session. This local optimization, however, does not hinder the ability to continue supporting circuit multiplexing
through the session ID for other L2TPv3 tunnels on the same router.
For more information on:
• Configuration procedures, see Configuring L2TPv3 over IPv6 Tunnels.
• Configuration examples, see Configuring L2TPv3 over IPv6 Tunnels: Example.
L2TPv3 over IPv4
Layer 2 Tunneling Protocol version 3 (L2TPv3) over IPv4 provides a dynamic mechanism for tunneling Layer
2 (L2) circuits across a packet-oriented data network, with multiple attachment circuits multiplexed over a
single pair of IP address endpoints, using the L2TPv3 session ID as a circuit discriminator.
The following figure shows how the L2TPv3 feature is used to set up VPNs using Layer 2 tunneling over an
IP network. All traffic between two customer network sites is encapsulated in IP packets carrying L2TP data
messages and sent across an IP network. The backbone routers of the IP network treat the traffic as any other
IP traffic and needn’t know anything about the customer networks.
Figure 15: L2TPv3 Operation
In the above figure the PE routers R1 and R2 provide L2TPv3 services. The R1 and R2 routers communicate
with each other using a pseudowire over the IP backbone network through a path comprising the interfaces
int1 and int2, the IP network, and interfaces int3 and int4. The CE routers R3 and R4 communicate through
a pair of cross-connected Ethernet or 802.1q VLAN interfaces using an L2TPv3 session. The L2TPv3 session
tu1 is a pseudowire configured between interface int1 on R1 and interface int4 on R2. Any packet arriving
on interface int1 on R1 is encapsulated and sent through the pseudowire control-channel (tu1) to R2. R2
decapsulates the packet and sends it on interface int4 to R4. When R4 needs to send a packet to R3, the packet
follows the same path in reverse.
L2TPv3 over IPv4 feature is supported only on Cisco ASR 9000 High Density 100GE Ethernet line cards.
Note
For more information on:
• Configuration procedures, see Configuring L2TPv3 over IPv4 Tunnels, on page 166.
• Configuration examples, see Configuring L2TPv3 over IPv4 Tunnels: Example, on page 197.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
91
Implementing Point to Point Layer 2 Services
L2TPv3 over IPv4
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