Cabletron Systems MMAC-Plus SFCS-1000, Configuration Manual

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2-2
Configuring a Classical IP ATM Network
2.1.1 Logical IP Subnets
An 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 this
sense, one LIS can be equated to one legacy LAN. It is possible to maintain
several overlaid LISs on the same physical ATM network. Therefore, in a
Classical IP ATM network, placing a host on a specific subnet is a logical
choice rather than a physical one. In this type of environment, communica-
tion between hosts in different LISs is only permitted by communicating
through 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 an
administrative issue. Limitations of IP addressing, IP packet filtering, and
administrative boundaries may guide a manager into establishing several
LISs onto a single ATM network. Keep in mind, though, that communication
between LISs must occur through IP routers.
2.1.2 Classical IP Interfaces
In order to support routing between multiple LISs, the host adapter software
allows a host to be configured as a member of (and a router between) up to
four distinct LISs per physical ATM interface. Each LIS membership is
through a separate Classical IP network interface. Existing system level IP
routing configuration tools are used to control routing through each of the
Classical IP interfaces in the same manner as routing among several physical
interfaces. Note that even though each Classical IP interface associated with a
given 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 next
letter. All of the Classical IP interfaces associated with physical unit one will
have
b
as the next letter, and so forth. Finally, each Classical IP interface has
its 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 is
qab2
.