what's up with ISL

[mark.mclaughlin]

Is it something that is not being utilized lately. I have been searching for why/when to use ISL vs. dot1q and I thought that if you had all Cisco devices you would use ISL. But through my searches I see that ISL is not available on 2950's. I have a couple of new 2960's I just started to configure and see that they do not have ISL as an option either.

Can someone clue me in.

Thanks

[Richard Burts · Accepted solution]

Mark

Many vendors, including Cisco, will recognize a need and introduce proprietary features in advance of a standard being developed and will then adopt the standard when it is developed.

ISL is an example of this. Cisco introduced ISL as it recognized the need to have a trunking protocol in Ethernet switching in advance of the adoption of dot1Q. After the standard was adopted Cisco began to move to it. Some products supported both products but increasingly Ethernet switches support only dot1Q and not ISL. So I believe that the reasons and opportunities to use ISL are getting fewer and fewer.

HTH

Rick

[williamwbishop]

Also, while some proprietary things are better than the standards version(ISL,EIGRP, etc), keep in mind that you don't know what hardware may enter your company over the next term, it's usually best to use a standards based method if one is available. It just so happens that standards take a while to develop and they have to rip someone off that has already beaten down the trail.

[millerlae]

In most scenarios it's safest to deploy VLANs and VLAN trunks using the IEEE 802.1q. This is due to the fact that if you mix your VLAN trunks with other brands of switches (whether Cisco or NON Cisco) they can "talk" together via the IEEE 802.1q protocol. Since ISL is Cisco proprietary and only limits you to certain models of Cisco switches ONLY then you can run it.

Other manufacturers don't have ISL and only can trunk 802.1q.

Unless you forecast your enterprise to employ the SAME model of Cisco switch across the board it's safer to deploy using 802.1q. That gives you the flexability to mix different types of Cisco switches and even different manufacturers.

Be aware that VTP is also Cisco Proprietary and can talk across ALL modern Cisco switches. VTP cannot (however) talk to a Foundry or Extreme Networks switch.