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Home Cisco® 642-887

Cisco® 642-887 Exam Practice Questions (P. 5)

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Question #21
Which statement is correct regarding the default MPLS TTL behavior?
  1. A
    When an ingress edge LSR receives an IP packet, it will decrement the IP TTL field by 1; then it will set the MPLS Label TTL field to 255.
  2. B
    When an ingress edge LSR receives an IP packet, it will decrement the IP TTL field by 1; then it will copy the decremented IP TTL field into the MPLS Label TTL field.
  3. C
    When an ingress edge LSR receives an IP packet, it will just copy the IP TTL field into the MPLS Label TTL field.
  4. D
    When an ingress edge LSR receives an IP packet, it will copy the IP TTL field into the MPLS Label TTL field first; then it will only decrement the MPLS Label TTL field by 1.
  5. E
    When an ingress edge LSR receives an IP packet, it will copy the IP TTL field into the MPLS Label TTL field first; then it will only decrement the IP TTL field by

Correct Answer:
B

Question #22
Which three statements are correct regarding ping mpls command operations? (Choose three.)
  1. A
    MPLS OAM has to be enabled on the router using the mpls oam command.
  2. B
    They use a 127/8 address as the destination address in the MPLS echo request packet.
  3. C
    They use ICMP echo request and ICMP echo reply packets.
  4. D
    They are used to test for a broken LSP.
  5. E
    If there is a broken LSP, instead of using label switching, the packet can still be forwarded based on the destination IP address in the mpls ping echo request

Correct Answer:
ABD
How MPLS Ping works?
MPLS Ping doesnt rely on ICMP echo messages. Instead it uses UDP protocol with both source and destination port as 3503 and relies on MPLS Echo request and MPLS Echo reply.
When MPLS ping is triggered from any MPLS router, it will generate UDP segment with source/destination port as 3503. The source address will be selected as usual while the destination address will be 127.0.0.1. The IP TTL will be set to 1.
Below is a sample IP format when MPLS Ping is originated from R5 to 150.1.6.6/32,

Now the originating LSR will look into the LFIB and populate the label header with respective labels to reach the FEC, in our case 150.1.6.6/32.
R4 on receiving the MPLS packet will be able to send to the actual destination only if the LSP is end to end. If the LSP is broken between R4 and R6, R4 will look into the destination IP address which will be 127.0.0.1 and wont be able to perform IP forwarding. As per RFC 1812, a router should not forward any packet that has destination address of 127.0.0.0/8

Question #23
What are the four fields inside the MPLS shim header? (Choose four.)
  1. A
    EXP
  2. B
    TTL
  3. C
    Version
  4. D
    S
  5. E
    Length
  6. F
    Label
  7. G
    Type
  8. H
    FCS

Correct Answer:
ABDF

Question #24
Which two of the following statements are correct regarding LSPs? (Choose two.)
  1. A
    An LSP is created for every routing protocol entry.
  2. B
    Each LSP is bidirectional, that is, packets traveling in the opposite direction use the same LSP.
  3. C
    An IGP is used to populate routing tables in all routers in an MPLS domain.
  4. D
    LDP is used to propagate labels and build LSPs.
  5. E
    The FIB is used to forward MPLS-labeled packets down an LSP.

Correct Answer:
CD

Question #25
Which four pieces of information are stored for each prefix in the LFIB? (Choose four.)
  1. A
    local label
  2. B
    outgoing label
  3. C
    next-hop IP address
  4. D
    outgoing interface
  5. E
    incoming interface
  6. F
    Layer 2 header rewrite information

Correct Answer:
ABCD
Forwarding Labeled Packets -
LSR forwards the packet based on:
Top Label value of the received packet
Corresponding entry in LFIB (LABEL <=> INTERFACE)
#show mpls forwarding-table - will show:
local label
outgoing label
prefix (network)
outgoing interface
next-hop

LSR expects packet to come with "top" label being "Local" (from show mpls forwarding-table).
If Outgoing label is "Aggregate", then that means that this is a summary route and more specific lookup is performed.
If LSR cannot find label/interface mapping in LFIB, then it drops the packet.
There are several "RESERVED" labels numbered from 0 to 15:
0 - explicit NULL - is used to preserver QoS info through EXP bits. It copies 'ip prec' or DiffServ.
1 - Router alert label - forces LSR to software switch the packet.
3 - Implicit NULL - this label is used for "connected" or "summary" routes. This way LSR signals its neighbors to execute "POP label" operation on "connected" routes. It is called PHP, Penultimate Hop Popping, and is used to make sure that LSR does not perform 2 lookups (label + ip).
14 - OEM alert label - is used for monitoring purpose.
In Cisco IOS, the default range is 16 through 100,000, but can be expanded by using "mpls label range" command.

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