
114056 Rev. A 3-1
Chapter 3
Understanding Classical IP over ATM Concepts
RFC 1577, Classical IP and ARP over ATM, describes an administrative entity
within an ATM network called a logical IP subnet (LIS). Each ATM LIS consists
of multiple network devices -- hosts and routers -- connected to the ATM network
and configured with interfaces to the same IP subnetwork.
Each LIS operates and communicates independently of every other LIS in an
ATM network. A host connected to an ATM network communicates directly with
other hosts in its own LIS. To communicate with hosts in another LIS, the host
must use an IP router. This router can connect to multiple LISs.
An ATM LIS must meet the following requirements:
• All members of the LIS (hosts and routers) must have the same IP network/
subnet number and address mask.
• All members must be directly connected to the ATM network, using SVCs.
• All members must access hosts outside the LIS through a router.
• All members must be able to communicate by means of ATM with every other
member of the LIS (that is, the virtual connection topology must be fully
meshed).
An ATM LIS can replace an IP LAN. In Figure 3-1, for example, three IP host
systems and an IP router have interfaces to an Ethernet LAN. To communicate
with each other on the LAN, the devices use the MAC addresses of the LAN that
they obtain using the ARP or by static configuration. For communications beyond
the LAN, the devices use IP addresses.
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