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Encapsulation Format of Ethernet Data
This section simply introduces the common used encapsulation format of Ethernet data to
understand the procedure for the switch to identify the protocol of packets. At present there are
two encapsulation formats of Ethernet data, Etthernet II encapsulation and 802.2/802.3
encapsulation, shown as follows:
Ethernet II encapsulation
802.2/802.3 encapsulation
DA and SA respectively refer to destination MAC address and source MAC address. The
number indicates the length of the field in bytes, for example, the length of source MAC
address is 12 bytes.
As the maximum length of Ethernet data is 1500 bytes, that is, 0x05DC in hexadecimal, the
Length field in 802.2/802.3 encapsulation ranges from 0x0000 to 0x05DC, but the Type field
in Ethernet II encapsulation ranges from 0x0600 to 0xFFF. The Type or Length field in the
range of 0x05DD to 0x05FF is recognized as illegal and will be directly discarded. The switch
identifies whether a packet is Ethernet II packet or 802.2/802.3 packet according to the
ranges of the two fields.
802.2/802.3 encapsulation contains the following three extended formats:
802.3 raw encapsulation
Only the Length field is encapsulated after source MAC address field and destination MAC
address field, followed by DATA field without other fields. Currently only IPX protocol supports
802.3 raw encapsulation format. The last two bytes of the Length field in 802.3 raw
encapsulation is 0xFFFF.
802.2LLC (Logic Link Control) encapsulation
The Length field, DSAP (Destination Service Access Point) field, SSAP (Source Service
Access Point) field and Control field are encapsulated after source MAC address field and
destination MAC address field. The value of Control field is always 3. DSAP field and SSAP
field in 802.2 LLC encapsulation are used to identify the upper layer protocol, for example,
when both the two fields are 0xE0, it indicates the upper layer protocol is IPX.
802.2 SNAP (Sub-Network Access Protocol) is encapsulated based on 802.3 standard
packets. In 802.2 SNAP encapsulation, the values of both DSAP field and SSAP field are
always 0XAA, and the value of Control field is 3. The Switch differentiates 802.2 LLC and
802.2 SNAP encapsulation formats according to the values of DSAP field and SSAP field.
The device determines the encapsulation format of its sending packets, and a device can send out
packets of two encapsulation formats. Ethernet II encapsulation format is the most common used
one currently.
802.3 and Ethernet II encapsulation formats are supported in IP protocol, ARP protocol and RARP
protocol, but not supported in all protocols. The switch identifies the protocol of the packets by
matching eigenvalues of two encapsulation formats.
The Procedure for the Switch to Identify Packet Protocol
57