Difference between layer 2 and layer 3 cisco switch
Difference between layer 2 and layer 3 Cisco switch
Layer 2 switch operates by utilizing Mac addresses in it's caching table to quickly pass
information from port to port. A layer 3 switch on the other hand, utilizes IP addresses to
pass information from port to port. Generally, LAN switching is a form of packet switching
used in local area networks.
Basically a layer 2 switch operates utilizing Mac addresses in it's caching
table to quickly pass information from port to port. A layer 3 switch utilizes
IP addresses to do the same. While the previous explanation is the "What", for folks in
networking the following "How" is far more interesting. Essentially, A layer 2 switch is
essentially a multiport transparent bridge. A layer 2 switch will learn about MAC
addresses connected to each port and passes frames marked for those ports. It also
knows that if a frame is sent out a port but is looking for the MAC address of the port it is
connected to and drop that frame. Whereas a single CPU Bridge runs in serial, todays
hardware based switches run in hybrid, as one can imagine, of a router and a switch.
There are different types of layer 3 switching, route caching andtopology-based. In route
caching the switch required both a Route Processor (RP) and a Switch Engine (SE).
The RP must listen to the first packet to determine the destination. At that point the
Switch Engine makes a shortcut entry in the caching table for the rest of the Switch
Engine makes a shortcut entry in the caching table for the rest of the packets to follow.
Due to advancement in processing power and drastic reductions in the cost of memory,
today's higher end layer 3 switches implement a topology-based switching which builds a
lookup table and and poputlates it with the entire network's topology. The database is
held in hardware and is referenced there to maintain high throughput. It utilizes the
longest address match as the layer 3 destination. Now when and why would one use a l2
vs l3 vs a router? Simply put, a router will generally sit at the gateway between a private
and a public network. A router can perform NAT whereas an l3 switch cannot (imagine a
switch that had the topology entries for the ENTIRE Internet!!).
In a small very flat network (meaning only one private network range for the whole site) a
L2 switch to connect all the servers and clients to the internet is probably going to suffice.
Larger networks, or those with the need to contain broadcast traffic or those utilizing
VOIP, a multi network approach utilizing VLANs is appropriate, and when one is utilizing
VLANs, L3 switches are a natural fit. While a router on a stick scenario can work, it can
quickly overtax a router if there is any significant intervlan traffic since the router must
make complicated routing decisions for every packet that it recieves.
The Characteristics between the layer 2 and layer 3
Layer 2 switches themselves act as IP end nodes for Simple Network Management
Protocol (SNMP) management, Telnet, and Web based management. Such management
functionality involves the presence of an IP stack on the router along with User Datagram
Protocol (UDP), Transmission Control Protocol (TCP), Telnet, and SNMP functions. The
switches themselves have a MAC address so that they can be addressed as a Layer 2
end node while also providing transparent switch functions. Layer 2 switching does not,
in general, involve changing the MAC frame. However, there are situations when
switches change the MAC frame. The IEEE 802.1Q Committee is working on a VLAN
standard that involves ?tagging? a MAC frame with the VLAN it belongs to; this tagging
process involves changing the MAC frame. Bridging technology also involves the
Spanning-Tree Protocol. This is required in a multibridge network to avoid loops.
The same principles also apply towards Layer 2 switches, and most commercial Layer 2
switches support the Spanning-Tree Protocol. The previous discussion provides an
outline of Layer 2 switching func-tions. Layer 2 switching is MAC frame based, does not
involve altering the MAC frame, in general, and provides transparent switching in par-allel
with MAC frames. Since these switches operate at Layer 2, they are protocol
independent. However, Layer 2 switching does not scale well because of broadcasts.
Although VLANs alleviate this problem to some extent, there is definitely a need for
machines on different VLANs to communicate. One example is the situation where an
orga-nization has multiple intranet servers on separate subnets (and hence VLANs),
causing a lot of intersubnet traffic. In such cases, use of a router is unavoidable; Layer 3
switches enter at this point.
Layer 3 Switches
Layer 3 switching is a relatively new term, which has been ?extended? by a numerous
vendors to describe their products. For example, one school uses this term to describe
fast IP routing via hardware, while another school uses it to describe Multi Protocol Over
ATM (MPOA). For the purpose of this discussion, Layer 3 switches are superfast rout-ers
that do Layer 3 forwarding in hardware. In this article, we will mainly discuss Layer 3
switching in the context of fast IP routing, with a brief discussion of the other areas of
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