[c-nsp] I-BGP/IGP question

Mark Tinka mark.tinka at seacom.mu
Tue Sep 16 02:10:05 EDT 2014

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
> ' via, where 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   so it can route it to my
> other IBGP router at my IGP doesn't have
> a route for 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 via as 
an entry, for example.

Your router will then do a look-up for how to get to 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 
(, for example). 

And so it goes until the packet gets to

It really is that simple.

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