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Re: BGP Default Route

  • From: Henry Yen
  • Date: Sat Sep 14 17:20:37 2002

On Sat, Sep 14, 2002 at 04:49:23AM -0400, Lupi, Guy wrote:
> Assume I am originating default for customers that only want a default
> route, or a default route and some portion of the full Internet routing
> table.  You're right, if I am the only gateway then it really doesn't

If you're the only gateway, why would you be running eBGP with the
customer at all (unless the customer has his/her own peers)?

> matter.  Obviously if there is more than one provider it would be better for
> the customer to accept full routes, but there are some customers out there
> that have 2 providers and don't want to assume the cost of purchasing a
> router that can accept 2 providers feeding it full tables (why you would

Perhaps the customer's upstreams are not in the same "tier" (e.g. one
provider is expensive tier 1 and is metered, and the other provider is
a local, cheap, but tier 2/3).  If the smaller provider is not as
well connected as the larger one, full routes can be sub-optimal, no?

> assume the cost of 2 providers and not a reasonably priced router that can
> handle it I don't know, but I have run into it before).  I am really just

There are customers who are multihomed at geographically distinct locations;
packets routed to any BGP border from interior (non-BGP) routers are often
better off just taking the nearest default 0/0 outbound.

> curious as to how people implement this and their reasoning for selecting a
> particluar method.  Is your method the one you stated before, default
> origination from the router that is directly connected to the customer?

FWIW, the large tier-1's we've had experience with do just that, and
assume that their POP's are "never" cut off from the rest of the 'net.

> -----Original Message-----
> From: Mike Leber []
> Sent: Saturday, September 14, 2002 4:48 PM
> To: Lupi, Guy
> Cc: ''
> Subject: RE: BGP Default Route
> The answer is you can do it all sorts of ways.
> Why are you originating default?
> If you are originating default because you are the only gateway for a
> customer, whatever partial connectivity your router has is better than
> effectively turning them off if you have a network partition.
> If your customer has more than one upstream they really should take full
> views so they have the ability to make routing decisions based on that
> information.  This fixes your concern and is the best engineering choice.
> A hack would be to conditionally announce default based on the presence of
> some specific other route.  This would be doing additional work to
> implement a suboptimal solution which a simpler use of BGP (full views)
> fixes automatically.
> Yes, as much as you can, your routers should be meshed with more than one
> connection each.
> Mike.
> On Sat, 14 Sep 2002, 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?
> > 
> > -----Original Message-----
> > From: Mike Leber []
> > Sent: Saturday, September 14, 2002 4:19 PM
> > To: Lupi, Guy
> > Cc: ''
> > Subject: Re: BGP Default Route
> > 
> > On Sat, 14 Sep 2002, Lupi, Guy wrote:
> > > I was wondering how people tend to generate default routes to customers
> > > running bgp.
> > 
> > Typically you would only originate default via BGP to a customer that
> > isn't taking a full view.
> > 
> >  neighbor default-originate
> >  neighbor filter-list 9 out
> > 
> > ip as-path access-list 9 deny ^.*$
> > 
> > >  Is it from the aggregation router that customers are directly
> > > connected to, or from one or more core/border routers?
> > 
> > In the example above the default originate is done via a specific BGP
> > session, so it isn't router wide on either core or border routers.
> > 
> > > If one is using a default route to null 0...
> > 
> > I'll leave the rest of this for somebody else to answer.
> > 
> > Mike.

Henry Yen                                       Aegis Information Systems, Inc.
Senior Systems Programmer                       Hicksville, New York

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