Cisco 10000 - Manual (Page 259)

C H A P T E R
9-1
Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
9
Configuring Multihop
In a Virtual Private Dialup Network (VPDN) environment, sessions generated from a remote host are
routed over an existing tunnel or a tunnel built to route a specific domain. Typically, sessions cannot
traverse more than one L2TP tunnel before reaching the ISP or corporate network. However, by using
the Multihop feature, you can configure the Cisco 10000 series router to terminate sessions arriving in
L2TP tunnels from a LAC and then route the remote traffic through new L2TP tunnels to an LNS device
in the ISP or corporate network.
The Multihop feature enables the Cisco 10000 series router to terminate sessions arriving in L2TP
tunnels from a LAC and to forward the sessions through new L2TP tunnels to the router’s peer L2TP
Network Server (LNS). The packets arrive at the router with L2TP encapsulation and the router forwards
the packets with a different L2TP encapsulation. The Cisco 10000 router maps the sessions to the new
tunnels based on the session’s domain or the tunnel in which the session arrived.
The Cisco 10000 router also supports the preservation of the IP type of service (TOS) field for tunneled
IP packets. Each L2TP data packet and IP packet has a TOS field. When the router creates an L2TP data
packet, the TOS field sets to zero (normal service), ignoring the TOS field of the encapsulated IP packet
being tunneled. To preserve quality of service for tunneled packets, the Cisco 10000 router supports the
configuration of accept-dialin and request-dialout VPDN groups using the l2tp ip tos reflect command.
When the router creates an L2TP data packet at a virtual-access interface (VAI), instead of ignoring the
IP packet TOS field, the router copies the field onto the L2TP data packet.
Note Typically, the Cisco IOS software reflects the TOS field from the inner packet header to the outer packet
header. However, the Cisco 10000 router propagates the TOS field from the ingress header to the egress
header.
Figure 9-1 shows an example of a multihop topology. On the access network side, the Cisco 10000 router
connects to access provider LACs. On the provider network side, the router connects to LNS devices in
other ISP or corporate provider networks. Multiple L2TP tunnels are carried over either multiple
interfaces or a single interface. Typically, the connection between the router and the LAC or the router
and the LNS is an ATM connection. However, this is not a requirement. You can use any interface that
can carry L2TP tunneled traffic.