350-401 Question #125: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. Which LISP component do routers in the public IP network use to forward traffic between the two networks?

Options

• AEID
• BRLOC
• Cmap server
• Dmap resolver

Explanation

RLOC (Routing Locator) is the address used by routers in the public IP network (the underlay/core network) to forward traffic between LISP sites. RLOCs are the actual routable IP addresses assigned to LISP tunnel routers (xTRs), and public network routers use these addresses to make standard forwarding decisions - they have no awareness of LISP EIDs at all.

Why the distractors are wrong:

• A. EID (Endpoint Identifier): EIDs are the source/destination addresses of end hosts within LISP sites; they are not routable in the public network and are hidden inside the LISP encapsulation.
• C. Map Server: The map server is a control-plane component that receives and stores EID-to-RLOC mappings from registered ETRs - it plays no role in the data-plane forwarding of traffic.
• D. Map Resolver: The map resolver handles map requests from ITRs to resolve EID-to-RLOC mappings - again a control-plane function, not a forwarding mechanism.

Memory Tip: Think of it this way - RLOC = Routable Location. The public internet only sees and routes based on locations (RLOCs), while EIDs are the "hidden" identities of endpoints tucked inside the LISP tunnel.

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