Table of Contents
Advertisement
Figure 3
Layer 2 Tunneling Protocol
Layer 2 Tunneling Protocol (L2TP) is an emerging Internet Engineering Task Force (IETF) standard
that combines the best features of two existing tunneling protocols: Cisco's Layer 2 Forwarding
(L2F) and Microsoft's Point-to-Point Tunneling Protocol (PPTP). L2TP is an extension of the
Point-to-Point Protocol (PPP), which is an important component for Access Virtual Private
Networks (VPNs).
Traditional dial-up networking services only supported registered IP addresses, which limited the
types of applications that could be implemented over VPNs. L2TP supports multiple protocols and
unregistered and privately administered IP addresses over the Internet. This allows the existing
access infrastructure such as the Internet, modems, access servers, and ISDN terminal adapters
(TAs) to be used.
L2TP can be initiated wherever PPTP or L2F is currently deployed, and can be operated as a client
initiated tunnel such as PPTP, or a network access server (NAS) initiated tunnel such as L2F.
The current implementation of L2TP in Cisco IOS software is dependent on a PPP connection
supported on one of the directly attached interfaces. A dial-up PPP connection is required in order
to initiate an L2TP Tunnel connection. This is a requirement of the L2TP Access Concentrator
(LAC). Currently the Cisco uBR900 series cable access router cannot function as the LAC; it can
only function as the L2TP Network Server (LNS), which terminates a tunnel created elsewhere in
the network.
Easy IP
Dynamic Host Configuration Protocol (DHCP) Server
Cisco uBR900 series cable access routers support Intelligent DHCP Relay and DHCP Client
functionality. A DHCP Relay Agent is any host that forwards DHCP packets between clients and
servers. A DHCP Relay Agent enables the client and server to reside on separate subnets. If the Cisco
IOS DHCP server cannot satisfy a DHCP request from its own database, it can forward the DHCP
request to one or more secondary DHCP servers defined by the network administrator using standard
Cisco IOS IP helper-address functionality.
Network Address Translation and Port Address Translation (NAT/PAT)
Network Address Translation (NAT) reduces the need for globally unique IP addresses. NAT allows
an organization with addresses that are not globally unique to connect to the Internet by translating
those addresses into globally routable address space.
Cisco uBR900 Series Cable Access Router in a Routing Configuration with a Hub
CATV
coaxial cable
Cisco uBR7246
CMTS
HFC network
Ethernet
Cable
Modem
HUB
Configuring the Cisco uBR900 Series Cable Access Routers 9
Data Operations
Ethernet
LAN
Advertisement
Table of Contents
