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Ethernet II Versus IEEE 802.3

The Ethernet Version II frame format is slightly different from the IEEE 802.3 frame format. The Ethernet Version II frame format is described in Figure 3.4.

Figure 3.4 Ethernet Version II frame format.

With Ethernet Version II, there is no SOF field and the preamble is extended to eight octets. The IEEE 802.3 Length field becomes a type field to describe the protocol type of the frame. There is no LLC PDU.

The Cisco router supports four different frame types via the encapsulation command:

  • ARPA for Ethernet Version II frames

  • NOVELL-ETHER for IEEE 802.3 frames without an LLC PDU (default)

  • SAP for IEEE 802.3 frames with an IEEE 802.2 LLC PDU

  • Subnet Access Protocol (SNAP), for IEEE 802.3 frames with an IEEE 802.2 LLC PDU and a SNAP header

The SNAP format is an extension of the IEEE 802.3 format to include a SNAP header after the LLC Header. The LLC header is specially encoded. The destination and source SAP are set to 0xAA, and the control field is one octet and set to 0x03. The SNAP header is five octets in length. The first three octets are set to 0 and the last two octets contain an EtherType that defines the appropriate protocol.

Key Concept

Ethernet allows multiple encapsulation types at the Data Link layer.

Following is a sample router configuration to set an Ethernet interface to SAP encapsulation and a Fast Ethernet interface to SNAP encapsulation:

interface Ethernet0 
  encapsulation sap 
 
interface FastEthernet0 
  encapsulation snap
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