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Re: Dynamically Changing Exit Policy (iBGP)

  • From: Benjamin Howell
  • Date: Tue Oct 30 10:26:26 2007

On Tue, Oct 30, 2007 at 12:55:24AM -0400, Jon Lewis wrote:
> On Mon, 29 Oct 2007, Benjamin Howell wrote:
> >On Mon, Oct 29, 2007 at 04:53:50PM -0400, Deepak Jain wrote:
> >>You can "nail" down your announcements to external peers by tying their
> >>network blocks to a route-of-last resort on one of your loopbacks. This
> >>will prevent flapping externally.
> >
> >Point taken, but it's actually difficult to nail down all of our
> >routes. We have some lone /24's that are not subnetted and thus cannot
> >be used with an 'ip route ... null0' statement. When WAN connectivity
> >drops, the routes flap if we don't have a stable iBGP session. Thus I'd
> >like to steer well clear of severing the iBGP session.
> Not subnetting them doesn't mean you can't
> ip route a.b.c.d null0 250
> while still routing the /24s internally (with lower metric) or having them
> connected on some interface.

Whoops, some oversights make you feel like an idiot. You're right.

> >Only a single internal /30 route will be removed when an interface goes
> >down. I can't come up with a route-map implementation that would
> >add/remove the weights to the routes already received from our eBGP
> >neighbors. If I'm missing something, please let me know.
> ...
> >>>I'd like to dynamically change from best-exit to a "hot potato" exit
> >>>policy when an internal DS3 fails. We fail over to a much lower
> >>>bandwidth link and would like to avoid sending anything but internal
> >>>traffic over that link. If it's not already clear, this change needs to
> >>>happen automatically.
> Are you talking about a single internal DS3, or the more general case of 
> "if any of our internal DS3s are down, we need to route differently"?
> If it's a simple case of two DS3 connected routers which are iBGP peers 
> and also have directly connected eBGP peers, could you use route-maps to 
> set ip next-hop on iBGP exchanged external routes (setting the ip next-hop 
> to be the IP of the other end of the internal DS3, with a second IP of an 
> eBGP neighbor interface)?  I haven't tried it, but it seems like it might 
> do what you want.

Indeed, I'll give it some thought. That seems like it should work. In my
case, it is just two DS3 connected routers. I figured I'd leave the
question open-ended though for other readers' benefit.

> Another possiblility (I've never tried) would be to configure multiple 
> iBGP using loopback IPs, the other using the DS3 interface 
> IPs, exchanging internal routes over both sessions, while exchanging 
> external routes over only the second.  If the DS3 goes down, the session 
> exchanging external routes dies.  I'm not sure you can do this, but I 
> think by having different peer/endpoint IPs (loopbacks for one session, 
> serial interface IPs for the other), it may work.

Actually this suggestion seems to be a common theme. I hadn't considered
this possibility and it seems like it should work fine. David Burns also
suggested this in an email that wasn't directed to the list.

Thanks for everybody's input. I should have some workable options now.

Ben Howell