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MPLS Questions


Here you will find answers to MPLS Questions
Question 1
Which statement correctly describes the disabling of IP TTL propagation in an MPLS network?
A. The TTL field from the IP packet is copied into the TTL field of the MPLS label header at the ingress edge LSR.
B. TTL propagation cannot be disabled in an MPLS domain.
C. TTL propagation is only disabled on the ingress edge LSR.
D. The TTL field of the MPLS label header is set to 255. 
E. The TTL field of the IP packet is set to 0.


Answer: D
Explanation
Time-to-Live (TTL) is a 8-bit field in the MPLS label header which has the same function in loop detection of the IP TTL field. Recall that the TTL value is an integer from 0 to 255 that is decremented by one every time the packet transits a router. If the TTL value of an IP packet becomes zero, the router discards the IP packet, and an ICMP message stating that the “TTL expired in transit” is sent to the source IP address of the IP packet. This mechanism prevents an IP packet from being routed continuously in case of a routing loop.
By default, the TTL propagation is enabled so a user can use “traceroute” command to view all of the hops in the network.
We can disable MPLS TTL propagation with the “no mpls ip propagate-ttl” command under global configuration. When entering a label-switched path (LSP), the edge router will use a fixed TTL value (255) for the first label. This increases the security of your MPLS network by hiding provider network from customers.
Question 2
Which three of these statements about penultimate hop popping are true? (Choose three)
A. It is used only for directly connected subnets or aggregate routes.
B. It can only be used with LDP.
C. It is only used when two or more labels are stacked.
D. It enables the Edge LSR to request a label pop operation from its upstream neighbors.
E. It is requested through TDP using a special label value that is also called the implicit-null value.
F. It is requested through LDP using a special label value that is also called the implicit- null value.

Answer: A D F
Question 3
Which of these tables is used by an LSR to perform a forwarding lookup for a packet destined to an address within an RFC 4364 VPN?
A. CEF
B. FIB
C. LFIB
D. IGP

Answer: C
Explanation
(Notice: The term Label Switch Router (LSR) refers to any router that has awareness of MPLS labels)
Label Forwarding Information Base (LFIB) is responsible for forwarding incoming packets based on label as it holds necessary label information, as well as the outgoing interface and next-hop information.
Question 4
A network is composed of several VRFs. It is required that VRF users VRF_A and VRF_B be able to route to and from VRF_C, which hosts shared services. However, traffic must not be allowed to flow between VRF_A and VRF_B. How can this be accomplished?
A. route redistribution
B. import and export using route descriptors
C. import and export using route targets
D. Cisco MPLS Traffic Engineering

Answer: C
Question 5
Multi Protocol Label Switching (MPLS) is a data-carrying mechanism that belongs to the family of packet-switched networks. For an MPLS label, if the stack bit is set to 1, which option is true?
A. The stack bit will only be used when LDP is the label distribution protocol
B. The label is the last entry in the label stack.
C. The stack bit is for Cisco implementations exclusively and will only be used when TDP is the label distribution protocol.
D. The stack bit is reserved for future use.

Answer: B
Explanation
MPLS_stack_bit.jpg
MPLS Header Packet Format
LABEL: 20 bits
EXP: Experimental, 3bits – are reserved for experimental use
S: Bottom of stack, 1 bit
TTL: Time to Live, 8bits – same as IP TTL
The bottom-of-stack bit, or “stack bit”, is just used to indicate it is the bottom of the label stack because it is possible (and common) to have more than one label attached to a packet. The bottommost label in a stack has the S bit set to 1, other labels have the S bit set to 0. Sometimes it is useful to know where the bottom of the label stack is and the S bit is the tool to find it.
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