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Cisco 819 Series Integrated Services Routers Software Configuration Guide
OL-23590-02
Chapter 8 Configuring the Serial Interface
Information About Configuring Serial Interfaces
PPP uses keepalives to monitor the link state, as described in the “Keepalive Timer” section on page 8-4.
PPP supports the following authentication protocols, which require a remote device to prove its identity
before allowing data traffic to flow over a connection:
• Challenge Handshake Authentication Protocol (CHAP)—CHAP authentication sends a challenge
message to the remote device. The remote device encrypts the challenge value with a shared secret
and returns the encrypted value and its name to the local router in a response message. The local
router attempts to match the remote device’s name with an associated secret stored in the local
username or remote security server database; it uses the stored secret to encrypt the original
challenge and verify that the encrypted values match.
• Microsoft Challenge Handshake Authentication Protocol (MS-CHAP)—MS-CHAP is the Microsoft
version of CHAP. Like the standard version of CHAP, MS-CHAP is used for PPP authentication; in
this case, authentication occurs between a personal computer using Microsoft Windows NT or
Microsoft Windows 95 and a Cisco router or access server acting as a network access server.
• Password Authentication Protocol (PAP)—PAP authentication requires the remote device to send a
name and a password, which are checked against a matching entry in the local username database
or in the remote security server database.
Use the ppp authentication command in interface configuration mode to enable CHAP, MS-CHAP, and
PAP on a serial interface.
Note Enabling or disabling PPP authentication does not effect the local router’s willingness to authenticate
itself to the remote device.
Multilink PPP
Multilink Point-to-Point Protocol (MLPPP) is supported on the Cisco 819 ISR serial interface. MLPPP
provides a method for combining multiple physical links into one logical link. The implementation of
MLPPP combines multiple PPP serial interfaces into one multilink interface. MLPPP performs the
fragmenting, reassembling, and sequencing of datagrams across multiple PPP links.
MLPPP provides the same features that are supported on PPP Serial interfaces with the exception of
QoS. It also provides the following additional features:
• Fragment sizes of 128, 256, and 512 bytes
• Long sequence numbers (24-bit)
• Lost fragment detection timeout period of 80 ms
• Minimum-active-links configuration option
• LCP echo request/reply support over multilink interface
• Full T1 and E1 framed and unframed links
Keepalive Timer
Cisco keepalives are useful for monitoring the link state. Periodic keepalives are sent to and received
from the peer at a frequency determined by the value of the keepalive timer. If an acceptable keepalive
response is not received from the peer, the link makes the transition to the down state. As soon as an
acceptable keepalive response is obtained from the peer or if keepalives are disabled, the link makes the
transition to the up state.