looping
Tony Li <tli@cisco.com> Wed, 20 May 1992 12:24 UTC
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Date: Tue, 19 May 92 16:21:28 -0700
From: Tony Li <tli@cisco.com>
Message-Id: <9205192321.AA16899@lager.cisco.com>
To: msteenst@bbn.com
Cc: yakov@watson.ibm.com, idpr-wg@bbn.com
In-Reply-To: Martha Steenstrup's message of Tue,
19 May 92 18:14:01 -0400 <9205192242.AA23955@jerico.usc.edu>
Subject: looping
Hi Martha,
Suppose X and Y both seek routes to Z. Z generates a distance vector
message at time 0 and begins propagating it to its neighbors.
By time 1: X's route is X .. Z, and Y's route is Y .. Z.
By time 2: X's route reaches Y, and Y's route reaches X.
X's route becomes X .. Y .. Z, and Y's route
becomes Y .. X .. Z.
By time 3: X's new route reaches Y, and Y's new route
reaches X. X rejects the route X .. Y .. X .. Z
because it contains a loop, and Y rejects the route
Y .. X .. Y .. Z because it contains a loop.
Let me first plainly state that we're purely talking BGP/IDRP here and
not just any random DV. When a route is advertised, it is implicitly
a replacement of the previous known route. Thus, when Y advertises
X .. Y .. X .. Z back to X, X is obligated to remove X .. Y .. Z.
This prevents:
However,
X still retains the route X .. Y .. Z, and Y still
retains the route Y .. X .. Z.
If you want to talk random DV protocols, I would be happy to do so,
but I don't see the point. ;-)
Tony