Let's play network guru - Multiple links between switches

[Jeff Golas]

Ok, here's a quasi-hypothetical (quasi because its real but not earth shattering). Long story short I'm doing a massive amount of changes/maintenance tomorrow and I'm trying to prepare beforehand as much as I can.

Right now I have 2 switches connected together with one link. I'm trying to add (or switch to) a link team with 2 links. Spanning-tree IS enabled (i know this has nothing to do with teams but read on).

I disabled 2 ports on each switch, enable lacp, created my teams, and enabled the first of two ports only to have the switch go bezerk with a loop, which thankfully I was able to turn back off quickly (did I mention these are production?). So the question is - if spanning tree is enabled, in which case it should have detected the loop and shut it down automatically, why did the loop still occur?

[Mark]

No clue, but I do have my switches connected with a team.  They are Dell and HP switches, though.

[Jeff Golas]

For what it's worth these are all HP. I'm really surprised that the loop occurred, although I *think* I have a clue as to what happened. Just sucks because I want to get this done.

[Jeff Golas]

I was able to work around it by adding the existing uplink to the trunk group as well. It did go down for a moment but not enough to cause any issues. The PITA part was doing all the VLAN commands of course while my boss just decides to come over and chat at that very moment.

Once I did that on both switches...I was able to bring up the second and third links in the group no problem. I'm a little leary about taking down that first one (because I really only need or want 2) but for now I'm leaving it until after hours.

[Mark]

Yeaqh, I pretty much prepped for it, knew exactly what I was going to do, and did it in the morning before everyone got here.  I didn't have any issues anywhere though.  I just used the current single link, added it to the lag group and then plugged in the 2nd link.

[Jeff Zylstra]

Since I know nothing about this, but I am always curious....  I'd say that it has something to do with the switch expecting a certain port or ports to be active in order to detect the links, and those port(s) aren't active, or aren't configured properly.  I'd say don't deactivate any ports, but rather let the switch "auto-discover" the ports. 

I usually have problems when I try to configure "smart" things myself and not let the equipment do it.  It never amazes me when I find that the equipment is way smarter than I am!  Sounds like you need to reserve your playtime for after hours in a non-production environment and let it do what it probably wants to do, but may not be allowed to do right now.   

[Mark]

Yeah, I didn't disable anything.  I set it all up and didn't plug in any cables until it was ready.

[Jeff Golas]

I disabled the ports to try to avoid what actually happened. Spanning-Tree is enabled, so that if two links between two devices are plugged in, there won't be a "loop" between those two devices. Same goes for trunks (so I thought). I enabled that second link, and instead of spanning-tree disabling (blocking) the second link, it enabled it and boom loop happened.

I *believe* it may be because I just realized this switch has older firmware on it. Going to update it tonight and see what happens.

[Jeff Zylstra]

I always like it when you IT gurus experiment and I get to benefit from your pain without having to experience any myself. Good luck. Hope all goes well!