On Tuesday, September 16, 2014 06:55:40 AM Wes Smith wrote: > I've often wondered how large ISPs handle some basic IGP > design issues re routing between I-BGP nodes in the > network. For example .. assume two BGP routers > connected over a backbone running some kind of IGP I-BGP > uses the local IGP to route to other I-BGP nodes within > the ASThe I-BGP next-hop address is the Egress I-BGP > node (normally) or the IBPG peer loopbackBest practice > says to keep the IGP small, for example just the I-BGP > loopbacks So a potential routing table entry in router > BGP1 would be something like Rtr BGP1 can reach > '8.8.8.8/32 via 10.10.10.10, where 10.10.10.10 is router > BGP-2s loopback address and BGP-2 is the egress router > So far so good. .. I understand all this part. But my > puzzle is .. for this to work, the IGP would also need > to have a route for 8.8.8.8. so it can route it to my > other IBGP router at 10.10.10.10But my IGP doesn't have > a route for 8.8.8.8 as I don't redistribute BGP into > IGPSo catch-22 . I've resolved this internally using > tunnels between my I-BGP routers and using > next-hop-self. But I'm pretty small scale .. How do > larger organisations do it? I'm guessing MPLS TE or > other tunnels between the POPs .. but would like to hear > from some folks that have done it.Thanks WS (apologies > for any dups..)
First, use NEXT_HOP=self; it's good practice. Your iBGP speakers will have 8.8.8.8/32 via 10.10.10.10 as an entry, for example. Your router will then do a look-up for how to get to 10.10.10.10. It will see that this is known via the IGP (IS- IS, OSPF, Static, e.t.c.), which will have an exit interface (TenGigabitEthernet0/2, for example) toward the next-hop (10.10.10.20, for example). And so it goes until the packet gets to 10.10.10.10. It really is that simple. Mark.
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