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MPLS LDP TDP Questions

Here you will find answers to MPLS LDP Questions

Question 1

For which purpose is the command mpls ldp maxhops used?

A. In large ATM-MPLS networks, the LFIB can become too large and it may be necessary to limit the maximum diameter of the MPLS LSPs.
B. Because downstream-on-demand label allocation uses hop count to control loop detection, it maybe necessary to limit the maximum diameter of the MPLS network.
C. Because end-to-end delay can cause problems with some voice applications, it may be necessary to limit the maximum diameter of the MPLS network.
D. When interconnecting large frame mode MPLS and cell mode networks it may be necessary to limit the maximum network diameter to prevent forwarding loops.


Answer: B


The “mpls ldp maxhops” command is used to limit the number of hops permitted in a label switched path (LSP) established by the Downstream on Demand (DoD) method of label distribution.

Note: When the downstream-on-demand method is used, an LSR advertises its locally assigned (incoming) labels to its LDP peer only when the peer requests them.

Question 2

Which two of the following statements regarding LDP are true? (Choose two)

A. LDP can also be used between nonadjacent routers using multicast LDP hello messages.
B. LDP does not require periodic hello messages once the LDP session has been established between the LDP peers.
C. LDP hello messages use TCP packets with a destination port number of 646.
D. Multiple sessions can be established between a pair of LSRs if they use multiple label spaces.
E. Per-platform label space can be identified by a label space ID of 0 in the LDP identifier field.


Answer: D E

Question 3

Refer to the exhibit. Based on the show outputs, which condition could be preventing the P1 router from establishing TDP adjacency with its neighbor over the s0/0.211 and s0/0.212 subinterfaces?

P1#sh mpls ldp neighbor

P1#sh mpls interfaces

Interfaces IP Tunnel Operational
Serial0/0.211 Yes (tdp) No Yes
Serial0/0.212 Yes (tdp) No Yes

A. The s0/0.211 and s0/0.212 subinterfaces line protocol are in the down state.
B. The P1 router cannot establish a TCP session with its neighbors.
C. The P1 router is missing the mpls label protocol LDP command.
D. The show mpls tdp neighbor command needs to be used to view the TDP neighbor status.


Answer: B


A is not correct as both of the serial interfaces are up (operational). If the “Operational” state is “yes”, labeled packets can be sent over this interface. It means that an MPLS protocol is configured on the interface and the required Layer 2
negotiations have occurred.

C is not correct as P1 router uses TDP (a label distribution protocol), not LDP.

D is not correct because the “show mpls tdp neighbor” command can be used to view the status of both TDP and LDP neighbor status.

Some information about neighbor establishment between two LDP/TDP routers:

LDP periodically sends hello messages (every 5 seconds). If the label switch router (LSR) is adjacent or one hop from its neighbor, the LSR sends out LDP link hello messages to all the routers on the subnet as User Datagram Protocol (UDP) packets with a multicast destination address of (“all routers on a subnet”) and destination port number of 646 (TDP uses destination port 711) .

After discovering any LDP neighbor using multicast UDP hello messages, a TCP session must be established for LDP to exchange labels over a reliable connection. If the TCP session cannot be established between two routers (for example there is an access-list that denies TCP sessions on the well-known port number 711 TDP), they cannot become neighbors. And you will see an empty output when using the “show mpls ldp neighbor” command.

(Reference: MPLS Student Guide)

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