Beware of learning the Internet Control Message Protocol (ICMP) just from
Cisco sites. ICMP is specified by the IETF, not Cisco. It's used by all IP
devices, not just Cisco devices. Cisco describes it in variously strange
ways that cannot be trusted. Learn it from the actual specification, which
is RFC 792. Here's the possible values for the code field that accompanies
a Destination Unreachable ICMP message, according to RFC 792:
0 Network unreachable
1 Host unreachable
2 Protocol unreachable
3 Port unreachable
4 Fragmentation needed and don't-fragment (DF) bit set
5 Source route failed
It's important to be able to distinguish the difference between not having
a route to a network, which could happen anywhere along the path, and not
being able to forward the packet to the final destination, which only
happens at the last hop. Not having a route would result in a Network
Unreachable which Cisco displays as N. When the last-hop router can't
forward the packet to the end node, the code is Host Unreachable, which
Cisco displays as H.
Generally what has happened when you see an H is that the final router sent
an ARP to try to forward the packet, didn't get a response, and sent back
to the source an ICMP Destination Unreachable, code = Host Unreachable.
Other situations can result in H, but this is the usual case.
Updated ICMP codes are listed in RFC 1700, the Assigned Numbers RFC. RFC
1700 adds some new codes that allow a router to be more specific in
describing the routing or forwarding problem. Administratively Prohibited
codes (which Cisco displays as A) are in this NEW list. That's why there's
some argument about it. Older implementations of ICMP don't use the A
codes. Administrative (i.e. security or access list) blocks could happen
anywhere in the path or at the end node. So you could see A instead of N or H.
Here's the new list of codes, according to RFC 1700:
0 Network Unreachable
1 Host Unreachable
2 Protocol Unreachable
3 Port Unreachable
4 Fragmentation Needed and Don't Fragment was Set
5 Source Route Failed
6 Destination Network Unknown
7 Destination Host Unknown
8 Source Host Isolated
9 Communication with Destination Network is Administratively Prohibited
10 Communication with Destination Host is Administratively Prohibited
Priscilla
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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