Encapsulation Format Of Ethernet Data - H3C S5100-SI Operation Manual

Encapsulation Format of Ethernet Data

This section introduces the common encapsulation formats of Ethernet data for you to understand the
procedure for the switch to identify the packet protocols.
Ethernet II and 802.2/802.3 encapsulation
There are two encapsulation types of Ethernet packets: Ethernet II defined by RFC 894 and
802.2/802.3 defined by RFC 1042. The two encapsulation formats are described in the following
figures.
Ethernet II packet:
Figure 1-4 Ethernet II encapsulation format
802.2/802.3 packet:
Figure 1-5 802.2/802.3 encapsulation format
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.
The switch identifies whether a packet is an Ethernet II packet or an 802.2/802.3 packet according to
the ranges of the two fields.
Extended encapsulation formats of 802.2/802.3 packets
802.2/802.3 packets have the following three extended encapsulation formats:
802.3 raw encapsulation: only the length field is encapsulated after the source and destination
address field, followed by the upper layer data. No other fields are included.
Figure 1-6 802.3 raw encapsulation format
Currently, only IPX supports 802.3 raw encapsulation, featuring with the value of the two bytes after the
length field being 0xFFFF.
802.2 Logical Link Control (LLC) encapsulation: the length field, the destination service access
point (DSAP) field, the source service access point (SSAP) field and the control field are
encapsulated after the source and destination address field. The value of the control field is always
3.
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