© Copyright IBM Corp. 2011 93
Chapter 8. VLANs
This chapter describes network design and topology considerations for using Virtual
Local Area Networks (VLANs). VLANs commonly are used to split up groups of
network users into manageable broadcast domains, to create logical segmentation
of workgroups, and to enforce security policies among logical segments. The
following topics are discussed in this chapter:
•
“VLANs and Port VLAN ID Numbers” on page 94
•
“VLAN Tagging” on page 95
•
“VLAN Topologies and Design Considerations” on page 99
This section discusses how you can connect users and segments to a host that
supports many logical segments or subnets by using the flexibility of the multiple
VLAN system.
•
“Protocol-Based VLANs” on page 102
•
“Private VLANs” on page 105
Note: VLANs can be configured from the Command Line Interface (see “VLAN
Configuration” as well as “Port Configuration” in the Command Reference).
VLANs Overview
Setting up virtual LANs (VLANs) is a way to segment networks to increase network
flexibility without changing the physical network topology. With network
segmentation, each switch port connects to a segment that is a single broadcast
domain. When a switch port is configured to be a member of a VLAN, it is added to
a group of ports (workgroup) that belong to one broadcast domain.
Ports are grouped into broadcast domains by assigning them to the same VLAN.
Frames received in one VLAN can only be forwarded within that VLAN, and
multicast, broadcast, and unknown unicast frames are flooded only to ports in the
same VLAN.
The RackSwitch G8000 (G8000) supports jumbo frames with a Maximum
Transmission Unit (MTU) of 9,216 bytes. Within each frame, 18 bytes are reserved
for the Ethernet header and CRC trailer. The remaining space in the frame (up to
9,198 bytes) comprise the packet, which includes the payload of up to 9,000 bytes
and any additional overhead, such as 802.1q or VLAN tags. Jumbo frame support is
automatic: it is enabled by default, requires no manual configuration, and cannot be
manually disabled.
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