OSPF External Routes Not Redistributing

I have seen many cases where external routes are not redistributing in OSPF or not getting install in the routing table. This happens when the next hop address is not reachable in the IGP. But the next question comes in mind that next hop address only plays role in BGP not in IGP. In IGP the next hop address of redistributed route is always ASBR address. We might be wrong this time; the immediate step is to check the “show ip ospf database external ”.

Output of show ip ospf database external 10.134.31.90
show ip ospf database external
OSPF Router with ID (10.10.10.1) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 298
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.134.31.90 (External Network Number )
Advertising Router: 10.10.10.2
LS Seq Number: 80003B51
Checksum: 0xF86
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 20
Forward Address: 10.10.2.7

See the above output and value of forward address which 10.10.2.7. (See previous post which states the reason for non zero forward address). In normal condition this values must be 0.0.0.0 but we are receiving 10.10.2.7.

According to RFC 2328 section 16.4 which clearly states that “If the forwarding address is non-zero, look up the forwarding address in the routing table. The matching routing table entry must specify an intra-area or inter-area path; if no such path exists, do nothing with the LSA and consider the next in the list”.

This means if either the forward address is not reachable in the IGP or it is coming via external route also. This is the reason for not installing the static route in IGP.

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