RE: BGP Default Route

["Stephen J. Wilcox" <steve () opaltelecom co uk>]

On Sat, 14 Sep 2002, 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

Why cant the customer point default at you? Makes more sense from an admin point
of view

> table.  You're right, if I am the only gateway then it really doesn't

Why bother running BGP in that case! If you're the only transit but they have
other peers they may as well default to you.

> 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

If they are clever about the 2 BGP feeds they can allow the 2 ISPs to send a
full table but filter inbound prefixes that you know are not going to be
installed eg if your transits are AS2 and AS3 then you can immedaitely filter
all routes from AS2 with AS3 in the path and vice versa.. if these are large
ISPs that will seriously reduce the routes received!

> router that can accept 2 providers feeding it full tables (why you would
> 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
> 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?

I've not done this but presumably you could either use default-originate or if
you want to guarantee some level of functionality in your network you could
insert a default into BGP at some other core router and then allow this route
out to this peer and not others, if your edge router loses connectivity with the
core router originating the route then it will withdraw it.. (assumes no default
in your routing table other than that of course)

Either way, as I say tho I'd suggest the customer does the engineering and not
you!

Steve

> -----Original Message-----
> From: Mike Leber [mailto:mleber () he net]
> Sent: Saturday, September 14, 2002 4:48 PM
> To: Lupi, Guy
> Cc: 'nanog () merit edu'
> 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 [mailto:mleber () he net]
> > Sent: Saturday, September 14, 2002 4:19 PM
> > To: Lupi, Guy
> > Cc: 'nanog () 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 () he net                                       http://www.he.net |
> +-----------------------------------------------------------------------+