even normal ports to a host can be configured to allow multiple VLANs, which is
especially necessary with virtualization, as different virtual machines on a single host
may be part of different VLANs. Realize that even in this basic configuration with only
two racks, the VLAN configuration can require changes on the network infrastructure
at multiple points, such as the ToRs and aggregation switches.
Imagine now that a new virtual machine is required for VLAN 20. There is no capacity
in the first rack, so the virtual machine must be created in the second rack, as shown
in Figure 3.8. This requires changes to the second rack ToR and both aggregation
switches. Now imagine that you have hundreds of racks and hundreds of VLANs. This
type of VLAN change can be very complex and take weeks to implement, because all of
the VLAN configuration is static and requires manual updating, which makes the
network a bottleneck in provisioning new services. You’ve probably heard of some
VLAN configuration problems, although you might not have known they were VLAN
configuration problems. Some of the major “outages” of Internet-facing services have
been caused not by hardware failure but by changes to network configuration that
“went wrong” and take time to fix, specifically VLANs! Suppose that you wish to use
Live Migration to easily move virtual machines between hosts and even racks; this
adds even more complexity to the VLAN configurations to ensure that the virtual
machines don’t lose connectivity when migrated.
Figure 3.8 New VM in VLAN 20 added to the host in the second rack, and the
changes to the switch VLAN configuration required
TAGGED VS. UNTAGGED CONFIGURATION
One thing regarding VLANs confused me when I first started with network
equipment (well, lots of things confused me!), and that was whether to configure
ports as tagged or untagged. Both are options when configuring a port on a switch.
When a port is configured as tagged, that port expects the traffic to already be
tagged with a VLAN ID. This means that the VLAN must be configured at the host