Implementing IBM System Networking 10Gb Ethernet Switches

52 Implementing IBM System Networking 10Gb Ethernet Switches

2.1 Virtual Local Area Networks

This section 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.

2.1.1 VLANs overview

Setting up VLANs is a way to segment networks, which increases 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 be forwarded only within that VLAN, and multicast, broadcast, and

unknown unicast frames are flooded only to ports in the same VLAN.

IBM System Networking switches support 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) comprises the packet,

which includes the payload of up to 9,000 bytes and any additional impact, 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.

2.1.2 VLANs and Port VLAN ID numbers

IBM System Networking switches support up to 1024 VLANs per switch. Even though the

maximum number of VLANs supported at any time is 1024, each can be identified by any

number 1 - 4095.

VLAN 1 is the default VLAN for the data ports. VLAN 4095 is used by the management

network, which includes the management port.

PVID numbers

Each port in the switch has a configurable default VLAN number, known as its PVID. By

default, the PVID for all non-management ports is set to 1, which correlates to the default

VLAN ID. The PVID for each port can be configured to any VLAN number 1 - 4094.

Each port on the switch can belong to one or more VLANs, and each VLAN can have any

number of switch ports in its membership. Any port that belongs to multiple VLANs, however,

must have VLAN tagging enabled.

VLAN tagging

IBM Networking OS supports 802.1Q VLAN tagging, providing standards-based VLAN

support for Ethernet systems.

Tagging places the VLAN identifier in the frame header of a packet, allowing each port to

belong to multiple VLANs. When you add a port to multiple VLANs, you also must enable

tagging on that port.