2-3 - Troubleshoot LSP and LDP

Question 1

Describe a label switched path (LSP).

Answer 1

• a sequence of label switch routers (LSRs) that forwards labeld packets for a particular Forwarding Equivalence Class (FEC)
• each intermediate LSR swaps the top label in a packet that traverses the LSP

Question 2

What is a FEC (Forwarding Equivalence Class)?

Answer 2

a group of packets that are forwarded along the same path and receive the same forwarding treatment

Question 3

T/F: When using LDP, an LSP is created for each entry in the main routing table, with the exception of BGP entries.

Answer 3

True

Question 4

In an MPLS enabled SP environment, do the core (P) routers NEED to have BGP running in order to transport traffic to destinations that the PEs know via BGP?

Answer 4

No. Only the PEs need to run BGP in that case, and the P routers technically only need to know how to label switch the traffic to the BGP next-hop (since the P routers aren’t forwarding based on Layer 3 header information, but rather MPLS labels).

Question 5

The steady state occurs and MPLS is fully functional when the routing protocol and LDP have populated these tables.

Answer 5

• Main IP routing table
• LIB table
• FIB table
• LFIB table

Question 6

T/F: LDP exchanges label information to its neighbors faster than routing protocols exchange routing information with their neighbors.

Answer 6

False

Question 7

Describe the MPLS label control methods.

Answer 7

• By default: LDP advertises labels for all the prefixes (except BGP routes) to all its neighbors
• LDP Label Advertisement Control: filter the labels that LDP announces outbound to remote peers
• LDP Label Acceptance Control: filter the labels that LDP accepts inbound from remote peers

Question 8

By default, how often does LDP send Hellos?

Answer 8

5 seconds

Question 9

The multicast addresses for LDP are:

Answer 9

• 224.0.0.2 (“all routers on a subnet”)
• FF02::2 (“all routers on a subnet”)

Question 10

What is the default port number used for LDP Hello messages? What is the default port number used for all other LDP control traffic?

Answer 10

A) UDP 646
B) TCP 646

Question 11

Describe the two parts of LDP neighbor establishment.

Answer 11

• LDP discovery: discover neighbors by sending Hellos via multicast
• LDP adjacency: receive Hellos and reply to them by opening a TCP session with the remote device

Question 12

Describe the overall LDP neighbor process.

Answer 12

1. Establish the TCP session
2. Exchange initialization messages that contain info like
   
   • label distribution methods
   • session keepalive time
   • fault-tolerant TLV
3. Exchange initial keepalive messages

After this, LDP keepalives transmit every 60 seconds (by default)

Question 13

Which of the following statements best describes LSP?

A. A sequence of LSRs that forwards labeled packets for a paricular FEC.
B. A group of packets that forward along the same path and receive the same treatment.
C. Populates the routing tables in all routers in an MPLS domain.
D. Swaps the top label in a packet that traverses the MPLS netowork.

Answer 13

A

Question 14

Which of the following statements best describes LDP?

A. Propagates routing information to all routers in an MPLS domain.
B. Determines the destination networks that are found in the main routing table.
C. Stores labels and related information inside a data structure called a LIB.
D. Standard protocol used to exchange labels between adjacent LSRs.

Answer 14

D