Re: BGP and OSPF

[Haesu <haesu () towardex com>]

< major snip >

> (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.

No. Don't.. Please. I've seen enough networks that break with IGP<->BGP redists.


> (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.

This is the way to do it. Recursive route lookup++

What you can even do is to reduce your IGP table entries:

        1) Have all of your 'edge'/'border' routers set next-hop-self on their IBGP peering to core routers.
           This will eliminate the need for 'DMZ' or '/30 pointopoint (whatever u wanna call it)' routes to exist in 
IGP tables. Smaller IGP = Faster convergence = more stability = more SLA guarantee = more revenue :)

        2) Have your edge/border routers become route reflector clients and the R0 or the routers sitting at the core 
would act as route reflectors. This way you don't have to keep adding up IBGP peers all over your network as you add 
more routers at your edge.


> Now source routing is obsolete in IPv4, does any one use it ?

Not that I know of... At least not me.

> (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.

Yikes.

> (Method 4) Same as method 2, but Tag Switching (or MPLS) is used
> instead of loose source routing.

Are we talking about IGP vs. EGP or are we talking about MPLS vs. other transport mechanisms?

> Can any one help me understand what is done in practice among
> Methods 1 to 4, or any other one that I missed ?

Method 2. Please for the love of god, don't even try Method 1, that's quite bad.

-hc

-- 
Haesu C.
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