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MPLS Questions

October 12th, 2010 in CCIE Written Go to comments

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


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?


Answer: C


(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



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.

  1. Nadir
    March 28th, 2011

    i think in question 2 options D,E & F are correct. because
    Penultimate hop popping is requested through TDP or LDP by using a special label value (1 for TDP, 3 for LDP) that also is called the implicit-null value.
    As for as option A is also true so what should be the best now????

  2. Ophiuchus
    March 29th, 2011

    I agree with Nadir. I also think Question 2 should be D,E,F instead of A,D,F.

  3. the furious five
    April 16th, 2011

    A,D,F true

  4. claudio
    April 23rd, 2011

    Q2, the answers D, E and F are correct, the answer A is not so accurate because of word “only”, as we can see in the link http://www.cisco.com/en/US/docs/ios/mpls/command/reference/mp_m1.html#wp1015111, on command “mls ldp advertise labels” the implicit null can be generated to choose the labels that we want to advertise.

  5. Bennet Eapen
    April 25th, 2011

    This may be a trivial issue, but questions 4 and 5 are repetitions.

  6. JackBlack
    May 2nd, 2011

    Question 2 is A, D, F — don’t be fooled by the incorrect answer in p4s!!!


  7. JackBlack
    May 2nd, 2011

    HA! After reading further all four answers are correct, now what??? 😉

  8. Mish
    May 4th, 2011

    Q2, p4s IS correct JackBlack,….according to ur link,
    “Penultimate hop popping is requested through TDP or LDP by using a special label value (1 for TDP, 3 for LDP) that also is called the implicit-null value.”
    so, that covers E & F. and D is obviously correct.

  9. JonathanArcher
    May 17th, 2011

    According to @JackBlack’s link the answers should be A, D, E, and F.

  10. implicit null
    May 18th, 2011

    Q2: am abit unsure about this as both TDP and LDP can request a penultimate hop pop. so why is answer F correct and not answer E?

  11. implicit null
    May 18th, 2011

    i found this this on cisco:


    so i will be answering D,E,F for Q2 if i get it in the exam.

  12. Morakh
    May 22nd, 2011

    Hey implicit null, answer is still A,D,E and F… 😉

    Note from your link: PHP is the default mode in Cisco IOS. In the case of IPv4-over-MPLS, Cisco IOS only advertises the implicit NULL label for directly connected routes and summarized routes.

  13. Anonymous
    June 30th, 2011

    Q2 answer A is wrong.

    Let’s say you have an MPLS core, and a Layer 3 only edge (with no MPLS) – the PE will then request PHP for destinations in the l3 edge. These destinations are does not have to be directly connected subnets or aggregate routes.

  14. Surya_Cisco
    July 24th, 2011

    NOTE PHP is the default mode in Cisco IOS. In the case of IPv4-over-MPLS, Cisco IOS only
    advertises the implicit NULL label for directly connected routes and summarized routes.

    I think ADF is aslo correct ans

  15. Anon192168
    August 6th, 2011

    I do a lot of MPLS, (teach the class and deployments) and it looks like they got this right from my MPLS bible (MPLS Configuration on Cisco IOS Software Page 17)
    “…(PHP)…This process is signaled by the downstream Edge LSR during Label distribution with LDP. The downstream Edge LSR distributes an implicit-null (POP) label to the upstream router, which signals it to pop the top label stack”
    So answers D and F are correct for sure.
    That leaves the question does TDP do this? My reference shows only LDP, but these two distribution protocols are very similar, so thats a tough one, so now ask yourself “Is there a better answer??”
    And the better answer is really ‘A’, PHP is essentially used to prevent an additional IP lookup, which comes from having to first look in the LFIB, and then the FIB. When the edge LSR knows the destination is ‘unlabled’ it can request the top label in the label stack to be popped, so it doesn’t have to look in the LFIB and LIB. Now it can only get an implicit-null label assigned to it if the destination network is local(directly connected) or an aggregate route or what we call an unlabeled destination. So I think ‘A” is a better answer. Overall not a well written question. But if it makes anyone feel better I see the exact same question on other study material saying ADF.

  16. Anon192168
    August 6th, 2011

    Q3. By saying the destination is in a VPN it is a labeled destination. Therefore LSR are going to first check the LFIB (Label forwarding information base) in the data plane. Answer C.
    CEF actually helps build these data structures but is not directly consulted in this process.
    FIB is the Forwarding information base used for normal or unlabeled destinations, it is in the data plane.
    IGP is the IGP which operates in the control plane which populates the RIB and then the FIB. The LIB is also in the control plane and then populates the LFIB in the data plane.

