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

[Oliver Boehmer (oboehmer)]

lee.e.rian at census.gov <> wrote on Friday, April 08, 2005 1:57 PM:

> Will a cisco router ever send a host unreachable because a host
> doesn't answer ARP requests?

It won't. 

RFC1812 states in 3.3.2:

   The link layer MUST NOT report a Destination Unreachable error to IP
   solely because there is no ARP cache entry for a destination; it
   SHOULD queue up to a small number of datagrams breifly while
   performing the ARP request/reply sequence, and reply that the
   destination is unreachable to one of the queued datagrams only when
   this proves fruitless

IOS does not implement the "SHOULD" clause, i.e. it does not queue the
datagrams (assuming CEF) and it also doesn't send unreachables.. This
has been day-one behaviour, and I'm not sure why this is needed.

Further down in the RFC it says:

4.3.3.1 Destination Unreachable

  If a packet is to be forwarded to a host on a network that is
  directly connected to the router (i.e., the router is the last-hop
  router) and the router has ascertained that there is no path to the
  destination host then the router MUST generate a Destination
  Unreachable, Code 1 (Host Unreachable) ICMP message.

Can we ascertain that there is no path if we don't receive an ARP reply?
I think this is debatable, unfortunately this hasn't been defined more
precisely.

	oli