BGP and OSPF

Thu Oct 9 15:02:49 UTC 2003

By definition, R0 should run BGP, or, R1-4 should be meshed and exchange
BGP with each other.  If R1-4 are meshed, then, it doesn't completely matter
which of R1-4 R0 defaults to, they will handle it from there.  If they are
not properly meshed, then, it gets messy, but, the protocol spec. says as
much.

Owen

--On Thursday, October 9, 2003 12:00 PM +0200 Jean-Yves Le Boudec 
<jean-yves.leboudec at epfl.ch> wrote:

>
> It is known that redistribution of routes learnt by BGP into an IGP is
> considered harmful, but I am still wondering how you can route
> without redistribution of BGP routes in an OSPF cloud that
> connects to several external networks. I have the following
> scenario.
>
>
>        RA (AS100)  |    RB (AS101)
>
>    ------------------------
>
>        R1       R2
>
>            R0        (AS559)
>
>        R3       R4
>
>     ------------------------
>
>        RC (AS201)  |    RD (AS202)
>
>
>
> All routers except R0 run BGP. R0--R4 are in the same AS and run
> OSPF. RA-RD are all in distinct ASs.  RA is BGP peer to R1, RB to
> R2, RC to R3 and RD to R4. The addresses and numbers are fake. The
> ASs are peers, not customers.
>
> Assume that R1 learns a route to a network in AS100, says 1.1/16,
> with next hop = 3.3.3.1 (the IP address of the p2p link R1-RA).
> Now assume a data packet with destination address in 1.1/16 is
> received by any router in AS559, say for example R0 or R4. The
> router has to know where to forward it. Since AS559 connects to
> different peer ASs in different locations, it does not seem
> feasible to use default routes.
>
>
> (Method 1) One way to is to assume that R1 redistributes the route
> 1.1/16 into OSPF, which will then propagate it as a type 4 LSA.
> Then R0 and R4 can build a forwarding table (using OSPF) and set a
> forwarding entry to 1.1/16. This method is what is described in
> Huitema's book "Routing in the Internet". Now I understand that
> this is not done in practice (I am right ?) since it forces OSPF
> to carry all the IP prefixes seen by BGP, which in that case might
> be all prefixes in the world.
>
> (Method 2) An alternative is to have recursive table lookup in
> forwarding entries at all border routers (R1 to R4). R4 writes
> that the destination address 1.1/16 is to be sent to NEXT-HOP =
> 3.3.3.1. R4 learns this over I-BGP from R1. The data packet with
> destination address in 1.1/16 uses loose source routing inside
> AS559 and is sent to the link R1-RA. The job of OSPF is only to
> propagate how to route to all addresses in AS559 (including
> 3.3.3.1) and there is  no redistribution of BGP into OSPF. Border
> routers need to update the forwarding tables using their RIB
> learnt from BGP.
>
> Now source routing is obsolete in IPv4, does any one use it ?
>
> (Method 3) Same as method 2, but IP in IP encapsulation is used
> instead of loose source routing. Seems heavy weight for a high
> speed backbone.
>
> (Method 4) Same as method 2, but Tag Switching (or MPLS) is used
> instead of loose source routing.
>
>
> Can any one help me understand what is done in practice among
> Methods 1 to 4, or any other one that I missed ?
>
>
> Thanks in advance,  JL
>