OSPF route is propagated as external route into other sites
Local subnet is announced to the PE router as type 1 or type 2 LSA
From the customer perspective, an MPLS VPN-based network has a BGP backbone with Interior
Gateway Protocol (IGP) running at customer sites.
Redistribution between IGP and BGP is performed to propagate customer routes across the MPLS VPN
In the case of MPLS-VPN Backbone as Area0
The OSPF superbackbone behaves exactly like Area 0 in regular OSPF:
PE routers are advertised as ABRs.
Routes redistributed from BGP into OSPF appear as interarea summary routes or as external routes
(based on their original LSA type) in other areas.
It is important to note that the MPLS/VPN backbone is not a real Ospf area 0 backbone.No adjacencies are formed between PE routers,and all ospf routes are translated into VPN-IPv4 routes.This means that the redist of routes into BGP does not cause these routes to become external ospf routes when advt to other member sites of the same VPN.
Whenever a PE router recieves an MP-BGP update that contains a prefix learned through OSPF by the originating PE router, it must be capable of identifying what type of ospf route is contained
within the update.This is necessary to allow the PE router to generate an appropriate LSA towards
the VPN cust CE router based on the ospf route type.To support this,when the PE router propogates
OSPF routes into MP-BGP thro redist,the BGP extended community attrib is used to preserve and
convey the ospf attrib of the route.
EXT Comm 0x8000
-4 bytes-Ospf area number
-1 byte-Ospf route type (1 thro 7)
-1 byte-Option (used for external metric type.
Here Ext comm shows the prefix is an inter-area with route-type 3.
Router#show ip bgp vpnv4 vrf Customer_A 10.0.1.0
BGP routing table entry for 1:10:10.0.1.0/24, version 64
Yes ,Very well explained thanks to you Aditya.What I feel every time the the routes receiving PE (remote PE)has to give update to it's attached CE as ONLY TYPE3 irrespective of the type at the orignating end.This means the OSPF super-backbone is completely transparent to customer OSPF Area's in the given VPN.
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