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ISL vs dot1q


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

Hall of Fame Super Silver

Re: ISL vs dot1q

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


Re: ISL vs dot1q

better stick with dot1q. cisco will soon obsolete isl

Community Member

Re: ISL vs dot1q

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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