3Com WX3000 Introduction to Protocol-Based Vlan, Encapsulation Format of Ethernet Data

The link type of a port on the device can be one of the following: access, trunk, and hybrid. For the three types of ports, the process of being added into a VLAN and the way of forwarding packets are different. For details, refer to the “Port Basic Configuration” part of the manual.

Port-based VLANs are easy to implement and manage and applicable to hosts with relatively fixed positions.

Protocol-Based VLAN

Introduction to Protocol-Based VLAN

Protocol-based VLAN is also known as protocol VLAN, which is another way to classify VLANs. Through the protocol-based VLANs, the switch can analyze the received packets carrying no VLAN tag on the port and match the packets with the user-defined protocol template automatically according to different encapsulation formats and the values of specific fields. If a packet is matched, the switch will add a corresponding VLAN tag to it automatically. Thus, data of specific protocol is assigned automatically to the corresponding VLAN for transmission.

This feature is used for binding the types of services provided in the network to VLANs to facilitate management and maintenance.

Encapsulation Format of Ethernet Data

This section introduces the common encapsulation formats of Ethernet data for you to understand well the procedure for the switch to identify the packet protocols.

Ethernet II and 802.2/802.3 encapsulation

Mainly, there are two encapsulation types of Ethernet packets: Ethernet II and 802.2/802.3, defined by RFC 894 and RFC 1042 respectively. The two encapsulation formats are described in the following figures.

zEthernet II packet:

Figure 1-4Ethernet II encapsulation format

DA&SA(12) Type(2) Data

z802.2/802.3 packet:

Figure 1-5802.2/802.3 encapsulation format

DA&SA(12) Length(2) DSAP(1) SSAP(1) Control(1) OUI(3) PID(2) Data

In the two figures, DA and SA refer to the destination MAC address and source MAC address of the packet respectively. The number in the bracket indicates the field length in bytes.

The maximum length of an Ethernet packet is 1500 bytes, that is, 0x05DC in hexadecimal, so the length field in 802.2/802.3 encapsulation is in the range of 0x0000 to 0x05DC.

Whereas, the type field in Ethernet II encapsulation is in the range of 0x0600 to 0xFFFF.

Packets with the value of the type or length field being in the range 0x05DD to 0x05FF are regarded as illegal packets and thus discarded directly.

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