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Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 1 Broadband Aggregation and Leased-Line Overview
Broadband Architecture Models
In the figure, PPPoX sessions are placed in the proper virtual routing and forwarding (VRF) instance
based on the virtual template to which they map. This model is identical to the one in
Figure 1-2 on the
access side. However, the two models differ on the network side. The model in Figure 1-3 uses MPLS
and a tag interface on the network side and separates traffic at Layer 3. The “PTA to VRF Architectural
Model” in Figure 1-2 uses VRFs does not use a tag interface, and separates traffic at Layer 2.
L2TP Architectures
Figure 1-4 shows an L2TP network server (LNS) model.
Figure 1-4 LNS Architectural Model
In the figure, the clients and the LACs exchange PPP packets that are typically encapsulated in PPPoA
or PPPoE and typically carried on ATM circuits. However, the protocols used between the clients and
the LAC do not affect LNS requirements. The LAC creates L2TP tunnels to all of the LNSs at which its
clients want to terminate. Multiple tunnels might exist between each LAC and each LNS. For each client
PPP session the LAC signals the LNS to add another session to a tunnel. The LAC forwards all traffic to
the LNS, including the PPP control traffic. The LNS terminates the PPP sessions and routes any client
IP packets on to the ISP or corporate network toward their final destination. The LNS performs
authentication, authorization, and accounting (AAA) actions on the PPP sessions.
L2TP to Virtual Routing and Forwarding Architecture
Figure 1-5 shows an L2TP to VRF model.
Cisco
10000 ESR
PPP sessions
in L2TP tunnels
PPP sessions,
typically PPPoA
and PPPoEoA
Typically OC-12 ATM or
OC-12 POS physical
links containing
L2TP tunnel flows
100K routed
subscribers
Typically GigEthernet
or OC-12 POS IP
routed traffic
AAA servers
ISP/corporate
network
Client
Client
ATM
network
EMS/NMS
76100
LAC
LAC
LAC
1000s of tunnels,
multiple physical
links