PART IX
WANs
Chapter 26 HDLC and PPP
Chapter 27 Frame Relay
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CHAPTER 26
HDLC and PPP
This chapter provides information and commands concerning the following topics:
• Configuring HDLC encapsulation on a serial line
• Configuring PPP on a serial line (mandatory commands)
• Configuring PPP on a serial line (optional commands), including those
commands concerning the following
— Compression
— Link quality
— Multilink
— Authentication
• Verifying or troubleshooting a serial link/PPP encapsulation
• Configuration example: PPP
Configuring HDLC Encapsulation on a Serial Line
NOTE: HDLC is the default encapsulation for synchronous serial links on
Cisco routers. You would only use the encapsulation hdlc command to return
the link to its default state.
Configuring PPP on a Serial Line (Mandatory Commands)
Router#cc
cc
oo
oo
nn
nn
ff
ff

ii
ii
gg
gg
uu
uu
rr
rr
ee
ee


tt
tt
ee
ee
rr
rr
mm
mm
ii
ii
nn
nn
aa
aa
ll
ll
Moves to global configuration mode
Router(config)#ii

ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee


ss
ss
ee
ee
rr
rr
ii
ii
aa
aa
ll

ll


00
00
//
//
00
00
//
//
00
00
Moves to interface configuration mode
Router(config-if)#ee
ee
nn
nn
cc
cc
aa
aa
pp
pp
ss
ss
uu
uu
ll
ll

aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


hh
hh
dd
dd
ll
ll
cc
cc
Sets the encapsulation mode for this
interface to HDLC
Router#cc
cc
oo
oo
nn
nn
ff
ff

ii
ii
gg
gg
uu
uu
rr
rr
ee
ee


tt
tt
ee
ee
rr
rr
mm
mm
ii
ii
nn
nn
aa
aa
ll
ll
Moves to global configuration mode
Router(config)#ii

ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee


ss
ss
ee
ee
rr
rr
ii
ii
aa
aa
ll

ll


00
00
//
//
00
00
//
//
00
00
Moves to interface configuration mode
Router(config-if)#ee
ee
nn
nn
cc
cc
aa
aa
pp
pp
ss
ss
uu
uu
ll
ll

aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


pp
pp
pp
pp
pp
pp
Changes encapsulation from default
HDLC to PPP
252 Configuring PPP on a Serial Line (Optional Commands): Authentication
NOTE: You must execute the encapsulation ppp command on both sides of the
serial link for the link to become active.
Configuring PPP on a Serial Line (Optional Commands): Compression
Configuring PPP on a Serial Line (Optional Commands): Link Quality
NOTE: In PPP, the Link Control Protocol allows for an optional link-quality
determination phase. In this phase, the link is tested to determine whether the link
quality is sufficient to bring up any Layer 3 protocols. If you use the command ppp
quality x, where x is equal to a certain percent, you must meet that percentage
of quality on the link. If the link does not meet that percentage level, the link

cannot be created and will shut down.
Configuring PPP on a Serial Line (Optional Commands): Multilink
Configuring PPP on a Serial Line (Optional Commands):
Authentication
Router(config-if)#cc
cc
oo
oo
mm
mm
pp
pp
rr
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ee
ee
ss
ss
ss
ss


pp
pp
rr
rr
ee
ee
dd
dd

ii
ii
cc
cc
tt
tt
oo
oo
rr
rr
Enables the predictor compression
algorithm
Router(config-if)#cc
cc
oo
oo
mm
mm
pp
pp
rr
rr
ee
ee
ss
ss
ss
ss



ss
ss
tt
tt
aa
aa
cc
cc
Enables the stac compression algorithm
Router(config-if)#pp
pp
pp
pp
pp
pp


qq
qq
uu
uu
aa
aa
ll
ll
ii
ii
tt
tt
yy

yy

x
Ensures the link has a quality of x percent.
Otherwise, the link will shut down.
Router(config-if)#pp
pp
pp
pp
pp
pp


mm
mm
uu
uu
ll
ll
tt
tt
ii
ii
ll
ll
ii
ii
nn
nn
kk

kk
Enables load balancing across multiple
links
Router(config)#uu
uu
ss
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ee
ee
rr
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nn
nn
aa
aa
mm
mm
ee
ee


rr
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oo
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ee

ee
rr
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pp
pp
aa
aa
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ss
ss
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ww
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oo
oo
rr
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dd
dd



cc
cc
ii
ii

ss
ss
cc
cc
oo
oo
Sets a username of routerb and a password
of cisco for authentication from the other
side of the PPP serial link. This is used
by the local router to authenticate the
PPP peer.
Router(config)#ii
ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee



