Difference between ISL & 802.1q

A single communication link called trunk is used between devices to carry traffic which may belong to multiple VLANS. We can configure the device to allow or deny particular VLAN through the trunk by its VLAN identifier.

VLAN identifier is a special tag that is encapsulated in a Ethernet frame. There are two main types of encapsulation protocols called ISL (Inter Switch Link) which is Cisco proprietary protocol and 802.1q which is an IEEE Standard.

ISL

  • ISL is an Cisco proprietary protocol.
  • Supports up to 1000 Vlans
  • Original frame is encapsulated and a new header is inserted during encapsulation process.
  • A 26 byte header and a 4 byte FCS (frame check sequence) are inserted. Hence a total of 30 Bytes of overhead.
  • ISL tags frames from native Vlans.
  • ISL is less preferred in networks because of its high overhead value which is added to each Ethernet frame.

Below picture shows the encapsulation process and ISL fields:

isl

Different fields of ISL encapsulation frame are defined as under:

DA (Destination Address):

Used to identify ISL frame format.

Type:

Identifies layer 2 protocol:  Ethernet (0000), Token Ring (0001), FDDI (0010), and ATM (0011).

User:

The USER field bits “0″ and “1″ refer to  the priority of the packet.

SA (Source Address):

Source address of the switch which transmitted the ISL frame.

Len:

Packet Length

SNAP:

Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC).

 HSA (High Bits of Source Address):

HSA represents the upper 3 bytes (the manufacturer ID portion) of the SA field.

VLAN (Destination VLAN ID):

Indicates VLAN ID of the packet.

BPDU:

Indicate whether a frame is 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.

802.1q

  •  It is an IEEE Standard.
  • 802.1q supports 4096 Vlans.
  • IN 802.1q encapsulation process, a 4 byte tag is inserted into original frame and FCS (Frame Check Sequence) is re-calculated.
  • 802.1q does not tag frames from native Vlans.

Below picture shows the encapsulation process and 802.1q fields:

802

 

Different fields of ISL encapsulation frame are defined as under:

TPID

Tag Protocol ID is used to identify frame as IEEE 802.1q frame.  The value is set to 0×8100

User Priority

Indicates frame priority level. Zero means best effort and 7 means highest and 1 represents the lowest priority.

CFI

Formerly knows as CFI (Canonical format identifier) and now knows as DEI (Drop eligible indicator) can be used with User priority to mark the packets that can be dropped in case of congestion.

VLAN ID

Specifies the VLAN to which frame belongs.

Configuration 

1. ISL

LAB_Switch(config)#interface Fas 1/1
LAB_Switch(config-if)#switchport trunk encapsulation isl
LAB_Switch(config-if)#switchport mode trunk

2. 802.1q

LAB_Switch(config)#interface Fas 1/1
LAB_Switch(config-if)#switchport trunk encapsulation 802.1q
LAB_Switch(config-if)#switchport mode trunk