3.0 PPP OPERATIONAL BACKGROUND
PPP is a protocol used for multi-plexed transport over a pointto-point link. PPP operates on all full duplex media, and is a symmetric peer-to-peer protocol, which can be broken into three main 
components:1.A standard method to encapsulate datagrams over 
serial links;2.A Link Control Protocol (LCP) to establish, configure, 
and test the data-link connection; 3. A family of Network Control 
Protocols (NCPs) to establish and configure different network la yer 
protocols.
In order to establish communications over a point-to-point link, 
each end of the PPP link must first announce its capabilities and 
agree on the parameters of the link’s operation. This exchange is 
facilitated through LCP Configure-Request packets.
Once the link has been established and optional facilities hav e 
been negotiated, PPP will attempt to establish a network protocol. 
PPP will use Network Control Protocol (NCP) to choose and configure one or more network layer protocols. Once each of the network layer protocols have been configured, datagrams from the 
established network layer protocol can be sent over the link. The 
link will remain configured for these communications until explicit 
LCP or NCP packets close the link down, or until some external 
event occurs.
The PPP Bridging Control Protocol (BCP), defined in RFC 
1638, configures and enables/disables the bridge protocol on 
both ends of the point-to-point link. BCP uses the same 
packet exchange mechanism as the Link Control Protocol 
(LCP). BCP is a Network Control Protocol of PPP, bridge 
packets may not be exchanged until PPP has reached the 
network layer protocol phase.
3.1 Applications
In situations where a routed network requires connectivity 
to a remote Ethernet network, the interface on a router can 
be configured as a PPP IP Half Bridge. The serial line to the 
remote bridge functions as a Virtual Ethernet interface, effectively extending the routers serial port connection to the 
remote network.The bridge device sends bridge packets 
(BPDU's) to the router's serial interface.The router will 
receive the layer three address information and will forward 
these packets based on its IP address.
Figure 2 shows a typical Cisco router with a serial interface 
configured as a PPP Half Bridge.The router serial interface uses 
a remote device that supports PPP bridging to function as a 
node on the remote Ethernet network.The serial interface on the 
Cisco will have an IP address on the same Ethernet subnet as 
the bridge.
For example, the customer site is assigned the addresses
192.168.1.0/24 through 192.168.1.1/24.The address
192.168.1.1/24 is also the default gateway for the remote network.The above settings remove any routing/forwarding intelligence from the CPE.The associated Cisco configuration will 
set serial interface (s0) to accommodate half bridging for the 
above example.
Authentication is optional under PPP. In a point-to-point 
leased-line link, incoming customer facilities are usually fixed 
in nature, therefore authentication is generally not required.If 
the foreign device requires authentication via PAP or CHAP, 
the PPP software will respond with default Peer-ID consisting 
of the units Ethernet MAC address and a password which 
consists of the unit’s Ethernet MAC address.
Some networking systems do not define network numbers 
in packets sent out over a network. If a packet does not have 
a specific destination network number, a router will assume 
that the packet is set up for the local segment and will not forward it to any other sub-network. However, in cases where 
two devices need to communicate over the wide-area, bridging can be used to transport non-routable protocols.
Figure 3 illustrates transparent bridging between two 
routers over a serial interface (s0). Bridging will occur 
between the two Ethernet Interfaces on Router A (e0 and e1) 
and the two Ethernet Interfaces on Router B (e0 and e1).
Patton 
IM1/I4 
Bridge 
Ethernet LAN
PEC Device w/ Serial I/F
Router
Figure 2. Cisco router with serial interface, configured as PPP Half Bridge.
Using Bridge-Groups, multiple remote LANs can be 
bridged over the wide-area.
! 
no ip routing 
! 
interface Ethernet0 
ip address 1.1.1.1 255.255.255.0 
bridge-group 1 
! 
interface Serial0 
ip address 1.1.1.1 255.255.255.0 
encapsulation PPP 
bridge-group 1 
! 
interface Serial1 
ip address 2.2.2.2 255.255.255.0 
bridge-group 1 
! 
bridge 1 protocol ieee 
!
Figure 3. Transparent bridging between two routers over a serial link.
Router A
Patton DSL Modem
with Ethernet Interface
Router B
e0
e0
S1
e1
S1
S0
S0
LAN
LAN
LAN
LAN
LAN
6
Serial Interface
Serial Interface
5