ss
ss
ee
ee
rr
rr
ii
ii
aa
aa
ll
ll


00
00
//
//
00
00
//
//
00
00
Moves to interface configuration mode.
Verifying or Troubleshooting a Serial Link/PPP Encapsulation 253
TIP: When setting authentication, make sure that your usernames match the
name of the router on the other side of the link, and that the passwords on each
router match the other. Usernames and passwords are case sensitive. Consider

the following example:
NOTE: Because PAP does not encrypt its password as it is sent across the link,
recommended practice is that you use CHAP as your authentication method.
Verifying or Troubleshooting a Serial Link/PPP Encapsulation
Router(config-if)#pp
pp
pp
pp
pp
pp


aa
aa
uu
uu
tt
tt
hh
hh
ee
ee
nn
nn
tt
tt
ii
ii
cc
cc

aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


pp
pp
aa
aa
pp
pp
Turns on Password Authentication
Protocol (PAP) authentication only.
Router(config-if)#pp
pp
pp
pp
pp
pp


aa
aa

uu
uu
tt
tt
hh
hh
ee
ee
nn
nn
tt
tt
ii
ii
cc
cc
aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


cc
cc

hh
hh
aa
aa
pp
pp
Turns on Challenge Handshake
Authentication Protocol (CHAP)
authentication only.
Router(config-if)#pp
pp
pp
pp
pp
pp


aa
aa
uu
uu
tt
tt
hh
hh
ee
ee
nn
nn
tt

tt
ii
ii
cc
cc
aa
aa
tt
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ii
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oo
oo
nn
nn


pp
pp
aa
aa
pp
pp


cc
cc
hh
hh
a

a
aa
pp
pp
Defines that the link will use PAP
authentication, but will try CHAP if PAP
fails or is rejected by other side.
Router(config-if)#pp
pp
pp
pp
pp
pp


aa
aa
uu
uu
tt
tt
hh
hh
ee
ee
nn
nn
tt
tt
ii

ii
cc
cc
aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


cc
cc
hh
hh
aa
aa
pp
pp


pp
pp
a
a
aa

pp
pp
Defines that the link will use CHAP
authentication, but will try PAP if CHAP
fails or is rejected by other side.
Router(config-if)#pp
pp
pp
pp
pp
pp


pp
pp
aa
aa
pp
pp


ss
ss
ee
ee
nn
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tt
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--

--
uu
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nn
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aa
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mm
mm
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rr
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rr
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pp
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dd


cc
cc
ii
ii
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ss

cc
cc
oo
oo

This command must be set if using PAP in
Cisco IOS Software Release 11.1 or later.
Edmonton(config)#uu
uu
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CC
CC
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ll
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aa
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pp
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cc
cc
ii
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cc
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oo
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Calgary(config)#uu
uu
ss
ss
ee
ee
rr
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aa
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ee
ee


EE
EE

dd
dd
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pp
pp
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d
d
dd


cc
cc
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cc
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oo
oo
Edmonton(config)#ii
ii
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ee
ee
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cc
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ss
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00
00
//
//
00
00
//
//
00
00
Calgary(config)#ii

ii
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nn
tt
tt
ee
ee
rr
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ff
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aa
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cc
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ee
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ss
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ll


00
00
//
//
00
00
//
//
00
00
Edmonton(config-if)#ee
ee
nn
nn
cc
cc
aa
aa
pp
pp
ss
ss
uu
uu
ll
ll
aa

aa
tt
tt
ii
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oo
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nn
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pp
pp
pp
pp
pp
pp
Calgary(config-if)#ee
ee
nn
nn
cc
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aa
aa
pp
pp
ss
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uu
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pp
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Edmonton(config-if)#pp
pp
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pp
pp


aa

aa
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uu
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tt
hh
hh
ee
ee
nn
nn
tt
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ii
ii
cc
cc
aa
aa
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ii
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oo
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nn
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cc

cc
hh
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aa
aa
pp
pp
Calgary(config-if)#pp
pp
pp
pp
pp
pp


aa
aa
uu
uu
tt
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hh
hh
ee
ee
nn
nn
tt
tt
ii

ii
cc
cc
aa
aa
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ii
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oo
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nn
nn


cc
cc
hh
hh
aa
aa
pp
pp
Router#ss
ss
hh
hh
oo
oo
ww

ww


ii
ii
nn
nn
tt
tt
ee
ee
rr
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ff
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aa
aa
cc
cc
ee
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ss
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ss
ss
ee
ee
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rr
ii
ii
aa
aa
ll
ll

x
Lists information for serial interface x
Router#ss
ss
hh
hh
oo
oo
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ww


cc
cc
oo
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nn
nn
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oo
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ss
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ee
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rr
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ii
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ll

