[nsp] ospf default routes and bgp injection

[Gert Doering]

Hi,

On Tue, Mar 30, 2004 at 10:35:44PM -0700, Christopher J. Wolff wrote:
> In the example below, Router A receives a full routing table via BGP.
> Router A, Router B and Router C are members of ospf area 0.  There are no
> static routes in this network at this time.
> 
> ISP---Router A---Router B---Router C
> 
> If I understand correctly, Router C can connect to the ISP through Router B
> if one of the following conditions are met:
> 
> 1. Router A redistributes its BGP table into the ospf area 0.

Don't do that.  Never ever.  OSPF will die a miserable death if fed 
113000 routes.

> 2. Router B advertises its default route to router C using
> default-information originate.
> 3. Router A advertises 0.0.0.0 to all members of area 0 since its
> configuration specifies default-information originate always.

Version 3 is similar to what we do.

In the setup above, there is no need for Router B and Router C to know
anything specific.  "The internet is over there (Router A)" is all they
need to know - and a default route accomplishes this perfectly well.

> Accordingly,
> 
> If we choose option 1, Router A redistributes its BGP table into the ospf
> area, all routers need to have magnificent amounts of memory.

And all routers will die.

> With option 2, If router B advertises its default route, it becomes default
> gateway for all current and new ospf area 0 members, which is not desirable.

It would work, but would lead to packet loops between A and B for every
network that's not in BGP.  Not useful.

> With option 3, If router A advertises 0.0.0.0, this example would be broken
> when I add router D and E to area 0, right?
> 
> ISP-A/BGP---Router A---Router B---Router C
> 		| (igp)
> ISP-B/BGP---Router D---Router E

In that case, "A" and "D" should speak iBGP among each other, to make sure
both know all available routes.

For all others, it doesn't matter whether "A" or "D" is "the default
gateway" - as soon as your packets hit either, it will be BGP-routed.

I'd actually have "A" *and* "D" inject a 0.0.0.0/0 default route into the
IGP.   Otherwise the default route will be gone if "A" dies.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de