ISL vs StackWise

Unanswered Question
Jun 16th, 2008

Is there a benifit in establishing an ISL trunk between to 3750G switches compared to utilizing the StackWise connections before accessing our core switch?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
michaelchoo Mon, 06/16/2008 - 15:29

ISL/802.1Q trunks only limit you to whatever highest port speed you have on the switch (max of 10Gbps assuming 3750E with X2), while StackWise gives you 32Gbps.

As far as trunking goes, why do you want to use ISL rather than 802.1Q?

fultont Mon, 06/16/2008 - 18:28

I really don't see the need in the situation. Our VMWare implemention team is requesting ISL between the 2 3705G switchs.

I'm already using 802.1Q across a 4GB EtherChannel between the 2 Stackwise'd 3750G to a 4507R

Mark Yeates Mon, 06/16/2008 - 16:17

Are you referring to connecting two 3750G's together via copper or fiber and trunking them together? I see little to no benefit to setting the two switches up this way. Using the stackwise cable will give you 32Gb/s vs 1Gb/s link between the two switches. Stacking them will also let you manage the stack with one management IP vs having two, and most importantly you can always add more switches for future expansion to the stack and not waste user ports.


hennigan Mon, 06/16/2008 - 16:42

Stackwise has several advantages in terms of performance and management. As others have mentioned, speeds of 32 Gbits/s inter-switch are possible. In addition, a single session can be used to configure and administer all switches in the stack.

The limitation of Stackwise is that all switches in the stack must be in close physical proximity, as the longest Stackwise cable is about 10 feet (3m).

Trunking, either 802.1q (IEEE standard) or ISL (Cisco-proprietary) allows a single Ethernet cable to carry traffic for multiple virtual LANs. It can be used among switches of any distance that the Ethernet medium permits, several miles with fiber.

Trunking doesn't offer the increased throughput or single-stack-management of Stackwise. It is used instead to allow multiple VLANs to ride on a single Ethernet connection.

A third technology which may enter into your decision is Etherchannel (PAGP/LACP) which allows multiple Ethernet links to load-balance traffic.

Which technology or combination to use is dependent on the problem that you are trying to solve.

fultont Mon, 06/16/2008 - 18:21

Thanks everyone for their response. I should have provided a little more detail to explain the request. So here goes...

We are implementing VMWare servers in a new dedicated cluster. This cluster (3 racks) contains two dedicated 3750G-48TS switches with Stackwise and an 4GB Etherchannel (2GB per switch) to a 4507R switch via fiber. Each VMWare server has 6 NICS with 1 dedicated to VMotion, 1 dedicated to VM Console, and 4 dedicated to data with 2 data NICS going to each 3750G switch.

The VMWare implementation team is requesting we implement ISL trunking between the two 3750G switches before connecting to the core switch.

Am I missing something?

hennigan Mon, 06/16/2008 - 18:46

They probably want separate networks for VMotion, VM Console, and data.

Trunking is needed to isolate the traffic to different IP networks per application.

I would think that 802.1q would be a better choice than ISL, especially if any of the server NICs are going to be participating in multiple VLANs. 802.1q has pretty much replaced ISL.

Joseph W. Doherty Tue, 06/17/2008 - 12:15

"The VMWare implementation team is requesting we implement ISL trunking between the two 3750G switches before connecting to the core switch.

Am I missing something?"

Don't think so, but they may be missing something. My guess is they don't realize with the 3750Gs all ready connected with stack cables, there's no reason to define a channel (or trunk) between them. Since stacked 3750s operate as a single virtual switch, trying to interconnect them would loop the two switches.

If they're asking for ISL between the 3750 stack and the VMWare servers, or between the 3750 stack and the 4507R, then the request might makes more sense, but why not use the newer channel standard vs. ISL?

h.parsons Tue, 06/17/2008 - 12:22

They must be referring to an old document, even though ISL is a Cisco protocol I have read Cisco documents were they recommend not using it in favor of the other trunking protocols.


This Discussion