Format of two frames of Ethernet

The upper application encapsulates the data into an IP packet and then encapsulates it into a frame (the router receives a data frame, forwards a data frame, not a packet, he simply takes the number of received frames apart, exposing the three-layer header, The IP address in the header in the packet is only to determine the destination of the forwarding.

frame = Frame Header control information + data part + Frame end

The format of the frame is determined by the encapsulation protocol of the interface + the upper application together.

For example: Cisco serial port (ICMP message test)

interface is encapsulated by default HDLC Frame

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interface-Encapsulated PPP's Frame

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Ethernet frame:

Default Encapsulation protocol: ARPA (the predecessor of Ethernet, which is used to represent the protocol of the two frames of the encapsulated Ethernet)

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two different formats :

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Ethernet is originally IBM the private agreement of the company, which was later publicly owned by the IEEE Public organization, and IBM He also made him public, so the two kinds of Ethernet frames were created

Application Scenario: Application by upper layer + ARPA decision

first of all, the encapsulation of your interface is ARPA protocol, you can use one of two types of frame formats

for E2 and the 802.3 the selection of frames is:

E2 : For the forwarding of real data, such as files, videos, images, etc.

802.3 : Frame encapsulation using some control plane information, like Cisco's cdp/vtp/p AGP/UDLD

E2

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The format of two types of frames

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EEE802.3 frame format similar to ethernet_ii frame, just ethernet_ii frame Span style= "font-family:arial;" >type field is 802.3 length field replaced, and occupied Span style= "font-family:arial;" >data field 8 LLC and snap field.

2. Length Field defines the Data the number of bytes that the field contains.

3. Logical Link ControlLLC(Logical Link Control) service access point by DestinationDsap(Destination Service Access Point), source service access PointSSAP(Source Service Access Point) andControlThe fields are composed.

4. SNAP(sub-network Access Protocol) by the agency code (Org Code) and type (Type) fields. ORG Codethree bytes are0. Typethe meaning of the fieldEthernet_IIin the frameTypefields are the same. IEEE802.3frame based onDsapand theSSAPthe value of a field can be divided into the following categories:

1) when dsap and SSAP Both take a specific value of 0xff ,802.3 the frame becomes netware- .

ETHERNET frames that are used to host NetWare type of data.

2 ) When Dsap and the SSAP take a specific value 0xaa when the 802.3 the frame becomes

Ethernet_snap frame. ethernet_snap frames can be used to transfer multiple

type of protocol.

3 ) Dsap and the SSAP the other values are pure IEEE802.3 frame.

differentiate:whenTypefield value is less than or equal to the(or a hexadecimal0X05DC), the frame uses theIEEE 802.3format. WhenTypefield value is greater than or equal to1536(or a hexadecimal0x0600), the frame uses theEthernet IIformat. Most of the data frames in Ethernet use theEthernet IIformat.

This article is from the "11001675" blog, please be sure to keep this source http://11011675.blog.51cto.com/11001675/1725599

Format of two frames of Ethernet