8-2 Cisco BPX 8600 Series Reference
ATM and Frame Relay SVCs and SPVCs
The ESP provides the BPX switch with the ATM or Frame Relay signaling function. It interprets
industry-standard signaling messages from ATM or Frame Relay CPE to provide the call setup and
tear down for switched virtual circuits across the ATM network. In addition to SVC signaling, the
ESP also performs PNNI routing, collects statistics, and processes alarms and billing records for
SVC connections through the BPX switch.
Figure 8-1 Wide Area Network with BPX Switch and ESP
PVCs and SVCs
Both permanent virtual circuits and switched virtual circuits are defined by ATM and Frame Relay
standards groups.
PVCs
After being added to a network, permanent virtual circuits (PVCs) remain relatively static. The PVC
only allocates a physical circuit and consumes bandwidth when there is data to send. However, the
permanent virtual circuit remains in place, always available for use, and is similar to a dedicated
private line in this respect.
SPVCs
Soft permanent virtual circuits (SPVCs) are PVCs which are routed using the Private
Network-Network Interface (PNNI) routing protocol. The “permanent” qualifier indicates that the
virtual connection is established administratively, through an operator’s command, rather than on
demand by signaling. A soft PVC is one where the establishment within the network is done by the
ESP signaling (in this case, PNNI signaling), just as it is done for Frame Relay and ATM switched
virtual circuits.
In the PNNI network, SPVC connections are established using the best available route. During a
network failure, SPVC connections could be rerouted and the newly selected path many not be the
optimal route. The ESP’s SPVC feature provides for auto-grooming of SPVCs. Auto-grooming is a
background management process that evaluates SPVC connections; if a better path for the
connection is found, the SPVC will be released and rerouted to the optimized path.
Cisco WAN switching network
with PNNI routing
ATM
CPE
SVC
PVC
WAN
Service
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