Internet-facing networks, hosting providers to separate clients, and more.
Administrative Help With VLANs, it’s possible to move servers between
locations but maintain their VLAN membership, avoiding reconfiguration of the
host.
Separation of Physical Networks from Virtual Networks This enables
virtual LANs to span physical network segments.Typically, a VLAN and IP subnet has a one-to-one mapping, although it is possible to
have multiple subnets within a single VLAN. Remember, though, that a VLAN
represents a broadcast boundary; a single subnet cannot cross VLANs because, by
definition, an IP subnet represents a group of machines with direct communication
that rely on broadcasts for translating IP addresses to MAC addresses using ARP.
While VLANs seem like a useful technology, and they are, their configuration has
some drawbacks and complexity. First, consider a typical datacenter network switch
configuration with numerous racks of servers. Typically, two types of switches are
involved: servers within a rack connect to the top-of-rack (ToR) switch in each rack
and then connect to aggregation switches. The configuration in Figure 3.7 shows three
VLANs in use by Hyper-V servers for different virtual machines, which in this example
are VLANs 10, 20, and 30. Notice that machines in VLAN 10 span racks, which
requires configuration of the VLAN in not just the ToR but also aggregation switches.
For VLANs 20 and 30, all of the VMs are in the same rack, so while the ports from the
hosts in the rack to the ToR require access for VLANs 10, 20, and 30, the aggregation
switches will see only VLAN 10 traffic passed to them. As a result, only VLAN 10 has to
be configured.
Figure 3.7 Three VLANs in a two-rack configuration. For redundancy, each ToR has a
connection to two separate aggregation switches.
Notice in Figure 3.7 that single ports can be configured to allow traffic from different
VLANs. (Ports between switches are known as trunk ports, because they are
configured for all of the VLAN traffic that has to be passed between them.) However,