2-2Configuring a Classical IP ATM Network2.1.1 Logical IP SubnetsAn important concept in Classical IP networks is that of a Logical IP Subnet(LIS). An LIS is a group of hosts configured as members of the same IP subnet(that is, they have the same IP network and subnetwork numbers). In thissense, one LIS can be equated to one legacy LAN. It is possible to maintainseveral overlaid LISs on the same physical ATM network. Therefore, in aClassical IP ATM network, placing a host on a specific subnet is a logicalchoice rather than a physical one. In this type of environment, communica-tion between hosts in different LISs is only permitted by communicatingthrough an IP router which is a member of both LISs (as per RFC-1577).The number of LISs, and the division of hosts into each LIS, is purely anadministrative issue. Limitations of IP addressing, IP packet filtering, andadministrative boundaries may guide a manager into establishing severalLISs onto a single ATM network. Keep in mind, though, that communicationbetween LISs must occur through IP routers.2.1.2 Classical IP InterfacesIn order to support routing between multiple LISs, the host adapter softwareallows a host to be configured as a member of (and a router between) up tofour distinct LISs per physical ATM interface. Each LIS membership isthrough a separate Classical IP network interface. Existing system level IProuting configuration tools are used to control routing through each of theClassical IP interfaces in the same manner as routing among several physicalinterfaces. Note that even though each Classical IP interface associated with agiven physical interface uses the same physical hardware, they are each con-figured separately with their own MTU, IP address, and ATM address.The name of each of the Classical IP interfaces begins with qa. All of the Clas-sical IP interfaces associated with physical unit zero will have a as the nextletter. All of the Classical IP interfaces associated with physical unit one willhave b as the next letter, and so forth. Finally, each Classical IP interface hasits interface number as a suffix. As an example of the above naming conven-tion, the name of the third Classical IP interface (unit 2) on physical unit one isqab2.
