Name_________________________
S4/C4:
Point-to-Point Protocol
Directions: The questions are in order according to the curriculum. Answer them.
4.1.1
What protocol did PPP replace?
SLIP (Serial Line Internet Protocol) 1980s
What type of circuits can PPP be used on?
Synchronous and asynchronous
What is the most widely used WAN
protocol?
List six features of PPP:
PPP
1. control of data link setup
2.dynamic assignment of IP addresses
3. Network protocol multiplexing
4. link configuration and quality testing
5. error detection
6. data compression negotiations
4.1.2
What does PPP use to encapsulate over
serial links? A method for encapsulating
datagrams over serial links.
1. HDLC (high-level data link control)
What is used to establish, configure and
test the data-link connection.
LCP (Link Control Protocol)
What is used to establish and configure
different network-layer protocols?
NCPs (Network Control Protocols)
Name three protocols that PPP supports.
1. IP
2. IPX
3. Appletalk
4.1.3
Does synchronous and asynchronous
physical media operate at the lower levels
or the upper levels?
PPP encapsulates several network-layer
protocols with NCPs. List three of them.
Lower levels (physical layer)
1. BCP (bridge control protocol)
2. IPCP (Internet Protocol Control Protocol)
3. IPXCP (Internetwork Pkt Exchg Cntrl Pro)
4.1.4
What begins or ends a frame?
S4/C4 Online Curriculum
A Flag (consists of 01111110)
PPP
1
Address - Consists of the standard
broadcast address, which is the binary
sequence 11111111. PPP does not assign
individual station addresses.
What part of the frame is l - 1 byte and
consists of the binary sequence 00000011?
Control: calls for transmission of
unsequenced user data
Is the link connection or connectionless?
What is the size of the protocol field?
connection-less
2 bytes (Ids protocol in data field of the frame)
What is the default maximum length of he
data field
1,500 bytes
What’s the purpose of the FCS?
4.2.1
Error control purposes
What are the four phases PPP goes
through to establish communications?
(Look at the graphic)
1. link establishment
2. link quality
3. network-layer protocol
4. link termination
What phase is the link tested?
2, link quality
optional
Is link testing optional or required?
What kind of frames does the originating
PPP node send to configure network-layer
protocols?
What are the four LCP phases?
4.2.2
1.
2.
3.
4.
In Phase 1 (link establishment and
configuration negotiation) what does each
PPP device send to configure and
establish the data link?
When is phase 1 complete?
4.2.3
What happens in the link-quality
determination phase?
If authentication is used, when does it take
place?
What are the two authentication protocols
PPP supports?
4.2.4
What is Phase 3?
What type of packets are sent to choose
and configure one or more Layer 3
protocols?
What is the command to check the LCP
and NCP states when PPP is configured?
4.2.5
List two ways how LCP can terminate the
link.
S4/C4 Online Curriculum
NCP
PPP
link establishment
link quality
network layer protocol
link termination
An LCP packet
When a configuration acknowledgement
frame has been sent and received
The link is tested to see if it’s good enough to
bring up network layer protocols
Before the network layer protocol config.
phase begins
PAP (Password Authentication Protocol)
CHAP (Challenge Handshake Auth. Proto)
Network-layer protocol configuration negotiat.
NCP packets
Show interfaces
User request
Physical event like loss of carrier, timeout
2
4.3.1
4.3.2
If authentication is used, when does it take
place?
After the link has been established and the
authentication protocol chosen
Who must enter authentication
information?
What is the preferred protocol?
After the PPP link establishment phase is
complete, what type of authentication
information is sent to the remote node until
it is acknowledged or terminated?
How are passwords sent across the link?
The calling side
Is there any protection from trial-and-error
attacks?
Why is CHAP more effective than PAP?
CHAP
A Username/password pair
repeatedly
NO, not with PAP
It does not allow a caller to attempt to
authenticate without challenge, handshake can
be done repeatedly after link established
Does CHAP allow a caller to attempt
authentication without a challenge?
4.3.3
NO
CHAP provides protection against playback
attacks through the use of a variable
challenge value that is unique and
unpredictable
The use of repeated challenges is intended
to limit the time of exposure to any single
attack.
Write the router commands for the 6 steps
to configure PPP.
For steps 2 & 5 – write what happens as
this point.
1. ROUTER(config)# username [ ] password [ ]
2.ROUTER(config)#interface E0
3. ROUTER(config-if)#encapsulation PPP
5.
ROUTER(config-if)#ppp authentication
{chap/pap}
(use 2: first and second choice)
5. first one specified is requested during link
negotiation, second tried if first refused
6. ROUTER(config-if)#ppp pap sent-username [username]
password [password] (Pap disabled by default)
4.3.4
What are two methods that can be used to
simplify CHAP configuration?
1. use same host name on multiple routers
What is the command to configure the
same host name on each router?
What is the command to authenticate a
password from an unknown host?
Router(config-if)#ppp chap hostname [hostname]
S4/C4 Online Curriculum
PPP
2. configure a password that will be sent to
hosts wanting to authenticate
Router(config-if)#ppp chap password [secret]
3