- Enterasys Security Router User's Guide

Features
XSR User’s Guide 13-3
Figure 13-2 PPPoA Network Diagram
This implementation is restricted as follows:
• Maximum MTU of 1500 bytes
• ATM SVCs are not supported
• Frame Relay/ATM internetworking (per FRF.8) is not supported
• PPP coding transitions - switching the method (VC-multiplexed PPP to LLC-encapsulated
PPP and back) - are not supported
• PPP will not request the following LCP configuration options:
– Protocol Field Compression
– Field Check Sequence Alternatives
– Address-and-Control-Field-Compression
– Asynchronous-Control-Character-Map
PPP over Ethernet over ATM (Routed)
The XSR’s PPPoE option, as shown in Figure 13‐3, encapsulates PPP PDUs in the form of Ethernet
frames which are encoded as bridged PDUs, as defined by RFC-2684. The router incorporates an
integrated PPPoE client which supports the features described below:
• PPPoE encapsulation according to RFC-2516
• PPPoE session management procedures according to RFC-2516
• Bridged Ethernet PDU (802.3) encapsulation according to RFC-2684
• Statically configured and negotiated IP addresses
• Although the XSR does not support bridging protocols and operations, since most ADSL
access concentrators support this encapsulation type the XSR implements the IP PDU
encapsulation portion of RFC-2684 as required to enable data traffic flows. Essentially, the
XSR implementation is an integrated PPPoE client supported over a router, not over a bridge
as specified in RFCs 2516 and 2684. PPPoE is invoked by the
encapsulation pppoe
command.
PC
PC
PHY
802.3
IP
1483
PPP
IP
ATM
ADSL
Access
Concentrator
Router / Modem with
integrated PPPoA client
PPPoA
Client
DSL
ATM
SONET
ATM
DSLAM
Routed encapsulation
- LLC/SNAP
IP
Frwd
Local ethernet frames
Note: Your choosing the correct PPP encapsulation method depends on the Customer Premises
Equipment (CPE) support offered by your DSL provider.