The IP over ATM Mailing List Archive by date

[Curtis Villamizar <curtis@ans.net>]

In message <199601302113.NAA25540@hubbub.cisco.com>, Yakov Rekhter writes:
> Curtis,
> 
> > This example is relevant to NHRP because it details the consequences
> > of the misuse of NHRP.  Such misuse has been discussed and advocated
> > by numerous people in the ROLC and IPATM WGs in the past, and
> > indications are NHRP continues to be misrepresented by industry
> > marketing and sales people who are not closely following the IPATM WG.
> > 
> > I think this is sufficient reason to 1) leave the example in the
> > document, 2) continue to reference the possibility of routing loops in
> > the context of misuse of NHRP.
> 
> If you'd like to keep the example, then please add the following text:
> 
>    It is well-known that truncating AS_PATH information in BGP or losing
>    metric information in Distance Vector protocols (e.g., RIP) could
>    result in persistent forwarding loops. These loops could occur
>    without ATM and without NHRP. The following example illustrates
>    how the use of NHRP in the environment where forwarding loops could
>    exist even without NHRP (due to either truncated AS_PATH information
>    or loss of metric information) could produce forwarding loops.
>    
> as this text clarifies the source of looping.
> 
> Yakov.


Sure.  I've added this.  The first two senteces were added after the
two paragraphs we previously agreed to and before the paragraph
explaining that the figure applies only to the improper use of NHRP.
The remaining sentece was added to the end of the paragraph which
introduces the example.

The text added to the -06 draft (.tex source) now reads:

   NHRP assumes that routers do run routing protocols (intra and/or inter
   domain) and/or static routing.  NHRP further assumes that forwarding
   tables constructed by these protocols result in a steady state
   loop-free forwarding.  Note that these two assumptions do not impose
   any additional requirements on routers, beyond what is required in the
   absence of NHRP.

   NHRP runs {\em in addition} to routing protocols, and provides the
   information that allows to eliminate multiple IP hops (the multiple IP
   hops result from the forwarding tables constructed by the routing
   protocols) when traversing an NBMA network.  The IPATM and ROLC WGs
   have both expended considerable effort in discussing and coming to
   understand these limitations.

   It is well-known that truncating path information in Path Vector
   protocols (e.g. BGP) or losing metric information in Distance Vector
   protocols (e.g., RIP) could result in persistent forwarding loops.
   These loops could occur without ATM and without NHRP.

   The combination of NHRP and static routing alone cannot be used in
   some topologies where some of the destinations are served by multiple
   routers on the NBMA.  The combination of NHRP and an intra-AS routing
   protocol that does not carry inter-AS routing path attributes alone
   cannot be used in some topologies in which the NBMA will provide
   inter-AS transit connectivity to destinations from other AS served by
   multiple routers on the NBMA.

   Figure~\ref{looping} provides an example of the routing loops that may
   be formed in these circumstances.  The example illustrates how the use
   of NHRP in the environment where forwarding loops could exist even
   without NHRP (due to either truncated path information or loss of
   metric information) would still produce forwarding loops.

This is all text added to the draft.  I think we now have agreement
that this text warns of the perils of improper use of NHRP with broken
routing, treating NHRP fairly by not putting the blame on NHRP, but
clearly dispelling any misconception that adding NHRP to such cases
might somehow provide adequate information to prevent looping.

Curtis

ps- If there are no further objections I'd like to submit a -07 draft
and ask that the IESG last call be concluded or repeated.