This article discusses:

• Basic Point-to-Point Protocol (PPP) design
• Default Network Control Protocol (NCP) configuration options in ScreenOS
• Change in behavior when IPv6 is enabled

Unable to select a specific protocol (IPCP, ATCP, NBFCP etc.,) when configuring NCP under PPP.   Additionally, "IPCP" or "IPCP with IPv6CP" for PPP communication is visible after enabling IPv6.

Basic PPP Design

In order to establish communications over a point-to-point link:

1. Each end of the PPP link MUST first send LCP packets (Over Layer 2) to configure and test the data link.  
2. After the link has been established, the peer MAY be authenticated. 
3. Then, PPP MUST send NCP packets to choose and configure one or more network-layer protocols. ScreenOS by default, uses only IPCP protocol to negotiate with the server. No other protocol is supported.

ScreenOS Design

ScreenOS by design, supports only IPCP under NCP. This is why there is no option to select an NCP protocol when configuring PPP.

ScreenOS with IPv6 enabled

When IPv6 is enabled, the user is presented with options to choose either IPCP or IPCPv6 when configuring PPP profiles.

1. As long as IPv6 is disabled, IPCP will be used by ScreenOS by default. This behavior cannot be changed.
2. If IPv6 is enabled, you can select one of the following options:
    
   • Try IPCP first, then IPv6CP
   • Try IPv6CP first, then IPCP
   • Try IPCP only
   • Try IPv6CP only

From the Web UI, navigate to Network > PPPoE > Edit > PPP Parameters. The following options will be visible:

IPCP/IPv6CP Initiating
 

• Try IPCP first, then IPv6CP
• Try IPv6CP first, then IPCP
• Try IPCP only
• Try IPv6CP only

For information on how to enable IPv6, refer to KB15692 - How do I enable IPv6 on ScreenOS devices, and what devices are supported?