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 255.255.255.0 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 sessions...one 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