Manual

75
User Guide
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• The maximum amount of Ethernet Data in a standard packet is 1500 bytes, the
Length fi eld in 802.2/802.3 encapsulation is 2 bytes ranging from 0x0000 to
0x05DC and the Type fi eld in Ethernet II encapsulation is also 2 bytes ranging
from 0x0600 to 0xFFF. The Type or Length fi eld in the Data range of 0x05DD to
0x05FF is illegal and any packets with a value in that range will be discarded. The
Switch will identify whether a packet is Ethernet II or 802.2/802.3 according to
the value in the Type or Length fi eld.
802.2/802.3 encapsulation contains the following three extended formats:

802.3 raw encapsulation
Figure 6-9 802.3 raw encapsulation
• Only the Length fi eld is encapsulated after source MAC address fi eld and desti-
nation MAC address fi eld, followed by DATA fi eld without other header fi elds.
Currently only the IPX protocol supports raw encapsulation format. The last two
bytes of the Length fi eld in 802.3 raw encapsulation is 0xFFFF.

802.2LLC (Logic Link Control) encapsulation
Figure 6-10 802.2LLC encapsulation
• The Length fi eld, DSAP (Destination Service Access Point) fi eld, SSAP (Source
Service Access Point) fi eld and Control fi eld are encapsulated after Source MAC
Address and Destination MAC Address fi elds. The value of Control fi eld will
always be 3 in a valid packet. The DSAP and SSAP fi elds in 802.2 LLC encapsula-
tion are used to identify the upper layer protocol. For example when both the
two fi elds are set to 0xE0, it indicates the upper layer protocol is IPX.

802.2: 802.2 SNAP (Sub-Network Access Protocol) is encapsulated based on the
802.3 standard. In 802.2 SNAP encapsulation,
• The values of both DSAP fi eld and SSAP fi eld will always be 0XAA in a valid
packet and the value of Control fi eld will be 3. The Switch differentiates 802.2
LLC and SNAP encapsulation formats according to the values of DSAP and
SSAP fi elds.
• The connected device determines the encapsulation format of its sent packets,
devices can send out packets of two encapsulation formats simultaneously.
Ethernet II encapsulation is the most common format used. 802.3 and Ethernet II
encapsulation formats are supported in IP, ARP and RARP protocols, but not sup-
ported in all other protocols. The Switch identifi es the protocol of the packet by
matching values of the encapsulation format.