Margaret,
Margaret Wasserman wrote:
>
> In an attempt to simplify my earlier example, I oversimplified
> it. Here is a better diagram:
>
> >The ICMP issue that you have described would also exist when using
> >multiple "conceptual" routing tables in the case of a partitioned
> >site. For example:
>
> ==========================================================
> SITEA
> Host1 Host2
> | |
> ______._|_________ ___.______._|_____________
> Link1 | | | Link2
> | (down) |
> | | |
> |+-----------------+ |
> || |
> ============R1========================R3===================
> SITEB | |
> | |
> _____|_________________________|________________
> Link3
>
> ===========================================================
>
> Now, if Host1 sends a packet with site-local source and
> site-local destination, it obviously won't reach Host2.
> R1 will receive the packet, and will be unable to route
> the packet because there are no routes available in the
> conceptual site-local routing table for SITEA. This
> will result in an ICMP/No Route to Destination error.
> The use of the conceptual routing table prevents R1 from
> noticing that there is a global route to Link2, so the
> potentially useful ICMP/Scope Exceeded error is not sent.
How will R1 know of a route to Link2 based on the site-local
addresses in the packet? Will the routing protocol be able
to correlate which site-local prefixes are co-located with
global prefixes? I am not sure that assumption can be made.
>
> The user may see that some applications can reach Host2
> and other can not, but no pertinent error message will
> be displayed. Also, "smart" applications do not receive
> the information that addresses of a greater scope
> could potentially reach the destination.
>
> But, if Host1 sends a packet with site-local source and
> global destination through the same path, the ICMP/Scope
> Exceeded message will be returned.
>
> Is it is important to maintain the "Scope Exceeded" message
> _only_ for packets where the destination scope is greater
> than the source scope? Even if it means that we may discard
> packets that could successfully reach the destination in
> some cases (as in my first example)?
>
> A mixed scope packet where the source is greater than the
> destination is the _only_ time that a Scope Exceeded message
> will be generated in the current architecture. And, it
Ah, the intent was that the Scope Exceeded message is used when
the destination is greater than the source. If the source
address is greater than the destination, the inability to route
the packet should be due to the lack of routing information for
the source prefix.
Brian
--
Brian Haberman
Nortel Networks
[EMAIL PROTECTED]
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