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ISL and DOT1Q –
ISL and DOT1Q are 2 encapsulation standards supporting Virtual LANs (VLANs) on an Ethernet network. These are the 2 frame tagging protocols that Cisco supports.
The tagging protocol can be manually specified on a trunk port, or dynamically negotiated using Cisco’s proprietary Dynamic Trunking Protocol (DTP).
A common query in LAN switching which we generally hear is the difference between ISL encapsulation and DOT1Q encapsulation.
Below table will showcase how both the terms differ from each other & details of ISL and Dot1Q Encapsulation –
| PARAMETER | ISL | DOT1Q |
|---|---|---|
| Abbreviation for | ISL (Inter Switch Link) | - |
| Standard | Cisco proprietary protocol | IEEE Standard |
| Vlans Supported | Supports up to 1000 Vlans | Supports 4096 Vlans |
| Encapsulation | 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 which makes it total of 30 Bytes of overhead. | 802.1q encapsulation inserts a 4 byte tag into original frame and FCS (Frame Check Sequence) is re-calculated. |
| PVST | Supported | Not Supported |
| Supported on Cisco NX-OS | Not Supported | Supported |
| Frame Format | ISL Header (Image in downloadable table) | 802.1Q Tag (Image in downloadable table) |
| CPU Intensive | ISL uses more processor cycles than 802.1q because of the modification of the header and recalculation of FCS. | Less intensive than ISL |
| Native frame | ISL does not tag frames from native Vlans | 802.1q does not tag frames from native Vlans |
| Configuration | Switch(config)#interface Fa1/1 Switch(config-if)#switchport trunk encapsulation isl Switch(config-if)#switchport mode trunk | Switch(config)#interface Fa 1/1 Switch(config-if)#switchport trunk encapsulation 802.1q Switch(config-if)#switchport mode trunk |
| Fields used | * DA (Destination Address) * Type * User * SA (Source Address) * Len * SNAP * HSA (High Bits of Source Address) * VLAN (Destination VLAN ID) * BPDU * Index * Res * ISL CRC: | * TPID * User Priority * CFI * VLAN ID |
| Supported models | ISL is supported in Cisco 1900 series switches only | All cisco and non-cisco models |
Download the difference table here.
