No.... But make SURE you are right.
Even in the example you are giving I understand that spanning tree is VLAN orientated and shouldn't care about layer 3. However if you are using the switch to hand out IPs he could have just misspoken or perhaps remembering an odd bug.
The reason you have to say something is:
you possibly look like a dumbass later if you spend 3 hours troubleshooting nonsense.
if your boss makes you continue even after you say your opinion and he is wrong, he is much more apt to trust you later. By saying nothing he hasn't gone through this process. Don't be a dick about it or argue, if anything joke about it or bet a soda.
if your boss believes you after you explain and you are right - well this is a really good sign for your relationship going forward. You aren't even giving him a chance right now.
your boss, being a boss might have 100 things going through his mind. I know when I had 40 guys reporting to me I could barely hold a conversation. Put me in front of a switch/router to configure and Iit might take a couple minutes to get going but was soon just fine. He may or may not get it. If he does he will appreciate that you are cutting him some slack. If he doesn't then he will want an ally to keep him true with tech issues.
if you end up being wrong, accept it. Still speak your mind. Your boss will learn what to trust you on and what not to. I had no issues with guys saying whatever to me. Sometimes I was wrong, sometimes right, a lot of times it was just miscommunication. Often these discussions are when you learn the most.
when you are disagreeing, do it one on one. Don't put boss on the spot.