Product specifications

Efficient Networks
®
Router family
Command Line Interface Guide
Chapter 6: Connection Management
Efficient Networks
®
Page 6-47
Transport (Layer-2) VPNs
Virtual Circuits
A Layer-2 VPN is typically an ATM or Frame Relay (FR) link over a high-speed DSL,
T1, or a T3 line. With both of these protocols (ATM and Frame Relay), a dedicated
line can actually be connected to multiple sites simultaneously by means of PVCs
(Permanent Virtual Circuits). A PVC allows multiple dedicated (point-to-point)
connections to exist over a single dedicated (physical) line.
ATM or Frame Relay transport
When using ATM or Frame Relay (FR) as the transport for network traffic,
connections between two (or more) locations are managed with ATM switches. These
switches make each PVC appear like a single point-to-point connection from one ATM
router (or bridge) to another. Here again, the ATM network is a public network, and it
is used to transport private network traffic.
Advantages
Higher bandwidth: ATM and FR are used as Layer-2 transport on higher speed
physical connections. Generally, the lowest speed for FR connections are 56 Kbps
dedicated. Speeds go up from there to well over 45 Mbps.
Permanent connections: Each PVC is permanently mapped through the ATM network
from one LAN to another. This gives the perception that all of the local networks are
connected together and are private.
Quality of Service: The quality of the network service can be guaranteed because of
the nature of the ATM protocol.
Disadvantages
Expensive long haul: Connecting a PVC from one location to another will often incur
“mileage” charges if the PVC endpoints are not within a few miles of each other.
Permanent connections: The PVC connections are indeed permanent and must be
provisioned through the ATM network. Users cannot simply choose to call up another
location when they want. This can be a problem if a company needs to add
connectivity to multiple sites on a sporadic basis.
Expensive to install: Using permanent connections can be expensive to install, and
may not be available in all locations. It requires a commitment to long term quality
access from LAN-to- LAN through a WAN connection.