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Advanced POWER Virtualization on IBM System p5 - Previous ...

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The VLAN ID is placed in the Ethernet header and c<strong>on</strong>stitutes no additi<strong>on</strong>al<br />

header. To be able to do this, the Ethernet frame size for tagged frames was<br />

increased from 1518 bytes to 1522 bytes and the Ethernet header format was<br />

slightly modified with the introducti<strong>on</strong> of IEEE802.1Q. Thus, in c<strong>on</strong>trast to, for<br />

example, Point-to-Point-Protocol-over-Ethernet (PPPoE), which is comm<strong>on</strong>ly<br />

used for xDSL with a MTU of 1492, you do not have to care about reducing the<br />

TCP/IP’s MTU of 1500 with respect to VLAN ID tagging.<br />

Note: You do not have to reduce the TCP/IP default MTU size of 1500 for<br />

Ethernet due to the additi<strong>on</strong>al 4 bytes introduced by IEEE 802.1Q VLANs.<br />

Attenti<strong>on</strong>: If you increase the TCP/IP’s MTU size for virtual Ethernet that are<br />

implemented by the <str<strong>on</strong>g>POWER</str<strong>on</strong>g> Hypervisor, as introduced in 2.8.2,<br />

“Inter-partiti<strong>on</strong> networking with virtual Ethernet” <strong>on</strong> page 79, you must take the<br />

additi<strong>on</strong>al 4 bytes introduced by IEEE 802.1Q VLANs into account: the<br />

maximum MTU is 65394 without VLANs and 65390 bytes with VLANs. This is<br />

due to a limit of 65408 bytes for virtual Ethernet frames transmitted through<br />

the <str<strong>on</strong>g>POWER</str<strong>on</strong>g> Hypervisor. (The Ethernet headers are 14 and 18 bytes<br />

respectively, but there is no need for the 4 byte CRC in the <str<strong>on</strong>g>POWER</str<strong>on</strong>g><br />

Hypervisor).<br />

A port <strong>on</strong> a VLAN-capable switch has a default Port virtual LAN ID (PVID) that<br />

indicates the default VLAN the port bel<strong>on</strong>gs to. The switch adds the PVID tag to<br />

untagged packets that are received by that port. In additi<strong>on</strong> to a PVID, a port may<br />

bel<strong>on</strong>g to additi<strong>on</strong>al VLANs and have those VLAN IDs assigned to it that indicate<br />

the additi<strong>on</strong>al VLANs the port bel<strong>on</strong>gs to.<br />

►<br />

►<br />

A switch port with a PVID <strong>on</strong>ly is called an untagged port.<br />

Untagged ports are used to c<strong>on</strong>nect VLAN-unaware hosts.<br />

A port with a PVID and additi<strong>on</strong>al VIDs is called a tagged port.<br />

Tagged ports are used to c<strong>on</strong>nect VLAN-aware hosts.<br />

VLAN-aware means that the host is IEEE 802.1Q compatible and can interpret<br />

VLAN tags, and thus can interpret them, add them, and remove them from<br />

Ethernet frames. A VLAN-unaware host could be c<strong>on</strong>fused by receiving a tagged<br />

Ethernet frame. It would drop the frame and indicate a frame error.<br />

Receiving packets <strong>on</strong> a tagged port<br />

A tagged port uses the following rules when receiving a packet from a host:<br />

1. Tagged port receives an untagged packet:<br />

The packet will be tagged with the PVID, then forwarded.<br />

Chapter 2. <str<strong>on</strong>g>Virtualizati<strong>on</strong></str<strong>on</strong>g> technologies <strong>on</strong> <strong>System</strong> p servers 75

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