On Thu, Sep 28, 2006 at 10:45:24PM +0300, peter dunaskin wrote: > Hello, > > We're a small ISP working on redesigning our network's topology and > the overall architecture before replacing the existing > infrastructure, which happens to be different variations of low cost > router-boards running Linux. The ultimate goal thus being to run > OpenBSD wherever we can. > > A job on top of our list would be to to migrate the core routers. > Such solution would involve carp(4) and pfsync(4) -- hopefully making > them fully redundant and a lot more solid network appliances than > they currently are. > > But we're having a tough time figuring out how BGP should operate. > In the existing scheme, an AS (autonomous system ) with 2 IP address > blocks (193.x.x.0/24 and 194.x.x.0/23) is what holds our networks > together. > > Our two optical lines, namely [upstream0] and [upstream1] are > separated geographically (as illustrated in Fig. 1), themselves being > interconnected by a wireless link. > > [upstream0] [upstream1] > | | > | | > [core0]---------(wireless)---------[core1] > |193.x.x.0/24 |194.x.x.0/23 > | | > [clients] [clients] > > Fig. 1 > > Question; Is it possible to make two BGP neighbors (one at core0 and > second at core1) by using only one AS? We could get a second AS, but > as we expect to have more networks around we would really like to set > it up by only using one AS, not bothering ripe.net every time we > expand the network. And yes, we plan to use OpenBGP. >
Yes, you can use one AS to announce two networks even if they are split. You need to have a IBGP session between core0 and core1 and you should configure core0 to announce 193.x.x.0/24 and core1 to announce 194.x.x.0/23. The clou here is that if the wireless link goes down your networks are still reliably reachable -- actually you can use any of the links without major effects. This should mostly do the trick. Perhaps you need to play around with localpref, med or prepend-self to tune the network so that traffic for 194.x.x.0/23 will prefer upstream1 over the wireless link but this mostly depends on your policy how you would like your traffic to be routed. Use prepend-self to steer incomming traffic and local-pref for outgoing traffic. -- :wq Claudio
