Ms Oppenheimer,
That is a vast amt of great info and is precisely what I needed to read
regarding this subject. It actually explains it all. And puts it in
prespective, including Cisco books and their info of which I have learned a
little of today. I am not saying that I feel Cisco is bad after reading the
below, but I can see that I need to expand my resources of reading to get a
better overall understanding instead of a one-sided view.
Thanks again,
Jennifer Cribbs
[EMAIL PROTECTED]
>===== Original Message From Priscilla Oppenheimer <[EMAIL PROTECTED]> =====
>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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