  17. Anon192168
    August 6th, 2011

    Q4. The Route Descriptor is a 64 bit prefix that makes a 96 bit unique VPNv4 prefix. RD is not correct because it doesn’t have VPN membership. The route target is what ties the prefix to a VPN. Its actually an extended BGP community and you control VPN reachability and membership through importing and exporting route targets. Redistribution and MPLS traffic engineering don’t apply.
    Answer C.

  18. Anon192168
    August 11th, 2011

    Q2: I’m kind of liking DEF too. Found more info on it.

  19. Expe0626
    September 26th, 2011

    which IOS security feature is configured by the inspect inspection-name {in | out} command?
    A) IPS
    B) IPsec site-to-site VPN
    C) Cisco AutoSecure
    D) Cisco IOS Firewall

    In Sanaja option D. My opinion option C.
    what is right?

  20. Cherlin
    October 16th, 2011

    I love these articles. How many words can a wordmstih smith?

  21. Vijay
    November 8th, 2011

    Option D is correct, inspection rules are creted using these commands.

  22. Sameera Khan
    December 4th, 2011

    Q2 D,E,F. Both TDP and LDP uses Implicit Null Value.

  23. John
    December 12th, 2011

    So, for Q2, is it ADF or DEF? Too many arguments…

  24. John
    December 12th, 2011

    For Q2, it’s DEF. Check this page http://books.google.com/books?id=-NlJnfxBXqwC&lpg=PA55&ots=lOuHgXZa1C&dq=In%20order%20to%20implement%20penultimate%20hop%20popping&pg=PA55#v=onepage&q=In%20order%20to%20implement%20penultimate%20hop%20popping&f=false

    It’s explicitly stated.

  25. jep
    December 28th, 2011

    Q2. rstut already gave best answer: ADF. i’ve confirmed it.

  26. ri017
    June 7th, 2012

    Q2: D – yes, F – yes, but the third answer is defenetely not A. Think logically, if you answer A, than B is true too, because you haven’t chosen E. But in that case there are 4 answers! And as was said previously LSR will request implicit-null for any route it has to unlabel (it means connecetd, aggregated + static and any IGP route). My version – DEF.

  27. jep
    July 22nd, 2012

    Q2: because someone ask me about this question, i revisit this page;-) so we are agree D and F area correct. and indeed they’re correct. how about E? TDP could be used to send implicit-null with special label 1 but this is an old protocol, currently IOS using LDP and special label 1 used for alert label, what happens if upstream neighbor LSR using LDP (default), and how about other PHP LSR Router, are you going to reconfigure all routers back to an old protocol? E is conditionally correct (correct with reconfiguration). while anwer D and F are unconditionally correct. could you guess one more answer that also unconditionally correct? right, A. It is used only for directly connected subnets or aggregated routes. Don’t be affraid with the word “only” as the sentence cover both directly connected and summary routes.

  28. Ben
    August 22nd, 2012

    Written Exam v2.0 Update Version 2.0 of the Cisco CCDE written exam will be reesaeld at the end of March 2012. Candidates who have written exams scheduled on March 31, 2012 or later should prepare using the CCDE Written Exam Topics v2.0 .

  29. Muhammad Faisal
    January 25th, 2013

    Question 2:

    I think answer A, E and F are correct. Because in D,it is mentioned that Upstream neighbors instead of “directly connected Upstream neighbor”….

  30. Andy
    May 21st, 2013

    Q3: is the answer not FIB… Cause the question states an ‘address’ meaning to me anIPv4 address. Which means unlabelled, and an LSR can be both a E-LSR and understand both labelled and unlabelled. An labelled packet never has its address checked by an LSR. it had stated a labelled packet fine but it doesn’t.

  31. routergeek
    July 25th, 2013

    I think Muhammad is right: Q2, A, E, and F are correct. The reason I do not believe D is correct is that penultimate pop hopping is what happens when an edge LSR requests a label pop from its upstream neighbor(s) – it does not enable the edge LSR to do so. Just my $0.02

  32. Sin82
    October 5th, 2013

    Hi Guys
    Is there any configuration in MPLS section in the exam or is that just about fundamentals?

  33. Lonesomeboy
    November 17th, 2013


    According to decent references A, D, E and F are true.


    Chose your poison.

  34. DiegoL
    December 1st, 2013

    Hi Guys,

    In regards to question 2. A is a valid answer based on this cisco book:


  35. Anonymous
    March 14th, 2014

    A nice explanation of TTL propagation is also provided here:

  36. Anonymous
    March 14th, 2014

    guys, take a look here:

    it explains mols forwarding, including PHP.


  37. Abel
    March 25th, 2014

    Thank you Anonumous 🙂

  38. sami
    April 25th, 2014

    hi all
    subscribe to that channel http://www.youtube.com/user/Joynetworks for free
    you will find very interesting video about MPLS

  39. axl
    May 15th, 2014

    I think D,E, and F are the correct answers. Because if E is false statement, that means B would have to be a True statement.

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    June 28th, 2022

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