Configuring the KeepAlive Timeout for an Interface

You can optionally specify a keepalive timeout value. The keepalive mechanism tracks the status of the connection. There are two keepalive modes of operation: high-density mode and low-density mode. High-density mode is automatically selected when PPP is layered over ATM, tunnel, or PPPoE. Low-density mode is selected when PPP is layered over HDLC. The keepalive mode selection is made per interface.

In high-density mode, 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.

In 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. To restore the default of 30 seconds, enter ppp keepalive without a value.

To specify the keepalive value for each interface:

• (Optional) From the Interface Configuration mode, specify the keepalive value:
  host1(config-if)#ppp keepalive 50

  Use the no version to disable keepalive.