[c-nsp] bgp/ospf default route interaction

[Randy]

Hello Michael,

Please re-read and take the time to understand.
<snip>.....
on to "default-information-originate *always*:
> > 
> > this causes localy router to inject a default into OSPF
> domain regardless of whether it has a default OR
> not.(remember it's default can never be via OSPF)
...<snip>

Yes, it can be a bit confusing....but now You *know*.

Regards,
./Randy

--- On Mon, 8/27/12, Michael Ulitskiy <mulitskiy at acedsl.com> wrote:

> From: Michael Ulitskiy <mulitskiy at acedsl.com>
> Subject: Re: [c-nsp] bgp/ospf default route interaction
> To: "Randy" <randy_94108 at yahoo.com>
> Cc: "Christian Meutes" <christian at errxtx.net>, "cisco-nsp" <cisco-nsp at puck.nether.net>
> Date: Monday, August 27, 2012, 3:47 PM
> Randy,
> Thanks for the explanation.
> I do understand the first part (without "always")
> I guess what confused me is that I didn't expect a router
> would always prefer a locally injected
> ospf default over the learned one. Since both LSAs are in
> the database (just checked it) I expected
> it to go ahead and compare the metrics. In my example I
> specifically lowered the metric of default
> advertised by CORE1 and it didn't make any difference. CORE2
> still ignored it.
> Anyway, it's much clearer now.
> Thanks Randy and Christian,
> 
> Michael
> 
> 
> On Monday 27 August 2012 17:32:04 Randy wrote:
> > Hi,
> > ..actually not quite!
> > 
> > case1: default-information-originate (core1 and core2)
> > 
> > core2 learns 3 defaults:
> > via iBGP from core1 AD 200 but loc pref 150
> > via eBGP from ISP2 AD 20 loc pref less than iBGP
> learned default
> > via OSPF ad 110.
> > 
> > so on the face of it, it installs OSPF learned default
> in rib....what has happended in the process is that
> "default-information-originate" on Core2 *is NOT actually
> working*!
> > 
> > "default-information-originate" within OSPF *requires*
> that local router have a default-route known via any other
> igp/egp BUT NOT OSPF.
> > 
> > on to "default-information-originate *always*:
> > 
> > this causes localy router to inject a default into OSPF
> domain regardless of whether it has a default OR
> not.(remember it's default can never be via OSPF)
> > 
> > so what happens in the process is core2 suppresses OSPF
> learned default via core1 and now has iBGP default or eBGP
> default to choose from. It as a result chooses iBGP learned
> default in its rib.
> > 
> > (hint: in your original output, you will notice both
> eBGP and iBGP routes suffered from rib-failure...)
> > 
> > HTH wrt what is happening behind the scene.
> > ./Randy
> > 
> > --- On Mon, 8/27/12, Michael Ulitskiy <mulitskiy at acedsl.com>
> wrote:
> > 
> > > From: Michael Ulitskiy <mulitskiy at acedsl.com>
> > > Subject: Re: [c-nsp] bgp/ospf default route
> interaction
> > > To: "Christian Meutes" <christian at errxtx.net>
> > > Cc: "cisco-nsp" <cisco-nsp at puck.nether.net>
> > > Date: Monday, August 27, 2012, 1:57 PM
> > > Thanks for the reply.
> > > So what you're saying is that if ospf router
> itself injects
> > > ospf default in its ospf database then
> > > it would ignore any other ospf defaults it might
> receive
> > > (regardless of metric) and also it wouldn't 
> > > install this default in the RIB. 
> > > Did I get it right? 
> > > I didn't know this rule and I'm a little
> surprised, but I
> > > ran a few experiments and they seem to confirm it.
> 
> > > Thanks,
> > > 
> > > Michael
> > > 
> > > On Monday 27 August 2012 16:06:14 Christian Meutes
> wrote:
> > > > Hiho,
> > > > 
> > > > On Aug 27, 2012, at 7:52 PM, Michael Ulitskiy
> <mulitskiy at acedsl.com>
> > > wrote:
> > > > 
> > > > > Now on CORE2 I add 'always' keyword to
> OSPF
> > > 'default-originate' command,
> > > > > making it:
> > > > > 
> > > > > CORE2: router ospf 100
> default-information
> > > originate always
> > > > > 
> > > > > For some reason I don't understand it
> changes the
> > > game completely.  Now
> > > > > IBGP route is preferred over OSPF
> > > > 
> > > > it's because you inject the 0/0 into OSPF on
> CORE2
> > > *always*, effectively
> > > > annihilating the other 0/0 advertised by
> CORE1s OSPF on
> > > CORE2.
> > > > 
> > > > Thus, the only remaining defaults are the
> ones learned
> > > via BGP.
> > > > 
> > > > Without the always-keyword it seems that the
> OSPF route
> > > is learned fast
> > > > enough to prevent the BGP one to be installed
> in.
> > > > 
> > > > 
> > > _______________________________________________
> > > cisco-nsp mailing list  cisco-nsp at puck.nether.net
> > > https://puck.nether.net/mailman/listinfo/cisco-nsp
> > > archive at http://puck.nether.net/pipermail/cisco-nsp/
> > > 
> > 
>