Juniper JUNOSE 11.1.X - LINK LAYER CONFIGURATION 4-7-2010 Configuration Manual page 311

ppp keepalive
Use to specify the IPCP netmask option (option 0x90) for each PPP interface. By
default, the IPCP netmask option is disabled on the interface.
The IPCP netmask option is a nonstandard option that enables a peer to request
the netmask associated with the assigned IP address.
The netmask can be specified via RADIUS attribute 9, Framed-Ip-Netmask. If the
netmask is 255.255.255.255, the option is not negotiated. See the radius ignore
framed-ip-netmask command.
You can enable the IPCP netmask option either in a profile or on a static interface.
Example
host1(config-subif)#ppp ipcp netmask
Use the no version to disable the IPCP netmask option on the interface.
See ppp ipcp netmask.
Use to specify the keepalive timeout value.
There are two keepalive modes of operation: high-density mode and low-density
mode.
High-density keepalive mode is automatically selected if PPP is layered over
ATM, L2TP, or PPPoE.
Low-density keepalive mode is selected if PPP is layered over HDLC.
Keepalive mode selection is made per interface.
High-density mode This mode is also known as smart keepalive. When the
keepalive timer expires, the interface first verifies whether any frames were
received from the peer in the prior keepalive timeout interval. If so, the interface
does not send an LCP echo request (keepalive). Keepalive packets are sent only
if the peer is silent (that is, no traffic was received from the peer during the
previous keepalive timeout interval). If both sides are configured with keepalive,
receipt of an LCP echo request by one end suppresses the transmission of an
LCP echo request by that end. Smart keepalive is disabled when the keepalive
timeout value is at least 60 seconds, even when in high-density mode. Smart
keepalive is always disabled when in low-density mode. This mode suppresses
transmission of unnecessary LCP echo requests.
For high-density keepalive mode, the range is 30–64800 seconds. The default
value is 30 seconds.
Low-density mode When the keepalive timer expires, the interface always
sends an LCP echo request, regardless of whether the peer is silent.
For low-density keepalive mode, the range is 1–64800 seconds for POS uplink
interfaces, and 10–64800 seconds for all other HDLC interfaces. The default
value for all interfaces is 30 seconds.
If the keepalive interval is 30 seconds, a failed link is detected between 90 and
120 seconds after failure.
Use ppp keepalive without a value to restore the default, 30 seconds.
Example
Chapter 8: Configuring Point-to-Point Protocol
Optional Configuration Tasks
279