From: Mark H. Degner (mark@xxxxxxxxxx)
Date: Mon Aug 14 2000 - 22:54:40 GMT-3
Brian,
BGP CAN generate a default route. Note the 'default-information originate' com
mand that is
available in the BGP router configuration mode. And you CAN redistribute a def
ault route into BGP.
This IS an option provided by ISPs, and for good reason. Most ISPs give you th
ree options when
setting up BGP. You get the full Internet routing table, that you guys are tal
king about, or you
get only their customer routes, and all other Internet routes need to be provid
ed for by some sort
of default, or the third option is to have them ONLY advertise a BGP default ro
ute to you. Sure,
you could program a static default route on a BGP router if you didn't have the
full routing table,
but this doesn't help you if you have multiple BGP routers, and a physical link
goes down. Your
router will still have a route to 0.0.0.0 and will happily advertise as such.
If you receive the
default route from your provider, and that link goes down, this router no longe
r knows about 0.0.0.0
through that link, and will send traffic to whatever other router is generating
a default route.
Everyone talks like using a default route with BGP is some sort of cardinal sin
. Sure, if a default
route is advertised to the Internet, bad things happen, but this is on the shou
lders of the network
designer to avoid, and like I said before, any ISP worth its salt will only acc
ept route
advertisements from you that have been predetermined, ie, whatever AS you plan
to advertise. So
even if you TRIED to inject a default route outbound, it would be rejected by t
he BGP neighbor.
Just because a router generates a default into BGP doesn't mean that every rout
er on the Internet is
going to see it.
A default route in BGP isn't instantly bad. It all depends on the needs of the
network, and the
network design.
Mark Degner
----- Original Message -----
From: "Brian S turner" <brian@theatlasgroup.net>
To: <ccielab@groupstudy.com>
Sent: Monday, August 14, 2000 1:53 PM
Subject: RE: BGP default network
> If I may add some to this discussion.
>
> You guys are simply talking about 2 different things. You are both right.
> A default route being generated by 1 router on the internet, and we are
> talking Def Con 1. That same router on the otherhand, if it has a default
> route configured for itself is perfectly feasable. Do you see the
> difference?
>
> To answer the original question, no you cannot generate a default route, and
> I don't think you can redistribute a all 0's route into BGP , check me on
> that one. And you wouldn't want to. BGP puts the default in the term
> Default route.
>
> On the other hand, If you had a BGP router, and were using some filters to
> block AS's more than 2 hops away, you could put a default route on the
> router, to cover the routing for all the other places on the internet. This
> wouldn't be advertised to the internet it would simply be used locally, as a
> gateway of last resort. As a matter of fact this is how I would prefer to
> setup an internet connected router, even if I had the biggest fastest, most
> memory router on the planet.
>
> Brian
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