Point-to-Point Leased Lines
523
Note:
Be careful not to confuse LAPB and LAPD. The D can help remind you that it is for an ISDN D channel, but
don't let that make you think that the B is for an ISDN B channel.
HDLC and PPP Configuration
One common task for CCNAs is to enable an appropriate point-to-point data link protocol.
The configuration is straightforward, with LAPB being the exception. (Be sure to configure
the same WAN data link protocol on each end of the serial link. Otherwise, the routers will
misinterpret the incoming frames, and the link will not work.) Tables 8-4 and 8-5 summarize
the configuration commands and the
show
and
debug
commands used for HDLC and PPP
configuration.
High-Level Data
Link Control
(HDLC)
No
No
HDLC serves as Cisco's default on
serial links. Cisco uses a proprietary
type field to support multiprotocol
traffic.
Point-to-Point
Protocol (PPP)
Enables the user to
choose whether
error correction is
performed;
correction uses
LAPB
Yes
PPP was meant for multiprotocol
interoperability from its inception,
unlike all the others. PPP also
supports asynchronous
communication.
Table 8-4
PPP and HDLC Configuration Commands
Command
Configuration Mode
encapsulation
{
hdlc
|
ppp
|
lapb
}
Interface subcommand
compress
[
Predictor
|
stac
|
mppc
[
ignore-pfc
]]
Interface subcommand
Table 8-5
Point-to-Point Related
show
and
debug
Commands
Command
Function
show interface
Lists statistics and details of interface configuration,
including the encapsulation type.
show compress
Lists compression ratios.
show process
Lists processor and task utilization. Is useful in watching
for increased utilization due to compression.
Table 8-3
Point-to-Point Data Link Protocol Attributes (Continued)
Protocol
Error
Correction?
Architected
Type Field?
Other Attributes
ch08.fm Page 523 Monday, March 20, 2000 5:17 PM