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QUESTION 22
Which three statements are correct regarding ping mpls command operations? (Choose three.)
A. MPLS OAM has to be enabled on the router using the mpls oam command.
B. They use a 127/8 address as the destination address in the MPLS echo request packet.
C. They use ICMP echo request and ICMP echo reply packets.
D. They are used to test for a broken LSP.
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 packet.
Answer: ABD
Explanation:
How MPLS Ping works?
MPLS Ping doesn’t 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 won’t 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
When a link flaps, an mpls ldp session will also flap. Which feature minimizes traffic loss and provides faster convergence after the link is re-established?
A. BFD
B. MPLS LDP IGP SYNC
C. graceful restart
D. LDP session protection
E. LDP nonstop routing
Answer: D
QUESTION 24
A DSCP value of 41 in decimal corresponds to which IP precedence value?
A. 3 – Flash
B. 4 – Flash Override
C. 5 – Critical
D. 6 – Internet Control
E. 7 – Network Control
Answer: C
QUESTION 25
Which four pieces of information are stored for each prefix in the LFIB? (Choose four.)
A. local label
B. outgoing label
C. next-hop IP address
D. outgoing interface
E. incoming interface
F. Layer 2 header rewrite information
Answer: ABCD
Explanation:
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.
QUESTION 26
Which three network services can be implemented using MPLS within the service provider IP NGN core? (Choose three.)
A. Layer 2 VPNs
B. Layer 3 VPNs
C. traffic engineering
D. IntServ traffic engineering tunnels
E. encrypted LSPs
Answer: ABC
QUESTION 27
LDP session protection uses which one to maintain the LDP session between LDP neighbors?
A. LDP NSF
B. LDP NSR
C. backup-targeted LDP hellos
D. BFD
E. LDP-IGP synchronization
Answer: C
QUESTION 28
You are tasked to enable LDP on many of the interfaces on the Cisco CRS-3 router, and because there are many interfaces that need to have LDP enabled, you mistakenly did not enable LDP on all the required interfaces. To prevent this issue from happening again in the future, what could you do the next time you need to enable LDP on many interfaces?
A. use the mpls ldp auto-config command under the IGP routing process
B. use the mpls ldp sync command under the IGP routing process
C. use the interface all command under the MPLS LDP process
D. use the discovery command under the MPLS LDP process
Answer: A
QUESTION 29
Which two statements are correct in describing ISP environments that are running IP/MPLS in the core network? (Choose two.)
A. On the PE routers, each BGP route must use a unique label to reach the BGP next hop.
B. The BGP next hops point to the PE routers, and only the PE routers are required to run BGP.
C. A full mesh of IBGP sessions are required between all of the PE and P routers to ensure proper packets
forwarding.
D. The PE and P routers run LDP to learn the labels for reaching the BGP next-hop addresses.
Answer: BD
QUESTION 30
When troubleshooting LDP operations on the Cisco IOS and IOS XE routers, what is one of the first things that should be verified?
A. if running OSPF as the IGP, ensure that OSPFv3 has been enabled
B. check if the ip cef command has been enabled
C. verify in the running configurations that all of the required LDP interfaces are defined under the mpls
ldp command configuration mode
D. verify if there are any access lists that are denying TCP and UDP port 464 packets
Answer: B
QUESTION 31
Referring to the Cisco IOS XR configuration exhibit, Which labels will be advertised by the router with this configuration?
A. Only the label for 10.1.1.1/32 will be advertised to all the LDP peers.
B. Labels for all prefixes will be advertised to the 10.1.1.1 LDP peer.
C. Labels for all prefixes will be advertised to all the LDP peers.
D. No labels will be advertised to any LDP peers.
Answer: A
QUESTION 32
What is the term that is used for the label that an LSR assigns and distributes to other LSRs in MPLS?
A. Local
B. Remote
C. Explicit
D. Explicit Null
E. Aggregate
Answer: A
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