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Re: BGP Default Route

  • From: Joe Abley
  • Date: Sat Sep 14 17:05:22 2002

On Sat, Sep 14, 2002 at 04:20:18PM -0400, Lupi, Guy wrote:
> I see what you are saying, and I understand that the default route would be
> originated per neighbor, or per peer group for all neighbors within that
> peer group.  My biggest concern is that if the aggregation router with this
> configuration was to lose connectivity back to the routers which provide it
> with external routing information, it would still announce the default to
> that neighbor.  Do you feel that this is an acceptable risk, taking into
> consideration that the aggregation router has redundant connectivity to
> those routers that provide it with it's external routing information and it
> is highly unlikely that the router would lose it's view of the world?

Taking just defaults from multiple transit providers affords some
protection against failures in the provider access circuits, but
probably not from routing or connectivity failures within the
providers.

In some configurations it can certainly be adequate to just take a
default. Consider the case where a customer has a single transit
provider (who sends default) but one or more peers (who send a peer
view). Taking a full table from the transit provider doesn't change
the basic behaviour of the network (undeliverable packets get
dropped).

There are 101 motivations for multi-homing, though. You can't even
always classify your peers as either "peers" or "transit" without
including variations like "partial transit", or "domestic transit",
or "transit to that one particular AS who refuses to peer with us
for business reasons, whose routes we otherwise have no way of
learning".

Hence, it's difficult to come up with a general rule of thumb for
when defaults are appropriate, and when they are not.

Just as with every other aspect of your network design, you need
to sit down and figure out your failure modes, and come up with a
strategy that protects you against the ones for which the cost of
failure is greater than the cost of protection (or some other
appropriate metric).


Joe




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