ISL vs dot1q

Unanswered Question
Aug 23rd, 2008


I am trying to implement the configuration shown below, but after reading some more information I am confused if better using ISL. Both devices are Cisco switches, a 3750 and 4507R. By now they will hold just one connection, a giga link, but later the 3750 will be connected to other 4507R wich is connected through a 2gigs port-channel with the first 4507R I mentioned before.

Pleas help me to decide, ISL or dot1q?



description **** 3750 to 4507R ****

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 4,50

switchport trunk native vlan 4

switchport mode trunk

logging event trunk-status

speed 1000

duplex full

udld port aggressive

storm-control broadcast level 30.00

storm-control multicast level 30.00

storm-control action trap

spanning-tree guard loop

I have this problem too.
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Giuseppe Larosa Sat, 08/23/2008 - 09:08

Hello Hugo,

you can stay with 802.1Q.

There are no issues with 802.1Q and PVST+ when using only Cisco routers: BPDUs are sent with the correct 802.1q tag and contain the Vlan-id and this provides a sanity check.

So there's no problem of only CST running this happens when connecting to a non Cisco 802.1D compliant device.

Actually the BDPUs are sent using a proprietary encapsulation and to a well-defined multicast MAC address both different then standard 802.1D so that they can travel via a 802.1D cloud (they are seen as user multicast traffic by 802.1D devices).

So stay with 802.1Q and STP will manage the link redundancy correctly on every Vlan.

Hope to help


haroon.shaikh Sun, 08/24/2008 - 18:54

Hi Hugo,

I work with an ISP, and if you ask me I havent used ISL in ages, dot1Q is the way to go. It is also a standard across different vendors and it is guaranteed to work between different vendor equipments.

Also, you will find more documentation on Internet for dot1Q than ISL. Good luck...



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