768 Appendix A: Answers to the "Do I Know This Already?" Quizzes and Q&A Sections
Answers to the Chapter 8 Q&A Section
1
Name two WAN data link protocols for which the standards define a protocol type field,
which is used to define the type of header that follows the WAN data-link header.
PPP and Frame Relay. The Frame Relay protocol field was added to the standard based on
efforts from the IETF.
2
Name two WAN data link protocols that define a method of announcing the Layer 3
addresses of the interface to other devices attached to the WAN.
PPP and Frame Relay. PPP uses control protocols specific to each Layer 3 protocol
supported. Frame Relay uses Inverse ARP.
3
What does the acronym LAPD stand for? Is it used as the Layer 2 protocol on dialed ISDN
bearer channels? If not, what is used?
Link Access Procedure, D-channel. It is not used on bearer channels, but instead is used
on the signaling channel. PPP is typically used on bearer channels.
4
"Frame Relay uses source and destination DLCIs in the Frame Relay header, with length
10, 11, or 12 bits." Which parts of this statement do you agree with? Which parts do you
disagree with? Why?
There is only one DLCI field in the Frame Relay header, but it can be 10, 11, or 12 bits in
length. For further information, refer to the section "Frame Relay Protocols."
5
Explain the purpose of Inverse ARP. Explain how Inverse ARP uses Frame Relay
broadcasts.
A router discovers the Layer 3 address(es) of a router on the other end of a VC when that
other router sends an Inverse ARP message. The message is not a broadcast. Broadcasts
are not supported over Frame Relay.
6
Would a Frame Relay switch connected to a router behave differently if the IETF option
were deleted from the encapsulation frame-relay ietf command on that attached router?
Would a router on the other end of the VC behave any differently if the same change were
made?
The switch does not behave differently. The other router, however, must also use IETF
encapsulation; otherwise, the routers will not be looking at the correct fields to learn the
packet type. The IETF-defined headers include the Protocol Type field options.
apA.fm Page 768 Monday, March 20, 2000 5:24 PM