The down bit in the options field of the OSPF LSA header.
The sending PE router redistributes the OSPF route into MP-BGP, copies the OSPF cost into the MED attribute,and sets the BGP extended community to indicate the LSA type from which the route was derived.
The receiving PE router redistributes the MP-BGP route back into OSPF and uses the original LSA type and the MED attribute to generate an interarea summary LSA, an interarea summary LSA or type 3 LSA is always generated because the receiving PE router acts as an ABR between the superbackbone and the OSPF area (or areas).
When OSPFv2 is used on a PE-CE link that belongs to a particular ×××, the PE router must redistribute to that ×××’s OSPFv2 instance certain routes that have been installed in the BGP routing table. Similarly, a PE router must redistribute to BGP routes that have been installed in the ×××-specific OSPF routing tables.
When the PE receives, from a CE router, a type 3, 5, or 7 LSA with the DN bit set, the information from that LSA MUST NOT be used during the OSPF route calculation. As a result, the LSA is not translated into a BGP route. The DN bit MUST be ignored in all other LSA types.
When the PE receives, from a CE router, a type 3, 5, or 7 LSA with the DN bit set, the information from that LSA MUST NOT be used during the OSPF route calculation. As a result, the LSA is not translated into a BGP route. The DN bit MUST be ignored in all other LSA types.
The down bit is used between the PE routers to indicate which routes were inserted into the OSPF topology database from the MPLS ××× superbackbone and thus shall not be redistributed back in the MPLS ××× superbackbone. The PE router that redistributes the MP-BGP route as an OSPF route into the OSPF topology database sets the down bit. The other PE routers use the down bit to prevent this route from being redistributed back into MP-BGP
This prevents routes learned via BGP from being redistributed to BGP. (This restriction is analogous to the usual OSPF restriction that inter-area routes that are learned from area 0 are not passed back to area 0.)
Note that the DN bit has no other effect on LSA handling. In particular, an LSA with the DN bit set will be put in the topological database, aged, flooded, etc., just as if DN were not set.
But to prevent the customer sites from acting as transit parts of the MPLS ××× network, the OSPF route selection rules in PE routers need to be changed. The PE routers have to ignore all OSPF routes with the down bit set, because these routes originated in the MP-BGP backbone and the MP-BGP route should be used as the optimum route toward the destination.
This rule is implemented with the routing bit in the OSPF LSA. For routes with the down bit set, the routing bit is cleared and these routes never enter the IP routing table—even if they are selected as the best routes by the shortest path first (SPF) algorithm. -no capability vrf –
The down bit stops the routing loops between MP-BGP and OSPF. The down bit cannot, however, stop the routing loops when redistribution between multiple OSPF domains is involved