x
Tells you what type of cable (DCE/DTE)
is plugged into your interface and whether

a clock rate has been set
254 Configuration Example: PPP
TIP: With frequent lab use, serial cable pins often get bent, which might prevent
the router from seeing the cable. The output from the command show controllers
interface serial x shows no cable even though a cable is physically present.
Configuration Example: PPP
Figure 26-1 illustrates the network topology for the configuration that follows, which shows
how to configure PPP using the commands covered in this chapter.
Figure 26-1 Network Topology for PPP Configuration
NOTE: The host name, password, and interfaces have all been configured as per
the configuration example in Chapter 6, “Configuring a Single Cisco Router.”
Boston Router
Router#dd
dd
ee
ee
bb
bb
uu
uu
gg
gg


ss
ss
ee
ee
rr
rr

ii
ii
aa
aa
ll
ll


ii
ii
nn
nn
tt
tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee
Displays whether serial keepalive counters
are incrementing
Router#dd
dd

ee
ee
bb
bb
uu
uu
gg
gg


pp
pp
pp
pp
pp
pp
Displays any traffic related to PPP
Router#dd
dd
ee
ee
bb
bb
uu
uu
gg
gg


pp

pp
pp
pp
pp
pp


pp
pp
aa
aa
cc
cc
kk
kk
ee
ee
tt
tt
Displays PPP packets that are being sent
and received
Router#dd
dd
ee
ee
bb
bb
uu
uu
gg

gg


pp
pp
pp
pp
pp
pp


nn
nn
ee
ee
gg
gg
oo
oo
tt
tt
ii
ii
aa
aa
tt
tt
ii
ii
oo

oo
nn
nn
Displays PPP packets related to the
negotiation of the PPP link
Router#dd
dd
ee
ee
bb
bb
uu
uu
gg
gg


pp
pp
pp
pp
pp
pp


ee
ee
rr
rr
rr

rr
oo
oo
rr
rr
Displays PPP error packets
Router#dd
dd
ee
ee
bb
bb
uu
uu
gg
gg


pp
pp
pp
pp
pp
pp


aa
aa
uu
uu

tt
tt
hh
hh
ee
ee
nn
nn
tt
tt
ii
ii
cc
cc
aa
aa
tt
tt
ii
ii
oo
oo
nn
nn
Displays PPP packets related to the
authentication of the PPP link
Router#dd
dd
ee
ee

bb
bb
uu
uu
gg
gg


pp
pp
pp
pp
pp
pp


cc
cc
oo
oo
mm
mm
pp
pp
rr
rr
ee
ee
ss
ss

ss
ss
ii
ii
oo
oo
nn
nn
Displays PPP packets related to the
compression of packets across the link
Boston>ee
ee
nn
nn
aa
aa
bb
bb
ll
ll
ee
ee
Moves to privileged mode
Boston#cc
cc
oo
oo
nn
nn
ff

ff
ii
ii
gg
gg
uu
uu
rr
rr
ee
ee


tt
tt
ee
ee
rr
rr
mm
mm
ii
ii
nn
nn
aa
aa
ll
ll
Moves to global configuration mode

Network 172.16.20.0/24 Network 172.16.20.0/30 Network 172.16.30.0/24
172.16.10.10
s0/0/1
DCE
172.16.20.2
fa0/0
172.16.10.1
s0/0/0
172.16.20.1
Boston-2811
Buffalo-2811
172.16.30.30
fa0/0
172.16.30.1
Configuration Example: PPP 255
Buffalo Router
Boston(config)#uu
uu
ss
ss
ee
ee
rr
rr
nn
nn
aa
aa
mm
mm

ee
ee


BB
BB
uu
uu
ff
ff
ff
ff
aa
aa
ll
ll
oo
oo


pp
pp
aa
aa
ss
ss
ss
ss
ww
ww

oo
oo
rr
rr
dd
dd



aa
aa
cc
cc
aa
aa
dd
dd
ee
ee
mm
mm
yy
yy
Sets the local username and password for
PPP authentication of the PPP peer
Boston(config-if)#ii
ii
nn
nn
tt

tt
ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee


ss
ss
ee
ee
rr
rr
ii
ii
aa
aa
ll
ll


00

00
//
//
00
00
//
//
00
00
Moves to interface configuration mode
Boston(config-if)#dd
dd
ee
ee
ss
ss
cc
cc
rr
rr
ii
ii
pp
pp
tt
tt
ii
ii
oo
oo

nn
nn


LL
LL
ii
ii
nn
nn
kk
kk


tt
tt
oo
oo


BB
BB
uu
uu
ff
ff
ff
ff
aa
aa

l
l
ll
oo
oo


RR
RR
oo
oo
uu
uu
tt
tt
ee
ee
rr
rr
Defines the locally significant link
description
Boston(config-if)#ii
ii
pp
pp


