I hope the following lines can help to clarify the point:
Layer-2 switching is hardware based,wich means it uses the MAC address from the
host´s NIC cards to filter the network. Switches use Application-Specific
intergrated Circuits( ASIC´s) to build and maintain filter tables instead of
that routers uses ip adresses wich is layer 3 information to buid and maintain
routing tables
Layer-2 switches are fast because they don´t look at the network layer header
information( like routers do), looking instead at the frame´s harware address
before deciding to either forward the frame or drop it.
What makes layer-2 switching so efficient is that there is no modification to
the data packet, only to the frame encapsulating the packet.since no
modification of the data packet is performed( routers make decitions after check
ip network address layer-3 information), the switching process is faster and
less-error prone than routing.
Bridged networks break up collision domains, but the network is still one large
broadcast domain. Layer-2 switches (bridges) cannot break up broadcast domains,
wich can cause less performance and limit the size of the network. Broadcast and
multicasts, along with the slow convergence of spanning tree, can cause major
problems as the network grows. Because of these problems, layer-2 switches
cannot completely replace routers( layer-3 devices) in the internetwork.
Atte: Claudio Bernal
IP Conectivity Specyalist
Alestra - ATT
Monterrey,Nuevo Leon, Mexico.
