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