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Re: seeing the trees in the forest of confusion

  • From: Tony Li
  • Date: Sat Apr 26 21:50:30 1997

[email protected] (Doug Junkins) writes:

> These cases seem to point to a problem with BGP route withdrawls that will
> continue to increase the time it takes to recover from network problems.
> Perhaps the router vendors would like to comment.

I'm not a router vendor, but I used to play one on TV.  Is that close
enough?  ;-)

Let me comment about BGP, the protocol, as opposed to BGP, the

The protocol dictates that a BGP speaker that receives a withdrawl for a
prefix _MUST_ promptly distribute that withdrawl.  The reason for this is
obvious: a router which has no route to a prefix is blackholing traffic or,
if it has selected a different path, is possibly contributing to a
forwarding loop.  We can argue about the definition of 'promptly', but I
hope it's clear that taking hours to withdraw the route is out of the

Now, please note that a BGP speaker that receives a reachability
announcement for a prefix MAY decide to not advertise it for an indefinite
period of time, for whatever reason.  However, this is subject to some
restrictions.  If the newly reachable prefix is installed in the routers
forwarding table and it chooses not to advertise this fact, the router MUST
NOT advertise a shorter overlapping prefix.  Again, this would be lying
about the forwarding path that packets might take, so there's possibly a
forwarding loop.

What does this mean for an implementation?  In particular, how MUST flap
damping work?  Flap damping MUST NOT damp out withdrawls.  Note that a
_history_ of withdrawals may well be data used by subsequent flap damping
computations, but the withdrawl itself should propagate.  Flap damping
SHOULD happen on reachability advertisement.  To simplify the
implementation, most folks are likely to choose to suppress newly
advertised routes for a time.  While the path is suppressed, the
implementation probably does NOT want to install the path in its forwarding
table.  That would be painful.  Only after the path finishes its
suppression period should it be installed and then promptly advertised.

If your router's implementation is significantly different than this, you
might wanna have a talk with them.

Sooner would be better than later.  ;-)

Please note that I'm not throwing stones or pointing fingers.  I have no
knowledge of the internals of what happened other than what's appeared on
this list.  However, the reports are disturbing and there seems to be some
considerable confusion about the internals of BGP, so I thought some
education was in order.

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