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ISL and DOT1Q –
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.
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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.
ABOUT THE AUTHOR
Founder of AAR TECHNOSOLUTIONS, Rashmi is an evangelist for IT and technology. With more than 12 years in the IT ecosystem, she has been supporting multi domain functions across IT & consultancy services, in addition to Technical content making.
You can learn more about her on her linkedin profile – Rashmi Bhardwaj
