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are commonly used in large-scale classical IP-over-ATM (CIP) scenarios. SVCs,
despite their signaling configuration requirements, are easier to manage in
large-scale deployments. Figure 9.7 illustrates this complexity.
Figure 9.7. ATM PVC topology versus ATM SVC
topology.
CIP with PVCs requires individual virtual circuits to be established between every
router involved. SVCs are only established when needed, as opposed to the PVC
scenario, where the virtual circuit is always active until removed from the interface
processor. In an SVC topology (and PVC), the routers involved relate to one another
as if all were connected to the same local LAN segment. The routers establish SVC
connections using the Q.2931 signaling protocol between themselves and their local
ATM switch. It is the local ATM switch, however, that is actually responsible for all
SVC call setups and teardowns. To facilitate this, SVC calls are established over
signaling PVC that is established between ATM end-point device and its adjacent
ATM switch. SVC's between ATM connected end-points are established using
network service access point (NASP) addresses. Since, SVC calls need to be
established when the end-point has layer 3 traffic to transmit to another end-point,
some type of NASP to Layer 3 address mapping needs to be provided, this mapping
is accomplished using map-lists. Each SVC router uses a map-list containing Layer
3 to NASP addressing mappings for its brother routers. These addresses are sent to
the local ATM switch, which in turn establishes the SVC (with a remote ATM switch)
and manages the call level routing, although the router forwards packets through
the established SVC.
In order to use SVC, follow these steps: