Skip to main content

Too much redundancy will kill you

A customer asked me to verify their vSphere implementation. Everything looked perfectly redundant, in the traditional elegant way: cross over between layers to avoid single points of failure. I had to break the bad news: too much redundancy can mean NO redundancy.
In this case: host has 4 network interfaces (2x dual port card). VM's connect to a vSwitch, which has redundancy over vmnic0 and vmnic2 (using 1 port of each card). Another vSwitch for the storage traffic, same level of redundancy, using vmnic1 and vmnic3. Looking good.
Then the physical level. 4 host interfaces, 2 interconnected network switches. The traditional |X| design connects the two interfaces of every card to different switches. Looking good.

But looking at both configurations together, you'll see that every vSwitch gets connected to one physical switch. The sum of two crossed redundancy configurations equals no redundancy at all.
Enabling CDP or LLDP can help you identify this problem, as you can identify on every interface which physical switch it connects to. In this case the CDP physical switch identifier was the same on vmnic0 and vmnic2, and again the same on vmnic1 and vmnic3.
I advised changing the cabling to four straight || || connections, vmnic0 and vmnic1 to the left switch and vmnic2 and vmnic3 to the right switch. That re-introduces the redundancy they thought they had.

Comments

Anonymous said…
That is actually quite funny.
18/10/11 15:12
Anonymous said…
Nice catch. Many designs have this kind of error...

Obviously you would have to change at the vSS level if you hosts are blade type since the X is physically wired in the back plane of the chassis; NICPort1->SW1, NICPort2->SW2
18/10/11 16:10
Post a Comment

Popular posts from this blog

Volkswagen UHV bluetooth touch adapter & its problems

My Volkswagen car has the "universal cellphone preparation" UHV built-in. This is the main part of a car kit, but requires an additional adapter for connecting to a cellphone. At first, I was using an adapter for my good old Nokia 6310, even after I changed to the Nokia E71. Connecting was easy: pair the phone with the "VW UHV" bluetooth entity, and done. This has the phone connected to the car kit at all times, so even non-call-related functions use the car audio system (e.g. voice recognition). But progress will have its way, no matter what happens. So in comes the "bluetooth touch adapter". Instead of a phone-specific adapter, this is a small touchscreen device that slots into the UHV dashboard mount. Connecting a phone is very different now: the Bluetooth Touch Adapter connects to the "VW UHV" device via bluetooth the phone connects to "Touch Adapter" device, also via bluetooth The device doesn't allow step 2 if step 1 didn'...
12 comments
Read more

Which vSphere version is my VM running on?

Several years ago, I created a list of ESXi versions with matching VM BIOS identifiers. The list is now complete up to vSphere 7. Your Linux runs on a VMware VM, but on which ESXi version? Even without access to the host nor vCenter, you can see for yourself: run "dmidecode" and look at lines 10, 11 and 12. The BIOS release date, the address and the runtime size are unique for each ESXi version. Starting from 7.0 U3, no address and size is reported, and the "Phoenix Technologies LTD" name changed to "VMware, Inc.". Look up your result in the following table: ESXi version   BIOS release date   Address   Size ESX 2.5 04/21/2004 0xE8480 97152 bytes ESX 3.0 04/17/2006 0xE7C70 99216 bytes ESX 3.5 01/30/2008 0xE7910 100080 bytes ESX 4 08/15/2008 0xEA6C0 88384 bytes ESX 4U1 09/22/2009 0xEA550 88752 bytes ESX 4.1 10/13/2009 0xEA2E0 89376 bytes ESXi 5 01/07/2011 0xE72C0 101696 bytes ESXi 5.1 06/22/2012 0xEA0C0 89920 bytes ESXi 5.5 07...
7 comments
Read more

Reset lost root password on vSphere ESXi 6.7

VMware's solution to a lost or forgotten root password for ESXi is simple: go to  https://kb.vmware.com/s/article/1317898?lang=en_US  and you'll find that "Reinstalling the ESXi host is the only supported way to reset a password on ESXi". If your host is still connected to vCenter, you may be able to use Host Profiles to reset the root password, or alternatively you can join ESXi in Active Directory via vCenter, and log in with a user in the "ESX Admins" AD group. If your host is no longer connected to vCenter, those options are closed. Can you avoid reinstallation? Fortunately, you can. You will need to reset and reboot your ESXi though. If you're ready for an unsupported deep dive into the bowels of ESXi, follow these steps: Create a bootable Linux USB-drive (or something else you can boot your server with). I used a CentOS 7 installation USB-drive that I could use to boot into rescue mode. Reset your ESXi and boot from the Linux medium. Ident...
Post a Comment
Read more