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Answer Posted / raju parsha
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
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 parallel,
translating to extremly fast switching.
Layer 3 switching is a 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 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.
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