CCNA FAQ: Frame Relay Concepts
Q1. Which of the following is a protocol used between the Frame Relay DTE and the Frame Relay switch?
Answer: C. The LMI manages the link between the DTE and the switch, including noticing when a virtual circuit (VC) comes up or goes down.
Q2. Which of the following statements about Frame Relay are true? (Choose two answers.)
a. The DTE typically sits at the customer site.
b. Routers send LMI messages to each other to signal the status of a VC.
c. A frame’s source DLCI must remain unchanged, but the frame’s destination DLCI is allowed to change, as the frame traverses the Frame Relay cloud.
d. The Frame Relay encapsulation type on the sending router should match the encapsulation type on the receiving router for the receiving router to be able to understand the frame’s contents.
Explanation: A and D. The DTE typically sits at the customer site, and the DCE sits at the service provider site. Frame Relay switches send LMI messages to DTEs (typically routers) to signal VC status. A Frame Relay frame does not have a source and destination DLCI, but a single DLCI field.
Q3. What does DLCI stand for?
a. Data-link connection identifier
b. Data-link connection indicator
c. Data-link circuit identifier
d. Data-link circuit indicator
Q4. Router R1 receives a frame from router R2 with DLCI value 222 in it. Which of the following statements about this network is the most accurate?
a. 222 represents Router R1.
b. 222 represents Router R2.
c. 222 is the local DLCI on R1 that represents the VC between R1 and R2.
d. 222 is the local DLCI on R2 that represents the VC between R1 and R2.
Explanation: C. The DLCI addresses a VC, not a router. The DLCI value in the frame as it crosses the local link represents that VC on that link. Because the question refers to a frame crossing the access link connected to R1, 222 is the local DLCI on R1 that identifies that VC.
Q5. A Frame Relay planning diagram shows the number 101 beside R1, 102 by R2, 103 by R3, and 104 by R4. No other DLCIs are listed. The lead network engineer tells you that the planning diagram uses global DLCI addressing and that a full mesh of VCs exists. Which of the following are true? (Choose two answers.)
a. Frames sent by R1 to R2, as they cross R2’s access link, have DLCI 102.
b. Frames sent by R1 to R2, as they cross R2’s access link, have DLCI 101.
c. Frames sent by R3 to R2, as they cross R3’s access link, have DLCI 102.
d. Frames sent by R3 to R1, as they cross R3’s access link, have DLCI 101.
Explanation: B and C. The global DLCIs represent the DLCI the other routers use when sending frames over their local access links. So, when R1 sends a frame to R2, when the frame crosses R2’s access link, the network has changed the DLCI to R1’s global DLCI, 101. Similarly, when R3 sends a frame to R1, as the frame crosses R3’s access link, the frame has R1’s global DLCI in it, 101.
Q6. FredsCo has five sites, with routers connected to the same Frame Relay network. Virtual circuits (VC) have been defined between each pair of routers. What is the fewest subnets that FredsCo could use on the Frame Relay network?
Explanation: A. A single subnet can be used in any Frame Relay topology, but with a full mesh, a single subnet can be used with no tricky issues related to routing protocols.
Q7. BarneyCo has five sites, with routers connected to the same Frame Relay network. VCs have been defined between each pair of routers. Barney, the company president, will fire anyone who configures Frame Relay without using point-to-point subinterfaces. What is the fewest subnets that BarneyCo could use on the Frame Relay network?
Explanation: D. BarneyCo has a total of ten VCs. With all of them configured on point-to-point subinterfaces, you need ten subnets, because you need one subnet per VC.
Q8. R1 sends a Frame Relay frame over a VC to router R2. About the same time, a Frame Relay switch notices that too many packets are trying to exit the Frame Relay network over the access link connected to R2. Which of the following is the most likely result that could be caused by this scenario?
a. R1 eventually receives a frame with BECN set.
b. R1 eventually receives a frame with FECN set.
c. R1 eventually receives a frame with DE set.
d. None of the other answers is correct.
Explanation: A. The frame that experienced the congestion was going from R1 to R2, so the frame with the Backward (opposite direction) Explicit Congestion Notification (BECN) bit set would go in the opposite direction, from R2 back to R1.