[c-nsp] how to get ICMP host unreachables?

[Oliver Boehmer (oboehmer)]

> Just out of curiosity tho...  it does seem like the router saves at
> least one packet  while waiting for the arp reply.
> If I do a 'clear arp' on the router only some of the arp entries
> disappear. 
> Picking one of the hosts that does have it's arp entry cleared out,
> pinging it gets a 100% response rate.  Is there something else going
on under
> the hood that keeps the first ping packet around while the router
sends
> the arp request & waits for the reply?  If it makes a difference, the
router
> in question is a cat6500/SUP720 running IOS 12.2(17d)SXB6 and, at
least
> as far as I can tell, CEF is enabled.

I bet CEF is enabled. Not sure about this specific platform, but on a SW
forwarding platform, we drop the first packet while doing ARP:

r3>ping 10.0.20.5

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.20.5, timeout is 2 seconds:
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 20/20/20 ms
r3>

reducing the timeout of 2 seconds down shows even more packet loss..
"show cef drop" reports these drops as encaps failures

	oli