The New CNR Research Area in Pisa chooses an ATM Local Area Network
by Marco Sommani
Starting in late spring of 1999, the 15 CNR institutes in Pisa
will move to a new campus, known as the ‘Area della Ricerca di
San Cataldo’. The backbone of the local area network of the new
campus has been designed using ATM LAN Emulation (LANE) and Multiprotocol
over ATM (MPOA). The benefits of an ATM based solution appeared
clear as recently as two years ago. Now, however, with the appearance
on the market of powerful layer-3 switches with gigabit Ethernet
interfaces, the advantages of ATM based LANs have become much
less evident, to the point that their deployment is discouraged
by many experts. This article claims that, even if layer-3 switching
techniques over gigabit Ethernet can minimise the price/throughput
ratio, an ATM-based LAN remains the best solution if multiple
broadcast domains are scattered over the same physical infrastructure.
Design Goals and Constraints
Currently the 15 CNR institutes in Pisa are scattered throughout
the city. Typically, each institute has its own internal LAN and
communicates with the rest of the world via a slow connection
(64 Kb/s or basic rate ISDN in most cases, with a few 2 Mb/s exceptions).
The router with the external Internet connection is located in
a block hosting four institutes: CNUCE, IAT, IEI and IMC. The
internal WAN topology is roughly a star centred at the site with
the Internet connection.
Inter-institute traffic is quite low. Most communications are
either internal within an institute or directed to remote Internet
destinations. Only IP is used for inter-institute traffic. Intra-institute
traffic is also based on other protocols, most commonly AppleTalk,
IPX and NETBEUI.
The LAN of the new campus must be able to meet two conflicting
requirements: it must keep a clear separation between the broadcast
domains of different institutes and enable high-bandwidth and
low-delay inter-institute communications. This is because (i)
it has been considered undesirable to force network users to change
their habits suddenly, and (ii) inter-institute co-operation is
expected to become increasingly important and some institutes
may be requested to host servers providing services to the whole
campus.
The Solution
A graphical description of the new LAN is available at http://soi.cnr.it/~mario/
piante/Default.html.
The figure gives a simplified schema representing the fibre connections
between wiring closets. The four big rectangles represent the
main buildings in the campus (A, B1, B2 and C). Smaller rectangles
represent the 27 wiring closets concentrating all copper cable
coming from the rooms.
The ATM switches (IBM 8260s) are located in the wiring closets
of the central hexagon (1.1 - 1.0 - 3.0 - 3.1 - 20.1 - 13.0).
LAN emulation, MPOA, routing and bridging services are provided
by equipment (IBM MSS) co-located with two ATM switches. All other
wiring closets are just one fibre hop away from the nearest ATM
switch. Every wiring closet contains Ethernet switches (IBM 8271s)
with fibre ATM uplinks to the nearest ATM switch. Fibre connections
between peripheral wiring closets shown in the figure are currently
not used.
Typically one institute spans from two to six wiring closets and
shares some closets with other institutes. Ethernet ports on the
Ethernet switches are statically assigned to broadcast domains;
every broadcast domain is associated with an emulated LAN (ELAN)
in the ATM cloud. The Ethernet switches now in place are not MPOA-capable.
As a result, all traffic between different broadcast domains must
transit through one of the two IBM MSS acting as a router, resulting
in an obvious bottleneck for all inter-institute traffic. This
will not be a real problem in the beginning, when inter-institute
traffic is expected to remain low; however, as soon as inter-institute
traffic starts to increase, it will become necessary to add MPOA-capable
edge devices such as the IBM 8371 to the infrastructure.
Comparison with a Gigabit Ethernet Solution
The alternative to the ATM solution is to place layer-2 and/or
layer-3 switches in every closet and to interconnect them via
point-to-point Ethernet links at appropriate speeds (100 Mb/s
or 1000 Mb/s).
The non-ATM solution is appropriate when there is a close correspondence
between broadcast domains and the physical topology of the network.
In all other cases, the answer in a non-ATM world is to use Virtual
LANs (VLANs), ie to use bridging to exchange traffic between hosts
of the same broadcast domain attached to different switches and
to “tag” frames sent over inter-switch links with their VLAN identifier.
With complex VLAN topologies, management may be a real nightmare.
In fact, if all inter-switch links are enabled for transporting
traffic for all the VLANs, then all broadcasts and Multicasts
of the VLANs are propagated to all switches; otherwise, if inter-switch
links are configured just for those VLANs that they should interconnect,
network management becomes too complex.
With complex VLAN topologies, it is also very difficult to configure
inter-VLAN routing in a way that minimises the number of hops
over inter-switch links. This problem is only partially reduced
by special features that are common on many layer-3 switches,
such as IP auto-learn.
Conclusions
When the LAN structure is simple, gigabit Ethernet and layer-3
switches can offer the best price/performance ratio. In all other
cases, ATM LAN emulation, with its two-level addressing structure,
provides a much better solution, because it optimises traffic
patterns and simplifies network management.
In the last two years, there has been a tendency in the academic
community to under-estimate the advantages that can be obtained
with ATM. This probably depends on the fact that typically broadcast
domains inside universities can easily be mapped over the physical
infrastructure.
Strangely enough, ATM is also quite uncommon on commercial sites.
A possible explanation is the fact that many network consultants
prefer to promote what they already know (Ethernet) rather than
spending time to learn something new (LAN emulation and MPOA).
If this trend continues, prices of Ethernet-based solutions will
continue to fall and ATM prices will remain stable. Even worse,
switch manufacturers might decide to stop investments in ATM products.
In order to stop this trend, every effort should be made to clarify
ATM advantages to as wide an audience as possible.
Please contact:
Marco Sommani - IAT-CNR
Tel: +39 050 593 313
E-mail: Marco.Sommani@iat.cnr.it
