3com 5500-ei pwr Instruccion De Instalación

 

1-7 

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.  

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.  

 

Presently, The switches recognize packets with the value of the type field being in the range 0x05DD to 

0x05FF as 802.2/802.3 encapsulated packets. 

 

802.2/802.3 packets have the following three extended encapsulation formats: 

z 

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 the IPX protocol supports 802.3 raw encapsulation, featuring with the value of the two 

bytes after the length field being 0xFFFF.