LAN Switching: A Strategic Decision
[SEMI-FINAL DRAFT • 11/21/95]
This white paper helps network managers evaluate the long-term functional and economic
aspects of deploying LAN switches. The material takes a total network perspective,
rather than examine the LAN switch out of its context, for only with this broader
perspective can a fully-informed decision be made. This perspective also permits LAN
switching to become a long-term strategic network element, instead of an interim, throwaway step to increasing bandwidth.
The document is organized into four sections. Section one describes why routing remains
vital in switched LANs. The second section compares the three dominant architectures
being touted for switched LANs. Section three highlights strategic plans from six major
network vendors, indicating the fundamental architecture preferred by each. The fourth
and final section employs a hypothetical network configuration, created by Data
Communications magazine, to compare price/performance and overall costs of the
different architectures.
Routing’s Role in Switched LANs
Routing’s Role in Switched Networks [SIDEBAR]
• Eliminates the hard-to-manage “flat” network topology
• Provides a more scalable and dependable hierarchical arrangement
• Delivers efficient utilization of bandwidth
• Facilitates transparent connectivity among diverse network types and protocols
• Adds excellent security and firewall protection safeguards
• Enables static and dynamic Virtual LANs (VLANs)
• Offers the best course for migration to ATM
Contrary to ambitious claims that switching heralds the end of routing, routing remains an
essential function as networks migrate from shared to switched media. For a strategic
approach to network planning, routing and switching must be considered concurrently.
Indeed, routing provides the very framework for understanding the myriad switching
permutations and combinations.
Routing remains vitally important for a number of reasons. Routing facilitates
transparent connectivity among diverse network types and protocols. Routing adds
stability by making a network more predictable, dependable and manageable. Routing
adds security and firewall safeguards. Routing maximizes efficiency of limited resources,
especially for access to common servers and in relatively low bandwidth WAN links.
The advent of ATM has further highlighted routing’s continued importance. The
emerging Multiprotocol Over ATM standard (MPOA), key to protecting a company’s
investment in LAN adapters, wiring and other equipment, depends on routing. So the
question of routing remains not whether, but how.
LAN Switching Architectures
There are three dominant LAN switching architectures being widely touted today:
centralized, split and distributed. All three are dependent on routing; they differ solely on
the location of routing functions. With the centralized architecture, all routing functions
are placed in a single location. The split architecture is a cross between the other two
with centralized route determination and distributed packet forwarding. In the truly
distributed architecture multilayer switches, which perform both routing functions, are
deployed throughout the network in a peer mesh or hierarchical topology.
Centralized Routing, the historically dominant architecture, employs a single or multiple
routers in a common location, frequently a facility’s computer center. LAN segments
from hubs, bridges, and cut-through or layer 2 switches throughout the facility feed the
centralized router(s). A separate router port is required for each LAN segment and/or
each Virtual LAN (VLAN). As a result, the centralized architecture requires traditional
routers that support dozens, and even hundreds of ports. The resulting topology is a
collapsed backbone at the centralized router(s).
This architecture is popular for one simple reason: until recently, central routers have
been the only commercial option available. The principal advantage of centralized
routing is easy management. Among its disadvantages are the single point of failure,
poor scalability, suboptimal performance and the high cost of central mega-routers.
Both cut-through and layer 2 switches, because they provide no internal routing functions,
must be deployed using a centralized architecture. To facilitate scaling such a network,
companies must invest in a router with sufficient long-term capacity, even though much
of that capacity goes unused initially. As the network grows beyond that capacity, the
router must be replaced with a larger one or supplemented with additional routers.
Performance is similarly handicapped. Under typical traffic patterns, packets regularly
leave one switch and traverse the central router on their way to other switches. The
external router and routes become bottlenecks that dramatically decrease overall network
throughput, thereby diminishing any ostensible performance advantage offered by these
switched network architectures.
Poor scalability and suboptimal performance undermine the very reasons for switching,
which is why many organizations are considering newer alternatives to the legacy
centralized architecture.
[DIAGRAM OF CENTRALIZED NETWORK]
Split Routing is one response to the scalability and performance issues of a centralized
architecture. Split routing separates the router’s path determination and table creation
function from the packet/frame forwarding function, and places these in separate devices.