aa
aa
dd

dd
dd
dd
rr
rr
ee
ee
ss
ss
ss
ss


11
11
77
77
22
22
..
..
11
11
66
66
..
..
22
22
00

00
..
..
11
11


22
22
55
55
5
5
55
..
..
22
22
55
55
55
55
..
..
22
22
55
55
55
55

..
..
22
22
55
55
22
22
Assigns an IP address and netmask
Boston(config-if)#cc
cc
ll
ll
oo
oo
cc
cc
kk
kk


rr
rr
aa
aa
tt
tt
ee
ee



55
55
66
66
00
00
00
00
00
00
Sets the clock rate to the data
communications equipment (DCE) side
of link
Boston(config-if) #ee
ee
nn
nn
cc
cc
aa
aa
pp
pp
ss
ss
uu
uu
ll
ll

aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


pp
pp
pp
pp
pp
pp
Turns on PPP encapsulation
Boston(config-if)#pp
pp
pp
pp
pp
pp


aa
aa
uu

uu
tt
tt
hh
hh
ee
ee
nn
nn
tt
tt
ii
ii
cc
cc
aa
aa
tt
tt
ii
ii
oo
oo
nn
nn


cc
cc
hh

hh
aa
aa
pp
pp
Turns on CHAP authentication
Boston(config-if)#nn
nn
oo
oo


ss
ss
hh
hh
uu
uu
tt
tt
dd
dd
oo
oo
ww
ww
nn
nn
Turns on the interface
Boston(config-if)#ee

ee
xx
xx
ii
ii
tt
tt
Returns to global configuration mode
Boston(config)#ee
ee
xx
xx
ii
ii
tt
tt
Returns to privileged mode
Boston#cc
cc
oo
oo
pp
pp
yy
yy


rr
rr
uu

uu
nn
nn
nn
nn
ii
ii
nn
nn
gg
gg
--
--
cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg


ss
ss
tt

tt
aa
aa
rr
rr
tt
tt
u
u
uu
pp
pp
--
--
cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg
Saves the configuration to NVRAM
Buffalo>ee
ee
nn

nn
aa
aa
bb
bb
ll
ll
ee
ee
Moves to privileged mode
Buffalo#cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg
uu
uu
rr
rr
ee
ee



tt
tt
ee
ee
rr
rr
mm
mm
ii
ii
nn
nn
aa
aa
ll
ll
Moves to global configuration mode
Buffalo(config)#uu
uu
ss
ss
ee
ee
rr
rr
nn
nn
aa
aa
mm

mm
ee
ee


BB
BB
oo
oo
ss
ss
tt
tt
oo
oo
nn
nn


pp
pp
aa
aa
ss
ss
ss
ss
ww
ww
oo

oo
rr
rr
dd
dd


a
a
aa
cc
cc
aa
aa
dd
dd
ee
ee
mm
mm
yy
yy
Sets the username and password for PPP
authentication
Buffalo(config-if)#ii
ii
nn
nn
tt
tt

ee
ee
rr
rr
ff
ff
aa
aa
cc
cc
ee
ee


ss
ss
ee
ee
rr
rr
ii
ii
aa
aa
ll
ll


00
00

//
//
00
00
//
//
11
11
Moves to interface configuration mode
Buffalo(config-if)#dd
dd
ee
ee
ss
ss
cc
cc
rr
rr
ii
ii
pp
pp
tt
tt
ii
ii
oo
oo
nn

nn


LL
LL
ii
ii
nn
nn
kk
kk


tt
tt
oo
oo


BB
BB
oo
oo
ss
ss
tt
tt
oo
oo
n

n
nn


RR
RR
oo
oo
uu
uu
tt
tt
ee
ee
rr
rr
Defines the locally significant link
description
Buffalo(config-if)#ii
ii
pp
pp


aa
aa
dd
dd
dd
dd

rr
rr
ee
ee
ss
ss
ss
ss


11
11
77
77
22
22
..
..
11
11
66
66
..
..
22
22
00
00
..
..

22
22


22
22
55
55
5
5
55
..
..
22
22
55
55
55
55
..
..
22
22
55
55
55
55
..
..
22

22
55
55
22
22
Assigns an IP address and netmask
Buffalo(config-if) #ee
ee
nn
nn
cc
cc
aa
aa
pp
pp
ss
ss
uu
uu
ll
ll
aa
aa
tt
tt
ii
ii
oo
oo

nn
nn


pp
pp
pp
pp
pp
pp
Turns on PPP encapsulation