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 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.
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????
I agree with Nadir. I also think Question 2 should be D,E,F instead of A,D,F.
A,D,F true
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.
This may be a trivial issue, but questions 4 and 5 are repetitions.
Question 2 is A, D, F — don’t be fooled by the incorrect answer in p4s!!!
http://fengnet.com/book/mpls_vpn/ch02lev1sec3.html
HA! After reading further all four answers are correct, now what??? 😉
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.
According to @JackBlack’s link the answers should be A, D, E, and F.
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?
Q2
i found this this on cisco:
http://www.ciscopress.com/articles/article.asp?p=680824&seqNum=2
so i will be answering D,E,F for Q2 if i get it in the exam.
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.
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.
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
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.
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.
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.
Q2: I’m kind of liking DEF too. Found more info on it.
@Anon192168
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?
I love these articles. How many words can a wordmstih smith?
Option D is correct, inspection rules are creted using these commands.
Q2 D,E,F. Both TDP and LDP uses Implicit Null Value.
So, for Q2, is it ADF or DEF? Too many arguments…
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.
Q2. rstut already gave best answer: ADF. i’ve confirmed it.
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.
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.
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 .
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”….
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.
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
Hi Guys
Is there any configuration in MPLS section in the exam or is that just about fundamentals?
Thanks
Q2:
According to decent references A, D, E and F are true.
http://etutorials.org/Networking/MPLS+VPN+Architectures/Part+I+MPLS+Technology+and+Configuration/Chapter+2.+Frame-mode+MPLS+Operation/Penultimate+Hop+Popping/
Chose your poison.
Hi Guys,
In regards to question 2. A is a valid answer based on this cisco book:
http://books.google.com.au/books?id=x_jKdmDVPH0C&pg=PA41&lpg=PA41&dq=penultimate+hop+popping+aggregate+routes&source=bl&ots=lUJ4Tm9ir_&sig=JzD4moO-1lU5u-hUlioWzYNVOSU&hl=en&sa=X&ei=s8ebUsXkPOXYigew7oGwCA&ved=0CCsQ6AEwAA#v=onepage&q=penultimate%20hop%20popping%20aggregate%20routes&f=false
A nice explanation of TTL propagation is also provided here:
http://blogbt.net/index.php/2013/07/mpls-the-case-of-ttl-propagation/
guys, take a look here:
http://blogbt.net/index.php/2013/07/mpls-forwarding/
it explains mols forwarding, including PHP.
enjoy
Thank you Anonumous 🙂
hi all
subscribe to that channel http://www.youtube.com/user/Joynetworks for free
you will find very interesting video about MPLS
thanks
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.
Welcome to the world of adult Dating loveawake.ru