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What is ISL(Inter-Switch Link)?

Last Updated : 22 May, 2020
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ISL stands for Inter-Switch Link which is one of the VLAN protocols. The ISL is proprietary of Cisco and is used only between Cisco switches. It operates in a point-to-point VLAN environment and supports up to 1000 VLANs and can be used over Fast Ethernet and Gigabit Ethernet links only.



  • ISL is used in Cisco switches for VLAN switching purpose.
  • Cisco has also introduced facility to carry Token Ring, FDDI, and ATM frames over Ethernet ISL.
  • ISL Trunking can be used to improve the performance, manageability, and reliability.

How it works ?

  • The ISL works by encapsulating a 26 byte header and a 4 byte trailer between an Ethernet frame. The trailer is a Frame Check Sequence (FCS) on the ISL packet to ensure that it is not corrupted.
  • The size of the ISL frame varies from 94 bytes to 1548 bytes.
  • The encapsulated frame remains unchanged throughout.
  • The source address and destination address in the ISL header are not inherited from the encapsulated frame. The source address in the ISL header is that of the switch sending the ISL packet. The destination address is one of two multicast MAC addresses specific to ISL.
  • When two connected Cisco switches automatically negotiate a trunk with DTP they will choose ISL over 802.1Q unless one switch does not support it or specifically configured not to use ISL.

Characteristics of ISL

The various fields of Inter Switch Link (ISL) protocol encapsulation are given below:

  • DA (Destination Address): The destination address uses the multicast MAC address 01-00-0C-00-00-00. The first 40 bits of the DA field signal the receiver that the packet is in Inter-Switch Link (ISL) format.
  • Type: This field denotes the type of frame encapsulated. The type of frame could be either – Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).
  • User: 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.
  • SA : SA indicates Source Address. The address of the switch transmitting the Inter-Switch Link (ISL) frame is the Source Address.
  • Len: Len field is used to denote the length of the packet.
  • SNAP: SNAP is abbreviation of Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC). The AAAA03 SNAP field is a 24-bit constant value of “AAAA03”.
  • HSA (High Bits of Source Address): The HSA field is a 24-bit value which represents the upper 3 bytes (the manufacturer ID portion) of the SA field.
  • VLAN (Destination VLAN ID): Indicates VLAN ID of the packet. VLAN ID is a 15-bit value that is used to distinguish frames on different VLANs. VLAN ID is also known as the “color” of the frame.
  • BPDU: Indicate whether a BPDU, or CDP or VTP frame.
  • Index: The port index of the source of the packet.
  • Res: Reserved field for additional information, for instance, Token Ring or FDDI Frame Check Sequence field. For Ethernet, this field should be zero.
  • Encapsulated Ethernet Frame: The actual Ethernet frame.
  • ISL CRC: Four-byte check on the ISL packet to ensure it is not corrupted.

Advantages of ISL

  • ISL is used for tagging information. This means, we can multiplex multiple VLANs over a trunk link and this, in turn, helps the switch to easily determine the membership of VLAN frame over the trunked link.
  • It facilitates the interconnection of multiple switches together at the same time thereby maintaining information about the VLAN.
  • It provides a full wire-speed performance and low latency as compared to a Fast Ethernet connection.
  • With ISL there is no need of router in order to interact/communicate and so the users can efficiently and quickly access to servers without any delay.

Disadvantages of ISL

  • ISL is compatible between only Cisco switches. If the user wants to trunk between two dissimilar trunk links he may have to use the 802.1Q encapsulation.
  • ISL has a large overhead as compared to 802.1Q.
  • ISL supports only 1000 VLANs whereas 802.1Q supports upto 4096 VLANs.

ISL is no longer in use today due to various underlying reasons and problems associated with the switching, particularly in VLAN Trunking. , Cisco has deprecated ISL and some of the newer switches manufactured by Cisco don’t even support them. 802.1q is used commonly instead and even Cisco promotes using it. Most modern-day switches operate on this, 802.1q VLAN switching protocol.

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