ISL
From Internetworkpro
ISL or InterSwitch Link is a Cisco-proprietary means of carrying VLANs between switches. The standard (and more widespread) alternative is IEEE 802.1q.
[edit] Frame Format
The basic mechanism is to encapsulate the original frame with an additional 26-byte ISL header and new FCS. The orginal header and FCS are untouched.
[edit] Header
The ISL header itself contains many fields many of which are either redundant or non needed for simple VLAN trunking.
| Field | Description | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| DA | Destination Address
This is set to either set to the multicast address 01-00-0C-00-00 or 03-00-0C-00-00. This field is what determines if the frame is ISL or not | ||||||||||
| Type | Frame Type
This is a 4 bit code that determines this encapsulated frame type
| ||||||||||
| User | 4-bit code to extend the TYPE field above. The default value is "0000". | ||||||||||
| SA | Source Address
This is set to the MAC address of the switch that transmits the ISL frame. | ||||||||||
| LEN | Length
The total length of the frame excluding the DA, TYPE, USER, SA, LEN, FCS values. The length of these fields are 18 bytes. | ||||||||||
| AAAA03 (SNAP) | This field is a 24-bit constant with the value '0xAAAA03' | ||||||||||
| HSA | High Bits of Source Address
This contantes the upper 3 bytes of the SA field. This is normally the manufacturer ID portion of the MAC address. This value must be '00-00-0C' | ||||||||||
| VLAN | Vlan ID
This is the acutal VLAN number assigned the the encapsulated frame. The lat 3 bits are used for CoS leaving 12 bits for the VLAN number. These mean that ISL can only carry VLANS numbered from 1-1024. Extended vlans (1025-4094) are not allowed on ISL | ||||||||||
| BDPU | This is bit is set when the encapsulated frame is a BDPU, VTP, or CDP frame. (ie: frames that should be processed by the switch) | ||||||||||
| INDEX | This field is set to the port index of the port as it leave the switch. This value is ignored by the receiving switch | ||||||||||
| RES | This this value is used to Token Ring or FDDI frames that are encapsulated with ISL. With Token ring the Access Control (AC) and Frame Control (FC) are placed in here. For FDDI FC and Least Significant Byte (LSB) are placed in this field. For ethernet the value is set to all zeros |
