Information Technology Reference
In-Depth Information
ISL
Cisco's ISL is a proprietary trunk encapsulation method for carrying VLANs over FE or GE
interfaces.
ISL tags each frame to identify the VLAN it belongs to. The tag is a 30-byte header and CRC
that is added around the FE frame. This includes a 26-byte header and 4-byte CRC. The header
includes a 15-bit VLAN ID that identifies each VLAN. Although ISL is a point-to-point
protocol (over FE and GE) between two Cisco devices, it can carry FDDI, Token Ring, and
ATM in its payload.
ISL Frame Format
As shown in Figure 4-31, ISL adds a 26-byte header and 4-byte trailer to a frame when it is sent
through a trunk.
ISP Frame Format
Figure 4-31
ISL Header
(26 bytes)
Original Frame
(variable)
ISL CRC
(4 bytes)
The ISL frame with header, data, and trailer includes the following fields listed in Table 4-10.
ISL Frame Fields
Table 4-10
ISL Field
Description
DA
40-bit destination address set to multicast address 0x01 00 c0 00 00.
Type
4-bit value that indicates the source frame type with the following values:
0000 = Ethernet
0001 = Token Ring
0010 = FDDI
0011 = ATM
User
4-bits, usually set to zero, but can extend the meaning of the Type field.
SA
48-bit source MAC address.
LEN
16-bit length of user data and ISL header, excluding DA, T, U, SA, LEN, and CRC
fields.
SNAP
3-byte field set to 0xAAAA03.
HSA
Upper 3 bytes, the manufactured ID portion, of the SA field; it must contain the value
0x00_00_0C.
VLAN ID
15-bit VLAN identifier.
BPDU
1-bit set for all BPDUs that are encapsulated by ISL.
INDX
16-bit index indicates the port index of the source of the packet as it exits the switch.
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