Spanning Tree turned on by default

[Wayne Fussell]

Is there a reason not to turn on STP/RSTP/MSTP on all our procurve switches. Recently a seemingly simple patch created a loop. A 10 sec broadcast storm flattened our VMWare servers. I just wonder if there is a reason it's not turned on by default.

[Matt Hobbs]

Hi Wayne,

It's a good question and something I honestly don't have the true answer to. The only reason I can think of as to why it's not enabled by default is "That's the way it's always been".

There are other factors though that have probably let to this decision.

If you go back to the 4000M days, it supported the older 802.1D-1998 flavour of STP which by default did not enable the edge-port status (a.k.a. PortFast). This of course is a problem when your machines bootup faster than 30 seconds, it also generates a topology change - if you've got enough switch ports generating TC's you're going to have an unstable spanning-tree. If there is anything scarier in an Ethernet network I'd like to hear it, oh that's right, an undetectable loop!

We then move on to the likes of the 2524 which began to support 802.1w (RSTP). Fortunately once spanning-tree was enabled, it also enabled edge-port status by default cutting down on those nasty TC's but at the same time, opening you up to the risk of brief transient loops (which may not always be that brief).

Fast forward to the 5300/2600/2800/etc, where HP started supporting multiple flavours of STP, 802.1D, 802.1w and 802.1s. Which one should HP enable by default? Also 802.1s brought us back to having the edge-port status set to no (remember those TC's?).

Then we arrive at today with the 8200/5400/3500 which only support 802.1s, but were then brought to the later 802.1Q-2005 standard (incorporates 802.1s) which now supports the auto-edge-port feature.

As you can see, it's an administrators nightmare!

I wouldn't say that ProCurve are completely alone in this boat either, Cisco for example do enable STP, however you generally need to enable PortFast on your edge-ports. Not that many people just install Cisco switches without configuring them correctly though.

So really, your best bet is to ensure that spanning-tree is enabled and configured properly for any given switch. I wouldn't trust spanning-tree to give you a satisfactory solution in a default configuration - there is just too much else that can go wrong with it (e.g. an old switch asserting itself as root somewhere near the edge of your network). That is better than a loop in the network though.

One other feature you may wish to look at - 'loop-protect' which is now available on some of the newer ProCurve software releases.

Going back to the original question, another possible answer as to why it's not enabled by default because in a way it's actually more consistent than trying to chase a standard that's constantly evolving.

I'd be interested in others thoughts on this topic, and any other features you'd like to see enabled by default. (Personally I'd like to see 'console inactivity-timer' enabled by default).

Matt