2-3Configuring a Classical IP ATM Network2.1.3 SPANS InterfaceWhile each of the qa interfaces for a given physical interface is designed tosupport Classical IP using Q.2931 signalling, a SPANS interface (usuallycalled fa, but user configurable) also exists for each physical interface. The fainterface supports FORE IP on top of SPANS signalling. FORE IP allows com-munication using AAL4 or AAL5 with no encapsulation, uses a broadcastARP for SPANS address resolution, and supports direct communication of allhosts on a physical ATM network without the use of IP routers. Since SPANSand Q.2931 signalling use different VCIs, a host can simultaneously supportFORE IP over SPANS as well as Classical IP over Q.2931 on the same physicalinterface.As a result of standard IP routing, all traffic sent out an fa interface will useFORE IP, while all traffic sent out a qa interface will use Classical IP. Each ofthe fa interfaces should be assigned an IP address on a subnet different thanthe subnets of any of the qa interfaces. It is permissible to place multiple fainterfaces on the same subnet, and the driver will load balance connectionsacross these interfaces.It is only necessary to configure the fa and qa interfaces if the specific serviceprovided by that interface is required. A host sending only Classical IP wouldnot need to configure the fa interfaces. Likewise, a host sending only FORE IPwould not need to configure the qa interfaces. Both the fa and qa interfacesmay be configured simultaneously, but they must be in separate subnets.Remember that Classical IP specific configuration changes can only be donewith the qa devices, while SPANS specific configuration changes can only bedone with the fa devices.