The concept exists only in theory today, but is being touted as a beneficial architecture by
several traditional router vendors. With split routing a centralized “route server”
determines routes for the entire network. These routes are then conveyed to distributed
“data forwarders” that perform the actual packet forwarding.
Split routing has its roots in centralized routing. As LAN switching becomes pervasive,
centralized routers become bottlenecks. Rather than obsoleting their big routers, router
vendors are developing software that converts a central routing giant into a central route
server. Users must also buy special-purpose switches to perform the packet forwarding
function, deploying these in a distributed fashion.
While no commercial route server is yet available, the concept is useful for portraying
investment protection for centralized routers. Of course, current investments in layer 2
switches are similarly jeopardized by the debut of proprietary data forwarders.
The disadvantages of split routing are virtually identical to those found with a centralized
architecture: poor scalability, single point of failure, suboptimal performance, and high
costs associated with equipment conversion and displacement. The poor scalability
forces companies to over-invest in a route server with adequate long-term capacity. As
the network grows beyond that capacity, the route server must be replaced or
supplemented adding substantially to the cost and management complexity. An
additional drawback is that, until a standard is accepted in the industry, route servers will
remain proprietary.
Network performance does improve going from centralized to split routing because no
actual traffic traverses the route server. The route server remains a bottleneck, however,
because all of the data forwarders regularly query the route server (and wait for its
response) every time a packet arrives with a destination address that is not listed in the
local cache of addresses and routes. This is the case both for new, unknown addresses
and “old” addresses that have timed-out or expired in the cache memory.
[DIAGRAM OF SPLIT NETWORK]
While split routing may be an acceptable compromise for companies dependent on a
particular product or vendor, its poor price/performance is pushing others toward a fully
distributed architecture.
Distributed Routing utilizes “multilayer” switches deployed throughout a network.
Multilayer switches are capable of independent switching at layer 2 and at layer 3. For
this reason, the multilayer switch operates as a full switch and router in one unit. Each
multilayer switch handles both route determination and packet forwarding. Multilayer
switches communicate with one another, using standard routing protocols, to create and
maintain the collective network routing configuration. The resulting topology is either a
hierarchy or peer mesh of switches. In the hierarchical arrangement, a large “master”
multilayer switch functions as a collapsed backbone serving smaller subordinate switches.
The peer mesh arrangement has no such master; all multilayer switches communicate
freely with one another as traffic patterns require.
Distributed routing has a number of advantages. The network is more dependable with
alternate routes that eliminate single points of failure. The topology is substantially more
flexible and scalable, and offers both multivendor interoperability and incremental
migration. It supports an unlimited number of static or dynamic VLANs. And
distributed routing facilitates migration to ATM by locating packet processing close to
the users. Two perceived disadvantages, dependent on specific implementations rather
than the architecture itself, are slightly more complicated management and higher costs.
The section on “Total Cost of Ownership” will show that these are indeed
misperceptions.
Flexibility and scalability are compelling aspects of a distributed architecture. Multilayer
switches can be deployed gradually, as needed to grow the network or its bandwidth,
alongside or as replacements for shared media hubs or non-routing switches. Because
each multilayer switch maintains its own routing table, the network is self-configuring.
Each multilayer switch can also be redeployed in another location or serving another role
just as easily. Such flexibility makes migrating from a purely centralized architecture
incremental, manageable and affordable.
The management complexity of a distributed architecture results from the need to keep all
multilayer switch software at a compatible revision level. Advances in network
management platforms and applications are centralizing and simplifying this oncedifficult task. Similar hardware and software technology advancements are also making
distributed routing attractive as a price/performance leader. Deployment flexibility,
combined with elimination of all network-wide bottlenecks, preserves the investment in
multilayer switches while delivering optimal real-world performance.
[DIAGRAM OF DISTRIBUTED NETWORK]
With both distributed and split routing, there remain two important functions delegated to
centralized routing: conversion among protocols not supported by the multilayer
switches and an interface to the wide area network. Routers performing these two
functions would be located normally in the common equipment room, with each
connected to the corporate backbone. Multilayer switches route and bridge packets as
needed to these routers just as they do to other multilayer switches in the network–all
using standard protocols for maximum interoperability.
