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VPN Technologies Practice Questions

15 original questions covering ENARSI 2.1 to 2.3. Answer every one before opening any explanation. Target: 12 of 15 (80%). These questions do not repeat the 2.0 section of Question Bank, Part 1, so do both.

1. Blueprint 2.1

Refer to the exhibit. R5 was just added to the MPLS core. OSPF advertises only Loopback0 and the core links. LDP discovery with R4 works, but no LDP session forms. What is the fix?

R5# show running-config | section interface Loopback|mpls
interface Loopback0
 ip address 10.255.255.5 255.255.255.255
interface Loopback1
 ip address 192.168.50.1 255.255.255.0
 mpls ip

R5# show mpls ldp discovery
 Local LDP Identifier:
    192.168.50.1:0
  • A. Add Loopback1 to OSPF area 0 or configure mpls ldp router-id Loopback0 force
  • B. Remove mpls ip from Loopback1
  • C. Change the label range on R5 so it does not overlap with R4
  • D. Configure mpls label protocol tdp
Answer: A

With no mpls ldp router-id, LDP picks the highest loopback address, 192.168.50.1. The transport address defaults to the router ID, and R4 has no route to it because Loopback1 is not in OSPF, so the TCP session cannot be built. Pinning the router ID to Loopback0 (with force so it applies now) is the clean fix; advertising Loopback1 also works. mpls ip on a loopback does nothing harmful (B). Label ranges are local (C).

2. Blueprint 2.1

A customer runs traceroute across a provider's MPLS L3VPN. The output shows the local PE, the remote PE and the destination, but none of the three P routers in between. Which statement is correct?

  • A. The P routers are forwarding the traffic with IP lookups, not labels
  • B. The P routers are dropping ICMP time exceeded messages because of CoPP
  • C. PHP is disabled in the core
  • D. The provider has configured no mpls ip propagate-ttl on the ingress PEs
Answer: D

Without TTL propagation, the ingress PE sets the label TTL to 255 instead of copying the IP TTL, so the TTL never expires inside the core and the P routers never reply. The core appears as one hop. Labels are still switched (A). PHP affects which router pops the label, not TTL behaviour (C). B could hide P routers but would also show timeouts (asterisks) for those hops rather than skipping them.

3. Blueprint 2.1

Which two statements about the LIB and LFIB on an IOS LSR are true? (Choose two.)

  • A. The LIB holds labels advertised by all LDP neighbors for a prefix, because IOS uses liberal label retention
  • B. The LFIB holds labels from all neighbors and load-shares across them
  • C. The LFIB uses the label advertised by the neighbor that is the IGP next hop for the prefix
  • D. The LIB is used to forward labeled packets in the data plane
  • E. The LFIB is built from the BGP table
Answer: A, C

Liberal retention keeps every binding in the LIB (control plane). The LFIB (data plane) combines the LIB with the RIB and uses only the next hop's label, or one per next hop when the IGP has equal-cost paths. B confuses the two tables. The LIB is not used for forwarding (D). The LFIB is built from the IGP routing table and LDP, not from BGP (E).

4. Blueprint 2.1

The egress PE applies QoS based on the MPLS TC (EXP) bits of arriving packets, but all packets arrive unlabeled. What change makes the TC bits reach the egress PE?

  • A. Configure mpls ldp explicit-null on the egress PE
  • B. Configure no mpls ip propagate-ttl on the ingress PE
  • C. Configure a larger MTU on the core links
  • D. Advertise the egress PE loopback as a /24 instead of a /32
Answer: A

By default the egress advertises implicit null, so the penultimate router pops the label and the TC bits are lost. Explicit null makes the penultimate router swap to label 0, and the egress receives a labeled packet with the TC bits intact before popping it. B only changes TTL handling. D would break the LSP.

5. Blueprint 2.1

After a core link between two P routers recovers from an outage, customer VPN traffic over that link is dropped for about a minute, while pings between the P routers' loopbacks succeed. OSPF converges in a few seconds. Which feature prevents this?

  • A. mpls ldp autoconfig
  • B. mpls ldp sync under the OSPF process
  • C. bfd interval on the core links
  • D. mpls ip propagate-ttl
Answer: B

OSPF chose the recovered link before LDP had exchanged labels over it. IP traffic (unlabeled pings) was fine, but labeled VPN traffic had no outgoing label on that path and was dropped. LDP-IGP synchronization makes OSPF advertise the link with maximum metric until LDP is operational. Autoconfig only enables LDP on interfaces (A). BFD detects failures faster, which does not help after recovery (C).

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