ISL and Layer 2 Encapsulation
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Switch ports configured as ISL trunk ports encapsulate each Layer 2 frame before sending it out the trunk port. With ISL, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. At the receiving end, the header is removed and the packet is forwarded to the assigned VLAN. For each frame that is received on a trunk port, the switch recalculates the total frame cyclic redundancy check (CRC) and checks it against the CRC value in the FCS. If the CRC values do not match, the frame is discarded. If the values match, the switch discards the FCS, then de-encapsulates or strips away the header to read in the field information for processing.
The field information contains values that define various attributes of the original Layer 2 data within the encapsulated frame. This information is used for forwarding, media identification, and VLAN identification. The actual frame header and the fields within the frame depend upon the type of VLAN and media that connects to that VLAN. Ethernet is encapsulated with a 26-byte ISL header and a four-byte FCS. The 30-byte ISL encapsulation overhead is consistent among the different media types supported on Catalyst switches, but the overall size of the frame may vary depending on the media type and the supported MTUs of those media types.
Note This course only covers the Ethernet technology.
The ISL Ethernet frame header contains these information fields:
■ DA (destination address): 40-bit destination address. This is a multicast address and is set at 0x01-00-0C-00-00 or 0x03-00-0c-00-00. The first 40 bits of the DA field signal the receiver that the packet is in ISL format.
■ TYPE: Four-bit descriptor of the encapsulated frame types: Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).
■ USER: Four-bit descriptor used as the TYPE field extension or to define Ethernet priorities; it is a binary value from 0, the lowest priority, to 3, the highest priority. The default USER field value is "0000." For Ethernet frames, the USER field bits "0" and "1" indicate the priority of the packet as it passes through the switch.
■ SA (source address): 48-bit source MAC address of the transmitting Catalyst switch port.
■ LEN (length): 16-bit frame-length descriptor minus DA, TYPE, USER, SA, LEN, and CRC.
■ AAAA03: Standard Subnetwork Access Protocol (SNAP) 802.2 logical link control (LLC) header.
■ HSA (high system availability): First three bytes of the SA (manufacturer or unique organizational ID).
■ VLAN ID: 15-bit VID. Only the lower 10 bits are used for 1024 VLANs.
■ BPDU (bridge protocol data unit): One-bit descriptor identifying whether the frame is a spanning tree BPDU; also identifies if the encapsulated frame is a CDP frame.
■ INDX (index): Indicates the port index of the source of the packet as it exits the switch. It is used for diagnostic purposes only and may be set to any value by other devices. It is a 16-bit value and is ignored in received packets.
■ ENCAP FRAME: Encapsulated data packet, including its own CRC value, completely unmodified. The internal frame must have a CRC value that is valid when the ISL encapsulation fields are removed. A receiving switch may strip off the ISL encapsulation fields and use this ENCAP FRAME field as the frame is received (associating the appropriate VLAN and other values with the received frame as indicated for switching purposes).
■ FCS (frame check sequence): Consists of four bytes. This sequence contains a 32-bit CRC value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames. The FCS is generated over the DA, SA, LEN, TYPE, and Data fields. When an ISL header is attached, a new FCS is calculated over the entire ISL packet and added to the end of the frame.
RES (reserved): 16-bit reserved field used for additional information, such as the FDDI frame control field.
When examining the frame formats for Layer 2 frames, notice that the ISL Layer 2 header comes before the other Layer 2 field information in the frame.
The ISL frame contains two FCS fields. The first FCS field is generated from the original transmitting device of the frame. The second FCS field is generated from the ISL trunk port.
ISL encapsulates the frame without modifying its contents.
Continue reading here: Importance of Native VLANs
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