Answers to the Chapter 8 Q&A Section 769
7
What does NBMA stand for? Does it apply to PPP links? What about X.25 networks or
Frame Relay networks?
Nonbroadcast multiaccess. PPP is nonbroadcast, but not multiaccess. X.25 and Frame
Relay are NBMA networks. Multiaccess really means more than two devices connected
to the data link; therefore, when one device sends data, the intended receiver is not
obvious.
8
Define the terms DCE and DTE in the context of the physical layer and a point-to-point
serial link.
DTE refers to the device that looks for clocking from the device on the other end of the
cable on a synchronous link. The DCE supplies that clocking. An X.25 switch is a DCE
in the X.25 use of the word but probably is a DTE receiving clock from a DSU/CSU or
Mux from the physical layer perspective.
9
What layer of OSI is most closely related to the functions of Frame Relay? Why?
OSI Layers 1 and 2. As usual, Frame Relay refers to well-known physical layer
specifications. Frame Relay does define headers for delivery across the Frame Relay
cloud, but it provides no addressing structure to allow VCs among multiple different
Frame Relay networks; thus, it is not considered to match OSI Layer 3 functions. With the
advent of Frame Relay SVCs, it could be argued that Frame Relay performs some Layer
3like functions.
10
When Inverse ARP is used by default, what additional configuration is needed to get IGRP
routing updates to flow over each VC?
No additional configuration is required. The forwarding of broadcasts as unicasts is
enabled on each VC and protocol for which an Inverse ARP is received.
11
Define the attributes of a partial mesh and full mesh Frame Relay network.
In a partial mesh network, not all DTEs are connected with a VC. In a full mesh network,
all DTEs are connected with a VC.
12
What key pieces of information are required in the frame-relay map statement?
Layer 3 protocol, next-hop router's Layer 3 address, DLCI to reach that router, and
whether to forward broadcasts. Frame Relay maps are not required if Inverse ARP is in
use.
13
When creating a partial mesh Frame Relay network, are you required to use
subinterfaces?
No. Subinterfaces can be used and are preferred with a partial mesh because this removes
split horizon issues by treating each VC as its own interface. Likewise, subinterfaces are
optional when the network is a full mesh. Most people tend to use subinterfaces today.
apA.fm Page 769 Monday, March 20, 2000 5:24 PM