MDS to MDS ISL configuration and behaviour

[Andrew C]

Hi,

I've been Googling for days and really getting anywhere with this so I'm hoping someone here has some knowledge I can tap into!

The situation:

We have a new SAN and MDS (9148) installed and just about to start migrating VMs/physical servers to it.

There is a requirement for a SAN Copy of block data from the old SAN to the new SAN to allow the migration of some machines. Once this is done, those physical machines will then be repatched into the new MDS and zoned accordingly.

We can't connect the old SAN to the new MDS (for several reasons), nor can we connect the new SAN to the old MDS (for several reasons).

This is a 24/7 business. Downtime has been negotiated on a per application basis and to derisk the migration, it will be taking place over a 8 week period.

My conclusion is that we need an ISL between the MDS's. I've found a couple of guides on Google and YouTube for this but I'm missing something as it's not quite making sense yet.

My questions:

• Is setting the interface mode to "E" on each side enough to make the connection an ISL? Or are other steps needed?

• I've seen something about telling the ISL which VSAN(s) are allowed over the ISL - but not seen how to do so. Any clues would be appreciated! For info, old MDS1 and new MDS1 both have a single VSAN (10), and old MDS2 and new MDS2 both have a single VSAN (20). There is no inter VSAN routing.

• I've seen reference to "Zone Merge Behaviour". Can someone confirm what this is please? The old MDS's have their zonesets which are appropriate for the devices connected to them. The new MDS's have their zonesets for the devices they have connected. Will creating an ISL cause these to merge into a single zoneset? If so, should I expect any downtime?

• Finally, would anything change if the "old MDS's" were actually Nexus 5K's (-5548UP which is what is at the DR site)?

I'm happy if you tell me CLI or DCNM based methods. I should add here I can do the "basics" (aliases, zoning, zonesets, etc) in MDS, but this is not something I've come across before. There is no test MDS devices to test with, and at this stage I'm writing the change request rather than trying to get it working on the fly.

I've attached "an excellent diagram" in case that helps!

Many thanks in advance.

Andrew

[jihicks · Accepted solution]

Hi Andrew,

I don't see where anyone has answered this.

• Is setting the interface mode to "E" on each side enough to make the connection an ISL? Or are other steps needed?

If you connect two MDS ports together that are in AUTO mode ( default ), then they will connect as an ISL.   Setting the mode to E is a good idea too.

• I've seen something about telling the ISL which VSAN(s) are allowed over the ISL - but not seen how to do so. Any clues would be appreciated! For info, old MDS1 and new MDS1 both have a single VSAN (10), and old MDS2 and new MDS2 both have a single VSAN (20). There is no inter VSAN routing.

By default, if trunk mode is on, then all configured VSANs that are on both switches will be trunked. If there is only VSAN 10 on the switches, this will be trunked, providing there is no merge failure.  FCDOMAINs need to be unique on each switch per VSAN.

• I've seen reference to "Zone Merge Behaviour". Can someone confirm what this is please? The old MDS's have their zonesets which are appropriate for the devices connected to them. The new MDS's have their zonesets for the devices they have connected. Will creating an ISL cause these to merge into a single zoneset? If so, should I expect any downtime?

Look at this reference, it has the information:

Zone Merge Behavior When Two MDS Switches Have Different Active Zoneset Names Are Connected

Also, you can use DCNM to perform a Zone Merge Analysis before you do this.

See "Analyzing a Zone Merge" in this document:

http://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/6_x/configuration/guides/fabric/DCNM-SAN_published/fm_fabric/zone.pdf

• Finally, would anything change if the "old MDS's" were actually Nexus 5K's (-5548UP which is what is at the DR site)?

No. 

[Andrew C]

Thanks for taking the time to comment. I actually went ahead and created the ISL and got the results you mention. I found a couple of old MDS's which I tested my processes on before doing it for real which built a lot of confidence in the process.

I'd love to list here a set of points to note for other people doing it but it was a lot more straight forward than I imagined it would be - just as you explain.