The table below offers a summary comparison of the three routing architectures:
Routing Architecture
Centralized
Advantages
Easiest to Manage
Split
Unlimited Number of VLANs
Facilitates Migration to ATM
Distributed
Most Scalable
Best Flexibility
Standards-based
Dependable Mesh Topology
Unlimited Number of VLANs
Facilitates Migration to ATM
Available & Field-proven
Disadvantages
Poor Scalability
Single Point of Failure
Performance Bottleneck
Requires Port for Each LAN
Segment & Virtual LAN
Most Expensive
Poor Scalability
Single Point of Failure
Performance Bottleneck
Route Servers Not Yet
Commercially Available
Proprietary
More Complex Management
LAN Switch Designs [SIDEBAR]
Although all LAN switches increase bandwidth by increasing network segmentation, there
are three different designs available: cut-through, layer 2 and multilayer.
Cut-through switches are simple devices that forward packets based on destination
addresses without any additional processing. They do not check for bad packets, must
buffer incoming data streams when outbound ports are congested, and cannot be used to
create hierarchical LANs. In effect, their simple design makes these devices fast and
inexpensive, but also inflexible. Cut-through switches also work exclusively with a single
MAC type and speed, so a 10 Mbps Ethernet cut-through switch can only forward packets
to another 10 Mbps Ethernet LAN, and not to FDDI or 100 Mbps Ethernet backbone
LANs.
Layer 2 and multilayer switches are more sophisticated devices that employ a store-andforward design. Store-and-forward switches check for bad packets, perform sophisticated
filtering and forwarding, and can translate a packet to a different LAN type on a higher
speed backbone LAN, then switch the packet either at the MAC layer (layer 2 and
multilayer) or the network layer (multilayer only).
Layer 2 Switches switch packets at layer 2, the media access control or MAC layer. A
layer 2 switch has many similarities with a multiport bridge. Both cut-through and layer 2
switches can be used only with a centralized routing architecture, which is why most
vendors and users alike are turning to multilayer switches.
Multilayer Switches, also known as intelligent switches, can switch packets either at the
MAC layer (layer 2) or the Network layer (layer 3). Because it can switch at layer 3, a
multilayer switch provides the full routing functionality needed in a distributed architecture.
Normally traffic within a virtual LAN segment is bridged, while traffic to other VLANs is
routed. The multilayer switch is the most flexible because it is the only design that can be
deployed in all three architectures: centralized, split and distributed. This is particularly
important for migration from a centralized to a distributed architecture in manageable and
affordable steps. Because the multilayer switch is a permanent building block for ATM, it
is also a strategic choice for switching.
A more detailed comparison of switch designs can be found in another white paper titled
LAN Switch Designs: A Tactical or Strategic Choice available from Alantec.
Leading Vendor “Marketectures”
With so many disadvantages of centralized routing, it is not surprising that five of the six
major vendor “marketectures” (IBM’s SVN, DEC’s EnVISN, Bay’s BaySIS, Cisco’s
Fusion and Cabletron’s Synthesis) feature either split or distributed architectures at the
heart of their strategic directions. The remaining major player, 3Com with HPSN, is
sticking with the traditional centralized architecture–at least for the time being.
IBM, the penultimate centralized vendor, has embraced split routing with its SVN
(Switched Virtual Networking) architecture for migrating to ATM. SVN features a
centralized route server with packet forwarding provided by special-purpose switches.
While IBM’s SVN is currently short on product details, the eventual rollout is certain to
have a profound impact on existing centralized networks.
DEC has embraced distributed routing with its EnVISN (Enterprise Virtual Intelligent
Switched Networks) strategy for migrating to ATM. EnVISN is purely distributed, with
no need for a route server. DEC expects to ship its first intelligent switching products
late in 1995.
Bay Networks is touting a purely distributed architecture, as well, under its BaySIS
strategy. This is a significant step for a company that has its roots in Wellfleet, a leading
vendor of large, centralized routers. Bay is currently integrating route processing into its
layer 2 switches for availability early in 1996.
Cisco, the leading router vendor worldwide, is moving to a split architecture under the
Fusion vision. Cisco plans to migrate the role for its large, centralized routers to that of a
route server, and is integrating route processing into some of its Catalyst switches.
Cabletron also plans to support a split architecture under its Synthesis and Securefast
Virtual Networking strategies. Cabletron is working on a route server, and already has at
least some multilayer switching capabilities in its MMAC-Plus product.
