SR basic inter-area with OSPF
Load basic.ospf.sr.enabled.cfg
OSPF with SR is already enabled.
Configure all of R1’s links in area 1
Configure R3 and R4 as ABRs for area 1
R3 and R4 should propagate inter-area prefixes automatically
Ensure that an end-to-end LSP can be formed between R1 and R7
Answer
Explanation
Just like with ISIS, OSPF inter-area prefix SID propagation works by default. Notice that R1 still has the same SID mappings:
Let’s examine R3’s ext-prefix LSA for 7.7.7.1/32:
The route-type 3 under the extended prefix TLV means that this is an inter-area route. The regular OSPF rules for inter-area routes apply. This ensures loopfreeness for the prefix SIDs, the same way it works for basic type 3 LSAs.
The extended prefix TLV flags of 0x40 indicate the prefix is not attached to the ABR, and it is a Node SID. Recall the flag definition below:
A means the prefix, although inter-area, is directly attached to the ABR advertising the prefix SID
N means Node SID
The SID sub TLV flags of 0x40 indicate that the NP bit has been set, so PHP should be disabled. This is necessary because R3 is advertising the prefix SID on R7’s behalf, so R1 must not pop the top label, as R3 is not the final destination. However, expnull flag is not set. Recall the SID sub TLV flags below:
NP means No-PHP. This is set to disable PHP.
M means mapping-server. The prefix SID was advertised by a mapping server.
E means explicit-null. Together, with NP, you get expnull without PHP behavior.
V means the absolute value is used. This is never set for prefix SIDs.
L means the SID is locally significant. This is never set for prefix SIDs.
Notice that while R3 and R4 are both adveritising R7’s prefix SID, they do not reinject these back into area 0 as inter-area routes. This is due to the basic OSPF loopfreeness principal: ABRs do not accept a type 3 LSA from a non-backbone area.
R7 also learns of R1’s prefix SID via both R3 and R4 in area 0 as a type 3 route. Again, the NP flag is set in the SID sub TLV.
If we enable MPLS OAM, we should see and end-to-end LSP between R1 and R7:
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