef would realise that the packet was destined for a local interface on its own router.
So why would cef forward the icmp echo out of its own interface?
Is the locally generated ping only using process switching when sending out the icmp echo (with a sh ip ro <destination>
showing the destination is directly connected VIA the interface, so therefore sends icmp out of the interface).
Then when icmp packet is received (via the loop), cef running on the input interface checks and sees that the packet is indeed destined for itself?
I understand that hdlc is needed in order for the interface to be in an up, up (looped) state in order for the routing table to be populated
with the directly connected interface, and hence cef table is updated, but some clarification as to how the ping to loop actually works
would be greatly appreciated.