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Difference between ISL or dot1q trunking on a 3550 switch

Posted on 2005-03-13
Medium Priority
Last Modified: 2008-01-16
I'm trying to determine what encapsulation I should use on the switch..I know ISL is cisco propierty, but what the is the difference between the two. what makes one better that the other.....

Question by:ritru
LVL 11

Expert Comment

ID: 13532696
To answer your questions, read Cisco website:


ISL is a Cisco proprietary protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. ISL provides VLAN trunking capabilities while maintaining full wire speed performance on Ethernet links in full-duplex or half-duplex mode. ISL operates in a point-to-point environment and will support up to 1000 VLANs. In ISL, the original frame is encapsulated and an additional header is added before the frame is carried over a trunk link. At the receiving end, the header is removed and the frame is forwarded to the assigned VLAN. .ISL uses Per VLAN Spanning Tree(PVST) which runs one instance of Spanning Tree Protocol (STP) per VLAN. PVST allows to optimize root switch placement for each VLAN and supports load balancing of VLANs over multiple trunk links.

802.1Q is the IEEE standard for tagging frames on a trunk and supports upto 4096 VLANs. In 802.1Q, the trunking device inserts a four-byte tag into the original frame and recomputes the Frame Check Sequence (FCS) before sending the frame over the trunk link. At the receiving end, the tag is removed and the frame is forwarded to the assigned VLAN. 802.1Q does not tag frames on the native VLAN. It tags all other frames transmitted and received on the trunk. While configuring 802.1 trunk, you must make sure that the same native VLAN is configured on both sides of the trunk. IEEE 802.1Q defines a single instance of spanning tree running on the native VLAN for all the VLANs in the network which is called Mono Spanning Tree (MST). This lacks the flexibility and load balancing capability of PVST available with ISL. However, PVST+ offers the capability to retain multiple Spanning Tree topologies with 802.1Q trunking.

Expert Comment

ID: 13533863
If you ever need to add a non-cisco switch or router (or IP phone) that has to participate in VLAN trunking, go with 802.1q. That is what everything else is compatible with.
LVL 50

Accepted Solution

Don Johnston earned 375 total points
ID: 13534254
PVST+ is also Cisco proprietary.

A better solution to the mono spanning-tree issue is 802.1s which allows for multiple instances of spanning-tree over 802.1q trunks.


Expert Comment

ID: 13554732
802.1Q is the way to go, if you are looking for future interoperability with non cisco equipment.

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