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Chapter 7
Routed PPPoA
Routed PPPoA vs
TCP/IP configuration
Protocol stack
Connection methods
60
Three popular TCP/IP scenarios exist for Routed PPPoA implementations:
The Routed PPPoA interface negotiates a local PPP peer IP address with the
remote PPP peer of the Broadband Remote Access Server (BRAS) (actually the
remote PPP peer will provide the local PPP peer IP address to use). By
enabling NAPT on the Routed PPPoA interface, IP Routing makes
communication possible between the interface and the nodes on the Local
Network. Whether the IP configuration of the local nodes is done manually or
via the DHCP server of the SpeedTouch™ is irrelevant for end-to-end
connectivity.
The Service Provider assigns an IP address to the Routed PPPoA connection.
When the Routed PPPoA session is started, the SpeedTouch™ will put this IP
address in a existing local DHCP pool. The next time a local node renews its IP
address, the SpeedTouch™ assigns the public IP address to the local node.
The SpeedTouch™ itself stays in unnumbered mode.
This scenario is referred to as DHCP spoofing.
The Service Provider assigns a subnet to the Routed PPPoA connection. When
this Routed PPPoA session is started, the SpeedTouch™ will populate an
existing local DHCP pool with this subnet. The next time a local node renews
its IP address, it receives an IP address from the PPP IPCP subnet masking
DHCP pool.
This scenario is often referred to as IPCP subnet masking.
The figure below shows the routed PPPoA protocol stack.
ip
ppp
mac
eth
PC
CPE
Three modes exist to start a Routed PPPoA session:
Dial-In:
The session is opened manually.
Always-On:
After the SpeedTouch™ is powered, it automatically tries to start the session.
Dial-on-Demand:
The session is started automatically, triggered by packets arriving from the
local network, with as destination the PPP interface.
The Dial-In connection mode is the only connection mode that requires an
intervention from the user. The user can open Routed PPPoA Dial-In
connections via the SpeedTouch™ Web interface.
PPPoA
adsl
DSLAM
IP
PPP
ppp
PPPoA
AAL5/ATM
PHY
Service
Gateway
E-DOC-CTC-20051017-0167 v1.0
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