If there are lots of core routers like R0 then MPLS is a nice way to go. Just turn on tag switching on all the routers and it more or less works without any further configuration.
Phil At 11:44 AM 10/9/03 +0200, you 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Now source routing is obsolete in IPv4, does any one use it ?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 > >---------------------------------------------- >[EMAIL PROTECTED] Maillist-Archive: >http://www.mail-archive.com/swinog%40swinog.ch/ ---------------------------------------------- [EMAIL PROTECTED] Maillist-Archive: http://www.mail-archive.com/swinog%40swinog.ch/
