HPE EVA Storage

Difference between ISL and Trunking.

 
Scorpianzzz
Advisor

Difference between ISL and Trunking.

Hello,

 

I would like to know the difference between ISL and Trunking.

For eg. in case of two FC cables connected between 2 SAN switches - 8gb/s ports, when I do trunking, I get 16gb/s bandwidth. Will only ISL configuration give me the same bandwidth or it is just one path active and the other path inactive (active when the other path fails) ???

 

 

Cheers

Scorzzz...

9 REPLIES 9
Johan Guldmyr
Honored Contributor

Re: Difference between ISL and Trunking.

Hi, 

 

with fibre channel both links would be available.

 

If you also happen to use Exchange-Based Routing Protocol the (most likely SCSI) exchanges will be load-balanced between the members of a trunk. If you use port-based then it would be good for utilization purposes to also enabled DLS - dynamic link sharing. See output of "aptpolicy" for which is configured (this can be different between switches).

 

Trunking also provides some protection. If a member in a trunk gets disconnected your servers shouldn't notice anything.

Scorpianzzz
Advisor

Re: Difference between ISL and Trunking.

Thank you Johan for your reply.

 

I only had a basic query regarding ISL and trunking.

 

When we trunk 2 ports of 8gb/s we get a bandwidth of 16gb/s.
What if we only configure ISL of 2 ports, will we get 16gb/s bandwidth ? Will both the paths be active when we configure only ISL? If so, then what is the difference between ISL and trunking ???

 

 

Cheers

Scorzzz...

Johan Guldmyr
Honored Contributor

Re: Difference between ISL and Trunking.

Are you wondering if servers will be able to use both paths, and possibly doubling the paths they see to their targets?

 

The switches will have two ISLs of 8Gbps each. Both will be active.

Depending on your aptpolicy/dls they may not be used equally - effectively providing <16Gbps.

 

Are you asking for the difference between two ISLs and a two-member trunk from a performance point of view only?

Scorpianzzz
Advisor

Re: Difference between ISL and Trunking.

Hello Johan,

I would like to know the difference in the working of both and also the difference with respect to performance.
Johan Guldmyr
Honored Contributor

Re: Difference between ISL and Trunking.

Performance is a wide subject.
There is information about trunking in the "Fabric OS Administrator's Guide" - I think this would be a good start.
Sheldon Smith
HPE Pro

Re: Difference between ISL and Trunking.

Brocade describes them as follows:

  • Inter-Switch Link (ISL) - A connection between two switches using the E_Port.
  • Trunking - A fabric feature that enables distribution of traffic over the combined bandwidth of up to eight ISLs between adjacent switches, while preserving in-order delivery.

See the attached Brocade paper on ISLs and Trunking.

 

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Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company

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ggurureddy
New Member

Re: Difference between ISL and Trunking.

Smith you are correct.

tbone654
Occasional Advisor

Re: Difference between ISL and Trunking.

or in other words... The Difference between "walking to work" and "carrying a bag lunch".  Hmmm... you say?

 

The first event is something that happens... The second event is something that takes advantage of the other event...  think of efficiencies of scale...  You would prefer to deliver multiple lunches to work if you could, but that's not always the requirement.  But walking to work is still a good thing, even if you don't have a single bag lunch to bring or deliver.  I try...

 

You can trunk across multiple ISL's (inter-switch links) if they are configured properly.  As stated earlier, trunking allows "in order delivery" of packets from an initiator connected to one switch to a target connected to a different switch.  There is a difference between trunking on a CISCO switch, and a BROCADE switch because of the VLAN concept on the CISCO, but the general result are the same, speed.  As an example, if you configure 8 ISL's (I think the limit is 16 ISL's per switch) between two switches, you can trunk across any 2 or more ISL's.

 

You may chose to dedicate multiple trunked ISL's to a particular array, cluster or even an particular application...  One application that comes to mind is FCIP links to a remote site for replication.  Lets say you have an array somewhere downstream in your infrastructure in Miami, that needs to replicate to Peru.  You will probably do whatever you can to "stream" data as quickly as possible to the switch that has the FCIP links to Peru.  Using multiple ISL's, you trunk across as many ISL's as you need for performance.  I know, it's a stretch, I would much rather move that storage to the switch with the FCIP connection, even if I had to leave the servers

 

But the heart of your question is whether by having 2ea. 8Gb FC cables, do you get 16Gb of bandwidth by trunking?  The answer of course is no.  Your bandwidth is limited by the overall network contention on all the ISL's at any given time.  Trunking by itself does not increase bandwidth.  You may have additional request queues for each fiber cable you add, yes, but the net performance result does not improve by trunking multiple ISL's by itself.  MPIO settings, queue depth, number of ISL's, production/development mix, service queues on the storage, separate backup SAN/TAN availability, replication requirements, etc. all affect the configuration items (CI) of the infrastructure.  In most cases, by trunking existing ISL's, you will see NO improvement of speed (bandwidth), in most environments.

 

So unless you are a hedge fund or a telco, or have some heavy replication requirements, the recommendation is to allow those ISL's to do what they need to do to move traffic themselves.  If you can see a small percentage of the ISL's being hit harder than others, there are other configuration items to be tweaked first before trunking.  Trunking can cause more problems if not done properly.  Example, P9500 configured with multiple cache partitions for Prod, Dev, Replication, external storage, etc. can leave you cache poor for any or all those environments at different times.  It's always recommended to eschew over-configuring for cache performance before implementing partitioning.  The same holds true for trunking.  Especially if your wondering if just by trunking, you could double bandwidth.  NOT!

 

Friends, don't let friends implement trunking...

bharathcloud
New Member

Re: Difference between ISL and Trunking.

Usually we connect two san switches using fiber optical cable, the link or connection between two switches we call it as ISL(inter switch link), without trunking there will be no load balancing between isls, for example isl1 port sending 2g data, isl2 port sending 4gb data, after completing it's task isl1 port doesn't share isl2 port load because there is no trunking, we need to enable trunking on isl ports on both switches.

Trunking in brocade meant for load balancing, but in cisco it has differenet meaning for trunking there is a portchannel concept is there for load balancing between isls

 

I gave this  based on SAN switches.