An engineer is implementing LFA FRR on PE routers to allow unicast traffic to rapidly converge around link or node failures. The engineer configured IS-IS on the newly deployed routers in the data…

Question

Options

• AConfigure IP Cisco Express Forwarding on each router.
• BConfigure the isis network point-to-multipoint command on each router.
• CConfigure MPLS on each router.
• DConfigure the isis network point-to-point command on each router.

• A
  3% (2)
• B
  8% (5)
• C
  12% (7)
• D
  76% (45)

Explanation

Option D is correct because IS-IS on Ethernet interfaces defaults to broadcast network type, which involves a pseudonode (Designated IS) in the SPF topology. LFA FRR requires IS-IS to operate in point-to-point mode - using isis network point-to-point on each interface - so the algorithm can accurately compute loop-free alternate next-hops without the pseudonode distorting the topology. Without this, LFA cannot be enabled on the PE routers.

Why the distractors are wrong:

• A (CEF): CEF is enabled by default on modern Cisco routers and is not an additional configuration step the engineer needs to take.
• B (point-to-multipoint): This network type is designed for hub-and-spoke topologies and does not satisfy the point-to-point adjacency requirement that LFA needs.
• C (MPLS): LFA FRR is a pure IP-layer mechanism and does not depend on MPLS; MPLS has its own separate FRR mechanism (TE FRR with RSVP).

Memory tip: Think "LFA loves P2P" - Loop-Free Alternates require Point-to-Point IS-IS. The broadcast pseudonode "lies" about the topology, so LFA insists on a clean, direct view of each link, which only point-to-point mode provides.

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