IBM SC30-3681-08 Frame Relay Switched Virtual Circuits, Frame Relay Interface Initialization

	Using Frame Relay
v Preserves the ordering of frame transfer on individual VCs
v Does not acknowledge or retransmit frames
Frame Relay Switched Virtual Circuits
Frame Relay Switched Virtual Circuits (SVCs) provide the ability to implement
²cut-through² routing in a Frame Relay network, minimizing or eliminating
intermediate router hops between DTEs. Network complexity can be simpli®ed and
the DTE may experience improved performance.
SVCs may replace PVCs to conserve network bandwidth, reducing bandwidth cost.
FR SVC standards are a subset of ISDN standards and provide many of the same
advantages as ISDN with less complexity.
The following protocols are supported over FR SVCs:
v	AppleTalk 2
v	ARP
v	Bridging
v	DECnet IV
v	DLSw
v	IP/OSPF/RIP/BGP4
v	IPX
SVCs cannot be required and cannot belong to a required group.
Frame Relay Interface Initialization
Local Management Interface (LMI) is used to determine the status of PVCs on a
Frame Relay interface. If an LMI is enabled, the FR interface is active when a
successful exchange of LMI frames occurs between the router and the FR switch;
however, no data can be received from or transmitted to another router until an LMI
status message indicates that the PVC status for the DLCI to the other router is
active. Also, there are instances where the FR interface state is tied to PVC states
and the interface does not come up even if LMI or Q.922 exchanges are
successfully occurring (for additional information, see ªCon®guring PVC States to
Affect the Frame Relay Interface Stateº on page 391).
If LMI is not enabled and SVCs are enabled, the Frame Relay interface is active
when a successful exchange of Q.922 frames occurs between the router and the
adjacent device. All PVCs are considered active at this point. However, SVCs are
active only after a successful Q.933 activation exchange.

PVC status appears for all PVCs as either active or inactive. An active PVC has a completed connection to an end system. An inactive PVC does not have a completed connection to an end system because either an end system or an FR switch is off-line.

For example, in Figure 22 on page 390 router B has a con®gured PVC to router D. Router B is successfully interacting with FR management through FR switch B. Because either another FR switch is down or the end system is down, the

Chapter 30. Using Frame Relay Interfaces 389