Virtual LANs
In an Ethernet LAN, a set of accessories that accept a advertisement beatific by any one of the accessories in the
same set is alleged a advertisement domain. On switches that accept no abstraction of basic LANs (VLAN),
a about-face artlessly assiduously all broadcasts out all interfaces, except the interface on which it received
the frame. As a result, all the interfaces on an alone about-face are in the aforementioned advertisement domain.
Also, if the about-face connects to added switches and hubs, the interfaces on those switches and hubs
are additionally in the aforementioned advertisement domain.
A VLAN is artlessly an administratively authentic subset of about-face ports that are in the aforementioned broadcast
domain. Ports can be aggregate into altered VLANs on a distinct switch, and on assorted interconnected
switches as well. By creating assorted VLANs, the switches actualize assorted advertisement domains.
By accomplishing so, a advertisement beatific by a accessory in one VLAN is forwarded to the added accessories in that
same VLAN; however, the advertisement is not forwarded to accessories in the added VLANs.
With VLANs and IP, best practices behest a one-to-one accord amid VLANs and IP
subnets. Artlessly put, the accessories in a distinct VLAN are about additionally in the aforementioned distinct IP subnet.
Alternately, it is accessible to put assorted subnets in one VLAN, and use accessory IP addresses on
routers to avenue amid the VLANs and subnets. Also, although not about done, you can
design a arrangement to use one subnet on assorted VLANs, and use routers with proxy ARP enabled
to advanced cartage amid hosts in those VLANs. (Private VLANs ability be advised to consist
of one subnet over assorted VLANs as well, as covered after in this chapter.) Ultimately, the CCIE
written exams tend to focus added on the best use of technologies, so this book will accept that
one subnet sits on one VLAN, unless contrarily stated.
Layer 2 switches advanced frames amid accessories in the aforementioned VLAN, but they do not forward
frames amid two accessories in altered VLANs. To advanced abstracts amid two VLANs, a
multilayer about-face (MLS) or router is needed. Affiliate 6, “IP Forwarding (Routing),” covers the
details of MLS.