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    CCIE R&S 350-001 Q&As – Implement MPLS Layer 3 VPNs (1-5)

    By admin | June 18, 2014

    Tagged with:

    Section 3 – Implement MPLS Layer 3 VPNs

    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: ADF
    Explanation:
    Pass4sure sure says DEF
    To implement penultimate pop the edge LSR requests a lable pop from upstream neighbor via LDP OR TCP using special implicit-null lable 3 for LDP 1 for TCP; others agree and reference http://www.ciscopress.com/articles/article.asp?p=680824&seqNum=2
    Other sources say AEF and refer to http://fengnet.com/book/mpls_vpn/ch02lev1sec3.html
    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.

    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
    Refer to the exhibit. According to the output of the command show tag-switching forwarding-table, which four of these statements are true? (Choose four.)

    113
    A.    Packets to the IP address 10.10.10.5/32 will be tagged with "17" toward the next hop.
    B.    Label "19" will be advertised to MPLS neighbors so that they can use this label to reach the IP address 10.10.10.6/32.
    C.    IP address 10.10.10.4/32 is directly connected to the neighbor router on serial 3/0.
    D.    Packets arriving with label "17" will be forwarded without any label toward serial 4/0.
    E.    Packets arriving with label "20" will be forwarded with label "21" after label-swapping.
    F.    Label "20" is advertised to MPLS neighbors so that they can use this information to reach the prefix 10.10.10.8/32.
    Answer: CDEF
    Explanation:
    Label stacking is the encapsulation of an MPLS packet inside another MPLS packet – that is, adding an MPLS header “on top of” (hence stacking) an existing MPLS header. The result of stacking is the ability to tunnel one MPLS LSP inside another LSP.
    The primary advantage of LDP is that is scales well. It signals LSPs hop-by-hop, and so routers along the path do not have to maintain state for each LSP. Therefore LDP is useful in edge applications such as VPNs where hundreds or thousands of LSPs are originated and terminated. But LDP has no traffic engineering capabilities; it just follows the IGP shortest path to find LSP end-points.
    A central concept to MPLS is the Forwarding Equivalence Class (FEC), and it’s something many people new to the technology struggle to understand. So in this post I’d like to discuss FECs and their role in MPLS.
    An FEC is a set of packets that a single router:
    (1) Forwards to the same next hop;
    (2) Out the same interface; and
    (3) With the same treatment (such as queuing).
    FECs are nothing new. Every router performing generic IP forwarding determines the next hop to which the packet is to be forwarded, the interface out which the packet is sent to get to that next hop, and how to queue the packet for that interface. But we don’t often hear those very basic procedures presented as “determining what FEC a packet belongs to.”

    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 to1, 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:
    Theoretically, the range is 0 through (220-1). Label values 0-15 are reserved, and values 4-15 are reserved for future use. Values 0-3 are defined as:
    A value of 0 represents the "IPv4 Explicit NULL Label". This label indicates that the label stack must be popped, and the packet forwarding must be based on the IPv4 header. This helps to keep Exp bits safe until the egress router. It is used in MPLS based QoS.
    A value of 1 represents the "Router Alert Label". When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual packet forwarding is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the "Router Alert Option" in IP packets (for example, ping with record route option)
    A value of 2 represents the "IPv6 Explicit NULL Label". It indicates that the label stack must be popped, and the packet forwarding must be based on the IPv6 header.
    A value of 3 represents the "Implicit NULL Label". This is a label that an LSR can assign and distribute. However, it never actually appears in the encapsulation. It indicates that the LSR pops the top label from the stack and forwards the rest of the packet (labeled or unlabeled) through the outgoing interface (as per the entry in Lfib). Although this value might never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved.

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