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Multihoming with requirement to have a transit autonomous system

  Hi All,

There is a scenario where there are 2 service providers - service provider A and service provider B. There is one major site which will interconnect to both service provider A  and service provider B via dual routers.

There are some sites currently over service provider A. Those sites are going to migrate to service provider B.

This will happen in step by step. So during this phase, it may happen that some sites will be over service provider A and some sites migrated to service provider B. Those sites will communicate via major interconnect site.

e-BGP is routing protocol between CE-PE and IGP is OSPF on LAN side.

For major interconnect site, the two routers (one connected to service provider A and another connected to service provider B) are not directly connected to each other but connected via LAN. Also each router will have a different AS no.

So here, I can not use iBGP between the 2 routers. But I need to do mutual redistribution between BGP and OSPF.

For this, I am thinking to use route tag and as-path access list to prevent the loops.

e.g. ip as-path access-list 1 permit ^$ on each router



       routers ospf 1

       redistribute bgp xxxx subnets metric 10 tag 1

       router bgp x

       redistribute ospf 1 route-map OSPF-BGP

        route-map OSPF-BGP deny 10

        match tag 1

        route-map OSPF-BGP permit 20


Can anyone pls suggest some other appropriate method for loop prevention and any overall suggestion for this scenario.



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