Layer 3 Switching
Difference Between Layer 3 Switching and Routing
Layer 3 switches tend to have packet switching throughputs
in the millions of packets per second (pps), while traditional general-purpose
routers have evolved from the 100,000 pps range to over a million pps. Aggregate
performance is one of the key differences between Layer 3 switches and traditional
routers. Traditional routers still offer key features used typically in WAN environments.
However, many of those features, such as multicast routing, multiprotocol routing,
IBM feature sets, routing protocol stability, are still key for Layer 3 switches/campus
A Layer 3 or a Layer 2 Switch?— Scalability Advantages
Let’s look more closely at when a customer
might choose a Layer 3 switch over a traditional Layer 2 switch.
Layer 3 switches offer considerable advantages depending on
the customer’s requirements.
Scalability— For customers
with large networks that need increased performance to handle
the changing traffic patterns of today’s new applications,
Layer 3 switches offer increased scalability. Clearly a network
of hubs does not scale. While bridges helped, they were not
sufficient to handle networks of many thousands of users and
devices. Routers were the solution as they kept broadcasts
local to a segment. Layer 3 switches avoid the problems associated
with flat bridged or switched designs using traditional routing
mechanisms allowing customers to scale their network infrastructure.
Layer 3 switches also utilize routing protocols thus avoiding
the slow convergence problem of Spanning Tree Protocol and
lack of load-balancing across multiple paths.
Advanced services— Layer 3
switches also offer the benefit of broader intelligent network
services. These services permit applications to run on the
network as well as enable the creation of a cost-effective,
operational environment to support day-to-day operations and
management of the enterprise intranet.
Other advantages include:
Security—Layer 3 switches
provide enhanced security functions to protect corporate information
while allowing appropriate access. Access control lists are
supported by Layer 3 switches with no performance degradation.
Layer 3 switching is able to enforce the multiple levels of
security traditionally only found on routers on every packet
of the flow at wire speed.
Management—Networks that use
a multilayer model are by nature hierarchical. This type of
infrastructure is easier to manage as problems are more easily
Layer 3 switches offer significant redundancy and resiliency
options not available with Layer 2 switches. Default gateway
redundancy is provided by HSRP that enables Cisco switches
to transparently switch over to the hot standby backup router
instantly when the primary router goes off line, eliminating
a single point of failure in the network. UplinkFast provides
alternative paths when a primary link fails. Load balancing
is achieved by intelligent Layer 3 routing protocols.
While there are obvious advantages to a Layer 3 switch over
a Layer 2 switch, other factors needed to be considered as
well. Layer 3 switches are more expensive than Layer 2 switches
and are more complex. Depending on the size of a customer’s
network, the cost and complexity may not justify a Layer 3
switch. However, for customers with larger networks in need
of enhanced scalability, Layer 3 switches will actually simplify
Not All Layer 3 Switches Are Created Equal
At its most basic, Layer 3 packet switching or forwarding is
common across all vendors platforms, with perhaps exceptions in their multicast
or DHCP services behavior.
The more scalable, flexible, and adaptable Layer 3 switches
also offer a variety of routing protocols and services for topology discovery,
load balancing, and resiliency. Buying a Layer 3 switch without the richness and
depth of routing protocols is somewhat akin to a driverless car. The car can certainly
travel very fast in the direction that it is pointed, but the intelligence lies
in the driver, who needs to make all the decisions about where it should go and
when to stop and turn. The more flexible and resilient these capabilities, the
better reliability and adaptability the switch offers.
Finally, there are services. All the queuing, filtering, classification,
multiprotocol, route summarization and redistribution functions, plus additional
debugging, statistics gathering and event logging services is what lets network
managers deploy solutions that rise to the future challenges of mobility, multiservice,
multimedia, and service level agreements for business critical applications.
- Summary -
- Layer 3 switching is ASIC-based routing
- Traditional routers are better for WAN aggregation
- Layer 3 switches are more appropriate for scaling
Layer 3 performance
- Layer 2 switches are more appropriate when the additional
cost and complexity are not warranted