3Com is the only major networking vendor attempting to hold onto the status quo of the
centralized architecture with its HPSN (High Performance Scalable Networking) strategy.
3Com may be hedging its bets, however, because the company’s LANplex switches are
multilayer devices that currently support routing in distributed topologies. Or perhaps
3Com is struggling with a heavy dependence on ASIC technology, which is complicating
a migration to distributed or split routing. Whatever the situation, if 3Com holds to its
centralized architecture, it will stand alone among the major network players.
Total Cost of Ownership
Data Communications magazine created a hypothetical application for the purpose of
comparing the three fundamental architectures (“Next Generation Routing: Making
Sense of the Marketectures” in the September 1995 edition). The mock request for
proposal (RFP) included three different configurations of 50, 250 and 500 switched
Ethernet ports, all connected to an ATM backbone. The RFP requested a complete
configuration of switches (LAN and ATM) and routers. Per-port prices were calculated
by dividing the total cost of all equipment (LAN switches, routers, ATM switches and
route servers, if applicable) by the number of switched Ethernet ports.
The range of per-port pricing is given below for the three different configurations and the
three different architectures. Pricing for the centralized architecture is the average of bids
submitted by three leading networking vendors. Only a single vendor submitted a
proposal for the split architecture. Pricing for the distributed architecture is Alantec’s
configuration. All pricing is US list.
Routing Architecture
Centralized
Split
Distributed
50 Node Network
$1,270
$1,920
$801
250 Node Network
$1,120
$1,520
$730
500 Node Network
$910
$1,435
$657
Alantec’s distributed solution consists of PowerHub 6000 Intelligent Switches
(multilayer), each with an ATM backbone interface and dual power supplies, and Fore’s
ASX-200BX ATM switch. For non-ATM applications, an FDDI module would be
substituted for the ATM backbone interface. All configurations also include Alantec’s
PowerSight network management software. The actual number of ports in each
configuration is 60, 252 and 504 for true per-port pricing of $667, $724 and $652
respectively. Because a single PowerHub 6000 handles up to 60 Ethernet ports, the ATM
backbone switch was not required for the 50 node network, making its per-port pricing
appear disproportionately low.
[DIAGRAM OF ALANTEC’S CONFIGURATION]
The fact that Alantec’s distributed routing solution represents an average savings of 33%
over centralized and 55% over split architectures belies the common misperception that
the distributed architecture is the most expensive of the three. Considering the greater
performance of a distributed architecture, the price/performance advantage is even
greater. The cause of this common misperception is perspective. In a simple box-for-box
comparison, it is not unusual for a multilayer switch to cost more than a layer 2 or cutthrough switch with the same number of ports. But this is an apples and oranges
comparison. More important is total cost of ownership, which can only be evaluated
using a full network perspective. Because the multilayer switch supports full routing
capabilities, it eliminates the need for expensive centralized routers or route servers. As a
result, the real cost of the total network is normally much lower.
While less tangible than total cost, network management considerations often fall victim
to the same kind of apples and oranges comparison. A multilayer switch, being a more
sophisticated device, should be expected to require somewhat more complex management
than a simple layer 2 or cut-through switch. But taking a full network perspective reveals
that switches are not the only pieces in the puzzle. A distributed architecture with
multilayer switches involves both fewer types of equipment and fewer nodes. The variety
of equipment required by centralized and split architectures, each with its own specialized
management application, makes maintaining support staff expertise a real challenge. In
most situations, managing less is easier than less management.
Summary
Distributed networks with multilayer switches not only offer substantial architectural
advantages over the alternatives, they offer the best price/performance ratio and the
lowest overall cost of ownership. The multilayer switch is also a strategic choice with
both short-term flexibility for migrating from a centralized architecture, and long-term
durability as a permanent building block for future migration to ATM. Multilayer
switches are standards-based for maximum multivendor interoperability, and offer the
optimal design for implementing Virtual LANs. When deployed in a fully distributed
architecture, the full network scales easily and incrementally, and operates reliably with
its mesh topology.
Multilayer switches are, and will remain, the state-of-the-art in LAN switching. And
Alantec, as the leader in multilayer switching, has more experience than other vendors
entering the market segment, along with the most comprehensive product line available.
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