The Routing Over Large Clouds Mailing List Archive by date

[Dave Katz <dkatz@cisco.com>]

   From: rcoltun@sura.net
   Date: Tue, 18 Oct 94 22:36:34 -0400



   NHRP looks quite reasonable to me except for the fact that routers
   will have to periodically send out requests:

   >8.1 Router-to-Router Operation
   >
   >   In practice, the initiating and responding stations may be either
   >   hosts or routers.  However, there is a possibility under certain
   >   conditions that a stable routing loop may occur if NHRP is used
   >   between two routers.  This situation can be avoided if there are no
   >   "back door" paths between the entry and egress router outside of the
   >   NBMA network, and can be ameliorated by periodically reissuing the
   >   NBMA network, and can be ameliorated by periodically reissuing the
   >   NHRP request.  If these conditions cannot be satisfied, the use of
   >   NHRP between routers is not recommended.

   In my mind this a bit of a hand-wave although I certainly don't have
   better solution; doesn't look like it will scale all
   that well. Does it give anyone else heartburn?

I think that ultimately the router-to-router case will involve running
a stripped-down BGP or IDRP across the short-cut path.  This will make
it possible to detect the looping case.  It's intentionally handwaved
for the moment, however.

   One additional (implementatin related) question - suppose a NHS has more
   than one ATM port.  How is it supposed to determine if both ports are on
   than one ATM port.  How is it supposed to determine if both ports are on
   the same NBMA network? It certainly can look at either IP or P-NNI and
   the same NBMA network? It certainly can look at either IP or P-NNI and
   make some determinations...

The fundamental issue is in how the NBMA network IDs (a.k.a. "fabric numbers")
The fundamental issue is in how the NBMA network IDs (a.k.a. "fabric numbers")
are assigned and whether they show up in other protocols.  Ultimately the
boundaries of a fabric have to be configured somewhere, and if a single
fabric is being administratively treated as multiple fabrics, then the
configuration point would seem to be at the IP layer.

Keep in mind that this is likely to be used for more than just ATM.
Keep in mind that this is likely to be used for more than just ATM.
X.25 still sells like crazy...