HDLC Encapsulation

  • Default encapsulation method used by Cisco devices on synchronous serial lines.
  • For connecting non-Cisco devices, uses synchronous PPP.
interface s0/0/0
  encapsulation hdlc

PPP Configuration

Set PPP for encapsulation:

interface s0/0/0
  encapsulation ppp

Configure optional compression:

  compress [predictor | stac]
  • predictor specifies predictor compression algorithm to be used.
  • stac specifies Stacker (LZS) compression algorithm will be used.

Configure optional link quality monitoring:

  ppp quality <percentage>
  • Line will go down if the link quality percentage is not being maintained.
  • Link quality is calculated by comparing the total number of packets and bytes sent, to the total number of packets and bytes received by the destination node.

Create a multilink bundle:

interface Multilink 1
 ip address <...>
 ipv6 address <...>
 ppp multilink
 ppp multilink group <number>

Assign interfaces to multilink bundle:

interface <intf-id>
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group <number>

Disable PPP multilink:

no ppp multlink   ! On each bundled interfaces

Verify PPP multilink:

show ppp multilink

PPP Authentication

ppp authentication { chap | pap | \
                     pap chap | chap pap }

Note that the hostname and password of target router must match the username-password of the communicating router for it to work.

PPP PAP Authentication

R1:

username R3 secret class
interface s0/0/0
  encapsulation ppp
  ppp authentication pap
  ppp pap sent-username R1 password cisco

R3:

username R1 secret cisco
interface s0/0/0
  encapsulation ppp
  ppp authentication pap
  ppp pap sent-username R3 password class
PPP CHAP Authentication

R1:

hostname R1
username R2 password <password>

interface <intf-id>
 ip address ...
 ipv6 address ...
 encapsulation ppp
 ppp authentication chap

R2:

hostname R2
username R1 password <password>

interface <intf-id>
 ip address ...
 ipv6 address ...
 encapsulation ppp
 ppp authentication chap
PPP CHAP Authentication through the interface:

R1:

hostname R1

username AuthUser2 password cisco

interface <intf-id>
 ip address ...
 encapsulation ppp
 ppp authentication chap
 ppp chap hostname AuthUser1
 ppp chap password cisco

R2:

hostname R2

username AuthUser1 password cisco

interface <intf-id>
 ip address ...
 encapsulation ppp
 ppp authentication chap
 ppp chap hostname AuthUser2
 ppp chap password cisco

Troubleshooting PPP Serial Encapsulation

debug ppp { packet |
            negotiation |
            error |
            authentication |
            compression |
            cbcp }