Understanding the Network Scenario:
R2 is the boundary router between OSPF and IS-IS.
OSPF and IS-IS do not automatically redistribute routes between each other.
R4's loopback (10.1.4.4/32) exists in the IS-IS domain.
To allow R1 (OSPF) to reach R4’s loopback, R2 must redistribute IS-IS routes into OSPF.
Why Option C Is Correct (✅)
✔default-route-advertise in OSPF on R2:
This advertises a default route into the OSPF domain, allowing OSPF routers (R1, R3) to forward unknown traffic to R2.
Since IS-IS routes are not automatically injected into OSPF, this ensures R1 can reach R4.
✔IS-IS Already Has Routes to OSPF
IS-IS routers (R3, R4) already learn routes from OSPF via R2 (assuming route redistribution is properly configured).
No need to run default-route-advertise in IS-IS.
Why Other Options Are Incorrect (❌)
❌A. Run default-route-advertise in both OSPF and IS-IS views on R2
Unnecessary in IS-IS. The IS-IS domain already learns OSPF routes from R2 via redistribution.
Only OSPF needs a default route to forward unknown traffic to R2.
❌B. No configuration is required
Incorrect, because OSPF does not automatically learn IS-IS routes.
A default route or redistribution is needed for R1 to reach R4.
❌D. Run default-route-advertise only in the IS-IS view on R2
Incorrect, because IS-IS already has routing information from OSPF.
The issue is with OSPF not learning IS-IS routes, so the command needs to be in OSPF view.
????Reference:Huawei HCIE Datacom – OSPF and IS-IS Route Redistribution and Interoperability
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