BGP Default Route

Sat Sep 14 20:47:47 UTC 2002

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 [mailto:mleber at he.net]
> Sent: Saturday, September 14, 2002 4:19 PM
> To: Lupi, Guy
> Cc: 'nanog at merit.edu'
> 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 10.10.10.2 default-originate
>  neighbor 10.10.10.2 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.
> 
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+----------------- H U R R I C A N E - E L E C T R I C -----------------+
| Mike Leber           Direct Internet Connections   Voice 510 580 4100 |
| Hurricane Electric     Web Hosting  Colocation       Fax 510 580 4151 |
| mleber at he.net                                       http://www.he.net |
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