SR Basic Inter-area with ISIS
Load basic.isis.sr.enabled.cfg
ISIS with SR is already enabled.
Configure R1 and R3 as L1-only routers
Configure R5 as an L1/L2 router
R5 should propagate L1 into L2 and L2 into L1
Ensure that an end-to-end LSP can be formed between R1 and R7
Answer
Explanation
We saw the “no-PHP” flag and “ExpNull” flag in the previous lab. You might have wondered why there are two separate flags. When would we see the “no-PHP” flag set, yet the “ExpNull” flag not set?
One case that we would see this combination of flags is for inter-area routing. In this topology, R5 is now propagating R1 and R3’s prefixes into the L2, as if they originate at R5 itself. However, R5 does not own these prefixes, so directly connected nodes should not do PHP. Therefore, R5 sets the “no-PHP” flag. We can see this in its L2 LSP:
Also notice that the R flag is set for these prefixes, meaning “re-advertisement,” because the prefix has been redistributed or re-advertised from another level.
Likewise, R5 sets the R and no-PHP flag on all L2 prefixes redistributed into the L1:
If we enable MPLS OAM, we should see and end-to-end LSP between R1 and R7:
It is interesting that inter-area SR essentially just works by default. There is nothing special you need to do except for propagating L2 into L1. The Prefix-SID and its attributes are automatically carried between levels.
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