I have read from 2 diff sources regarding OSPF down bits for LSA Type 5
The down bit helps prevent routing loops between MP-BGP and OSPF, but not when external routes are announced, such as when redistribution between multiple OSPF domains or when external routes are injected in an area that is dual-homed to the provider network. The PE router redistributes an OSPF route from a different OSPF domain into an OSPF domain as an external route. "The down bit is not set because LSA Type 5 does not support the down bit". The redistributed route is propagated across the OSPF domain.
process steps for routing loops across OSPF domains
1. The PE router redistributes a non-ospf route into an OSPF domain as an external route. The down bit is set because the route should not be redistributed back into MP-BGP
2. A CE router redistribute the OSPF route into another OSPF domain. The down bit is lost if the CE router does not
understand this OSPF extension
3.The OSPF route is propagated through the other OSPF domain with the down bit cleared
4. A PE router receives the OSPF route;the down bit is not set,so the route is redistributed back into MP-BGP backbone resulting in a routing loop.
Question : it seems that source 1 is saying the down bits is not supported in OSPF type 5 LSA while source 2 states that it is supported. Please advise which is the correct one.
yes, source 1 is correct. Instead of the down bit a route tag is used to avoid routing loops with LSA5. The PE sets the route tag according to the rule described in my first posting and does not redistribute any external route with a route tag containing its AS number as described above.
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