Patton electronics 2720/I Series User Manual

Also see for 2720/I Series: Specification sheet User manual

Contents

63.0 PPP OPERATIONAL BACKGROUNDPPP is a protocol used for multi-plexed transport over a point-to-pointlink. PPP operates on all full duplex media, and is a symmetric peer-to-peerprotocol, which can be broken into three main components: 1. A standardmethod to encapsulate datagrams over serial links; 2. A Link Control Protocol(LCP) to establish, configure, and test the data-link connection; 3. A family ofNetwork Control Protocols (NCPs) to establish and configure different net-work layer protocols.In order to establish communications over a point-to-point link, each endof the PPP link must first announce its capabilities and agree on the param-eters of the link’s operation. This exchange is facilitated through LCPConfigure-Request packets.Once the link has been established and optional facilities have beennegotiated, PPP will attempt to establish a network protocol. PPP will useNetwork Control Protocol (NCP) to choose and configure one or more net-work layer protocols. Once each of the network layer protocols have beenconfigured, datagrams from the established network layer protocol can besent over the link. The link will remain configured for these communicationsuntil explicit LCP or NCP packets close the link down, or until some externalevent occurs.The PPP Bridging Control Protocol (BCP), defined in RFC 1638,configures and enables/disables the bridge protocol on both ends ofthe point-to-point link. BCP uses the same packet exchange mecha-nism as the Link Control Protocol (LCP). BCP is a Network ControlProtocol of PPP, bridge packets may not be exchanged until PPP hasreached the network layer protocol phase.3.1 APPLICATIONIn situations where a routed network requires connectivity to aremote Ethernet network, the interface on a router can be configuredas a PPP IP Half Bridge. The serial line to the remote bridge functionsas a Virtual Ethernet interface, effectively extending the routers serialport connection to the remote network. The bridge device sends bridgepackets (BPDU's) to the router's serial interface. The router will receivethe layer three address information and will forward these packetsbased on its IP address.Figure 1 shows a typical Cisco router with a serial interface configuredas a PPP Half Bridge. The router serial interface uses a remote device thatsupports PPP bridging to function as a node on the remote Ethernet net-work. The serial interface on the Cisco will have an IP address on thesame Ethernet subnet as the bridge.For example, the customer site is assigned the addresses192.168.1.0/24 through 192.168.1.1/24. The address 192.168.1.1/24 isalso the default gateway for the remote network. The above settings