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ETHERNET FRAME CONSTRUCTION. LAYER 2. 64 to 1518 Bytes long. preamble/SD. destination mac address. source mac address. TYPE/LENGTH. 802.2 SSAP/DSAP. DATA. FCS. 8. 6. 6. 2. 4 (+5). 4. 41 to 1500 Bytes long. L E N G T H I N B Y T E S.

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  1. ETHERNET FRAME CONSTRUCTION LAYER 2 64 to 1518 Bytes long preamble/SD destination mac address source mac address TYPE/LENGTH 802.2 SSAP/DSAP DATA FCS 8 6 6 2 4 (+5) 4 41 to 1500 Bytes long L E N G T H I N B Y T E S ISL ETHERNET FRAME CONSTRUCTION LAYER 2 64 to 1548 Bytes long preamble/SD multicast ISL DA mac TYPE USR source mac address LENGTH chk SNAP HSA VLAN SP IND T DATA CRC FCS 64 40 4 4 48 16 24 24 15 1 16+ 8 32 32 11 to 1500 Bytes long L E N G T H I N B I T S NOTE.. The ISL header adds up to 26 bytes to the standard Ethernet frame and 4 bytes to the trailer, extending the frame size by upto 30 bytes. Multicast ISL DA address is defined as Cisco Catalyst ISL = 0x0100.000C.0000 TYPE is a 4 bit value to indicate the source (0000=Ethernet, 0001=Token, 0010=FDDI, 0011=ATM….) USR is a 4 bit value used on Token Ring switching LENGTH chk is a 16 bit value to indicate the length of user data and ISL header less DA, TYPE, USR, SA, LENGTH chk and CRC SNAP is a 3 byte fixed value 0xAA-AA-03 HSA is 3 byte value that duplicates the high order of the ISL SA field VLAN is the VLAN id as a 15 bit value bu tonly 10 bits are used (VLAN = 0 to 1023) SP is a single bit flag to indicate VTP, CDP or Spanning Tree Protocol content IND is the port value of the source as a 16 bit value T 2 bits for AC/FC Token Ring flags (T=0 for Ethernet) CRC is the 32 bit CRC used for double check of data integrity

  2. ETHERNET FRAME CONSTRUCTION LAYER 2 64 to 1518 Bytes long preamble/SD destination mac address source mac address TYPE/LENGTH 802.2 SSAP/DSAP DATA FCS 41 to 1500 Bytes long 4 8 6 6 2 4 (+5) L E N G T H I N B Y T E S 802.1Q ETHERNET FRAME CONSTRUCTION LAYER 2 64 to 1522 Bytes long preamble/SD destination mac address source mac address VLAN TAG TYPE/LENGTH 802.2 SSAP/DSAP DATA FCS 4 8 6 6 4 2 4 37 to 1500 Bytes long L E N G T H I N B Y T E S NOTE…. IEEE 802.1Q VLAN Tagging extends the Ethernet frame by 4 bytes; 16 bit as media TYPE = 0x8100 = Ethernet 3 bit as PRIORITY = 802.1P 0 to 7 1 bit CFI bit = Canonical Format Identifier (0= canonical, 1= non-canonical) 12 bits for VLAN-ID (VLAN 0 to 4095)

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