What is PPP?

Point-to-Point Protocol (PPP) is a data-link layer protocol that establishes a direct connection between two network nodes over serial lines, dial-up circuits, or T1/E1 links. PPP negotiates the link with LCP, authenticates peers with PAP or CHAP, and carries network-layer traffic – most commonly over Ethernet through PPPoE.

Key takeaways

• PPP is a data-link layer protocol for direct node-to-node links.
• LCP establishes, configures, and tests the point-to-point link.
• PAP and CHAP authenticate PPP peers before user traffic flows.
• PPPoE encapsulates PPP frames inside Ethernet for DSL and broadband.
• VOS supports PPP on T1/E1 interfaces and PPPoE on WAN interfaces.

On this page: Why it matters · How it works · Types · PPP & Versa · FAQ

Why is PPP important?

Before Ethernet and IP networks displaced most dedicated circuits, WAN links – leased lines, dial-up modems, ISDN, and T1/E1 circuits – needed a standard way to carry more than one network-layer protocol over a single connection, negotiate link parameters, and verify the peer. PPP filled that role, and decades later it still does.

Its importance today is concentrated in two places. T1/E1 and other leased-line WAN interfaces still use PPP as their standard encapsulation, with PAP or CHAP authenticating the connection before traffic flows. And PPP survives at enormous scale inside PPPoE: most DSL and a good share of fiber broadband connections are still PPP sessions underneath the Ethernet framing, complete with LCP negotiation and CHAP authentication against the provider’s access concentrator.

What is PPP?

PPP operates in phases. First, Link Control Protocol (LCP) establishes the point-to-point link: it negotiates frame size, detects link loops, and can require authentication before the link comes up. If configured, peers exchange credentials using PAP (Password Authentication Protocol) or CHAP (Challenge Handshake Authentication Protocol) – CHAP is stronger because it never sends the password itself, instead hashing a fresh random challenge with a static shared secret on every exchange.

Once the link and authentication succeed, one or more Network Control Protocols (NCPs) configure the network-layer protocols riding over the link – IPCP for IPv4, IPv6CP for IPv6. Only after LCP and the relevant NCP complete does user traffic flow. PPP frames use HDLC-like framing, so multiple network-layer protocols can share one link unambiguously.

Types of PPP

Several PPP link types and extensions matter in practice. Point-to-point serial and T1/E1 encapsulation is the classic case: PPP runs directly over a leased line or a T1/E1 channel group, most often with CHAP authentication. Multilink PPP (MLPPP) bundles two or more physical links into a single logical connection, fragmenting and reassembling packets across the members to combine bandwidth and provide failover.

PPPoE (PPP over Ethernet) encapsulates PPP frames inside Ethernet frames so PPP’s negotiation and authentication can run over a shared Ethernet segment instead of a dedicated wire – the mechanism behind most DSL and some fiber broadband access. A PPPoE client discovers an access concentrator, establishes a session ID, and then runs standard PPP inside that session. A related variant, PPPoEoA, carries the same PPPoE session over an ATM-based DSL line instead of native Ethernet.

PPP & Versa

VOS devices support PPP encapsulation directly on T1/E1 WAN interfaces, where it is the default alongside Frame Relay and HDLC; administrators select PAP, CHAP, or no authentication per interface, and Multilink PPP bundles T1/E1 channel groups. NIC modules such as the CSG700 series’ T1/E1 card explicitly support PPP, HDLC, and Frame Relay for branch WAN connectivity.

PPPoE is configured as its own WAN interface subcategory, both directly on an appliance and through PPPoE interfaces in Concerto, where a dedicated VNI interface carries the session. Configuration covers the access concentrator name, username and password, and LCP echo settings that detect a dead peer. Full interface configuration – T1/E1 PPP encapsulation, authentication selection, PPPoE WAN interfaces – is documented in Configure Interfaces.

Frequently asked questions

What is the difference between PPP and PPPoE?

PPP runs directly over a serial line, T1/E1 circuit, or other point-to-point medium. PPPoE encapsulates the same PPP frames inside Ethernet frames so a PPP session can run over a shared Ethernet network, such as a DSL modem’s port to a provider’s access concentrator.

What is the difference between PAP and CHAP authentication?

PAP sends a username and password in clear text once, at connection setup. CHAP never sends the password itself – it hashes a random challenge with a shared secret and repeats the exchange periodically, so a captured exchange cannot be replayed. CHAP is the stronger, preferred option.

What does LCP do in a PPP connection?

LCP (Link Control Protocol) is the first phase of a PPP session. It negotiates link options such as maximum frame size, detects link loops, and can require authentication before allowing network-layer traffic onto the link. LCP also sends periodic echo requests to detect a dead peer.

What is Multilink PPP?

Multilink PPP (MLPPP) combines two or more physical PPP links into one logical connection. Packets are fragmented and distributed across the member links and reassembled at the far end, increasing available bandwidth and providing automatic failover if one member link fails or is disconnected.

Does PPP require authentication?

No. LCP can bring up a PPP link with no authentication at all. When authentication is configured, PAP or CHAP verifies the peer before the link passes traffic – common on WAN circuits and PPPoE sessions where a provider must confirm the subscriber’s identity.

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