ISL is a Cisco proprietary protocol for the interconnection of multiple switches and maintenance of VLAN information as traffic goes between switches. ISL provides VLAN trunking capabilities while it maintains full wire-speed performance on Ethernet links in full-duplex or half-duplex mode. ISL operates in a point-to-point environment and can support up to 1000 VLANs. In 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 frame is forwarded to the assigned VLAN.
ISL Frame
The ISL frame consists of three primary fields: the encapsulation frame (original frame), which is encapsulated by the ISL header, and the FCS at the end.| ISL Header | Encapsulation Frame | FCS |
This example shows the further expansion of the ISL header. The expansion includes the field acronyms and the number of bits for each field:
| No. of bits | 40 | 4 | 4 | 48 | 16 | 24 | 24 |
| Frame field | DA | TYPE | USER | SA | LEN | AAAA03(SNAP) | HSA |
| No. of bits | 15 | 1 | 16 | 16 | 8 to 196,600 bits (1 to 24,575 bytes) | 32 |
| Frame field | VLAN | BPDU | INDEX | RES | ENCAP FRAME | FCS |
Field Descriptions
This section provides detailed descriptions of the ISL frame fields.DA—Destination Address
The DA field of the ISL packet is a 40-bit destination address. This address 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—Frame Type
The TYPE field consists of a 4-bit code. The TYPE field indicates the type of frame that is encapsulated and can be used in the future to indicate alternative encapsulations. This table provides definitions of different TYPE codes:| TYPE Code | Meaning |
|---|---|
| 0000 | Ethernet |
| 0001 | Token Ring |
| 0010 | FDDI |
| 0011 | ATM |
USER—User Defined Bits (TYPE Extension)
The USER field consists of a 4-bit code. The USER bits are used to extend the meaning of the TYPE field. 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. Whenever traffic can be handled in a manner that allows it to be forwarded more quickly, the packets with this bit set should take advantage of the quick path. It is not required that such paths be provided.| USER Code | Meaning |
|---|---|
| XX00 | Normal Priority |
| XX01 | Priority 1 |
| XX10 | Priority 2 |
| XX11 | Highest Priority |
SA—Source Address
The SA field is the source address field of the ISL packet. The field should be set to the "802.3" MAC address of the switch port that transmits the frame. It is a 48-bit value. The receiving device may ignore the SA field of the frame.LEN—Length
The LEN field stores the actual packet size of the original packet as a 16-bit value. The LEN field represents the length of the packet in bytes, with the exclusion of the DA, TYPE, USER, SA, LEN, and FCS fields. The total length of the excluded fields is 18 bytes, so the LEN field represents the total length minus 18 bytes.AAAA03 (SNAP)—Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC)
The AAAA03 SNAP field is a 24-bit constant value of "0xAAAA03".HSA—High Bits of Source Address
The HSA field is a 24-bit value. This field represents the upper 3 bytes (the manufacturer ID portion) of the SA field. The field must contain the value "0x00-00-0C".VLAN—Destination Virtual LAN ID
The VLAN field is the VLAN ID of the packet. It is a 15-bit value that is used to distinguish frames on different VLANs. This field is often referred to as the "color" of the frame.BPDU—Bridge Protocol Data Unit (BPDU) and Cisco Discovery Protocol (CDP) Indicator
The bit in the BPDU field is set for all BPDU packets that are encapsulated by the ISL frame. The BPDUs are used by the spanning tree algorithm in order to determine information about the topology of the network. This bit is also set for CDP and VLAN Trunk Protocol (VTP) frames that are encapsulated.INDX—Index
The INDX field indicates the port index of the source of the packet as it exits the switch. This field 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.RES—Reserved for Token Ring and FDDI
The RES field is a 16-bit value. This field is used when Token Ring or FDDI packets are encapsulated with an ISL frame. In the case of Token Ring frames, the Access Control (AC) and Frame Control (FC) fields are placed here. In the case of FDDI, the FC field is placed in the Least Significant Byte (LSB) of this field. For example, an FC of "0x12" has a RES field of "0x0012". For Ethernet packets, the RES field should be set to all zeros.ENCAP FRAME—Encapsulated Frame
The ENCAP FRAME field is the encapsulated data packet, which includes its own cyclic redundancy check (CRC) value, completely unmodified. The internal frame must have a CRC value that is valid after the ISL encapsulation fields are removed. The length of this field can be from 1 to 24,575 bytes in order to accommodate Ethernet, Token Ring, and FDDI frames. 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
The FCS field consists of 4 bytes. This sequence contains a 32-bit CRC value, which is created by the sending MAC and is recalculated by the receiving MAC in order to check for damaged frames. The FCS is generated over the DA, SA, Length/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.Note: The addition of the new FCS does not alter the original FCS that is contained within the encapsulated frame.
Frame Size
The ISL frame encapsulation is 30 bytes, and the minimum FDDI packet is 17 bytes. Therefore, the minimum ISL encapsulated packet for FDDI is 47 bytes. The maximum Token Ring packet is 18,000 bytes. Therefore, the maximum ISL packet is 18,000 plus 30 bytes of ISL header, for a total of 18,030 bytes. If only Ethernet packets are encapsulated, the range of ISL frame sizes is from 94 to 1548 bytes.The biggest implication for systems that use ISL encapsulation is that the encapsulation is a total of 30 bytes, and fragmentation is not required. Therefore, if the encapsulated packet is 1518 bytes long, the ISL packet is 1548 bytes long for Ethernet. Additionally, if packets other than Ethernet packets are encapsulated, the maximum length can be greatly increased. You must consider this length change when you evaluate whether a topology can support ISL packets size.
Another system implication is that ISL packets contain two FCSs. The first FCS is calculated for the original data. The second FCS is calculated after the packet has been encapsulated in ISL. If the original data does not contain a valid CRC, the invalid CRC is not detected until the ISL header is stripped off and the end device checks the original data FCS. This typically is not a problem for switching hardware, but can be difficult for routers and network interface cards (NICs).
No comments:
Post a Comment