Is flanking useless in D D 5E

Ok... so "flanking" came up last night. One of the other players (we're all seasoned 3.5E players) believed that flanking a target with an ally grants both allies Advantage against the target.

That didn't seem right to me, as I didn't recall "flanking" (or an equivalent) anywhere in the 5E PHB. There's the "Help" action, which IIRC DOES grant Advantage against a mutual target within 5 feet, but using "Help" requires one of the flankers to spend an Action instead of attacking.

Anyhow - The DM and the rest of the players (myself included) rolled with the "Advantage for flanking" ruling for the rest of the encounter, but that feels both OP, as well as in the bounds of Rogue-specific ability more than anything else.

So - assuming I'm wrong...

Does Flanking exist?
What constitutes "Flanking," positionally?
What does it actually do?

Assuming I'm right...

Do I understand the "Help" action correctly? If two allies are both within 5 feet of the target, one of the allies can spend an action on his turn to grant the other ally Advantage against that target on their turn?

Flanking in 4e gave combat advantage on attack rolls, but it is pretty specific - you must occupy opposite corners or opposite sides. One corner and one side won't do it. Also, combat advantage in 4e was only +2, whereas advantage in 5e is approximately equivalent to +5.

Flanking doesn't exist in the standard 5e rules, but I believe there is an optional rule in the DMG for it. I don't know what that rule is, but if I were DMing, I think I would have it grant +2 rather than advantage.

(edit: one issue with my houserule would be whether or not the +2 would stack with advantage, becoming even more powerful in some situations... I don't think it really matters that much, and I would argue that there are some cases where 1 advantage doesn't reasonably model the improved attack position of the situation. However, it would be pretty simple to say that it doesn't stack with advantage (but probably should still get you a bonus when you have disadvantage).)
  • Last edited Fri Jan 23, 2015 4:41 pm (Total Number of Edits: 1)
  • Posted Fri Jan 23, 2015 4:14 pm
Flanking does not exist in the core rules.

However, There is a flanking variant in the DMG which grants advantage to the attacker in flanking situations (I think you have to be on opposite faces of the target). IMO, it's pretty unbalanced and over powered.
I guess that's why it didn't tip the DM off as "odd," since it's in the DMG (the players have all made a conscious decision not to look in the DMG). But yeah... that rule seems realllllllly OP to me, plus it almost completely invalidates the Help action, which is core rules.
We've been using the flanking rule in gridless combat, without being too fussy about the exact positioning of the flankers. In addition to making the Help action pointless, as you say, it also invalidates monster traits like Pack Tactics. So I'm considering abandoning it.