For what it's worth:
https://forum.atlas-games.com/t/the-mighty-forest/5049/49
Spells and supernatural abilities don't generally have MR so most of Aegis's effects don't need to penetrate. So the major concern is really just aegis's ability to exclude creatures with a might score. This makes your ideal penetration total equal to the Aegis's level just like wards.
I did the math and wizards communion really helps with penetration totals it's surprising. With an Aegis specialist and even a modest sized casting group high level Aegises with equivalent penetrations totals aren't that hard. If he can reliably hit say a 60 (for the sake of argument and easy math) then with one other Magus to help him he can put up a Aegis of 40 with a penetration of 40. 2 helpers it's 45. 3 helpers it's 48. 4 helpers is 50. So on and so forth and this is without figuring in the penetration ability or a mastery with a penetration specialization. Of course with small casting groups the wizards communions might be prohibitively high but once your dealing with a casting group of around 4 or 5 the communions only need to average in the twenties.
Assuming both magus know Wizard's Communion at level 40, or some other repartition. With 2 helpers, 40+20+20 is easier to reach.
I have to echo this arguement entirely. We play that an Aegis needs to penetrate
I think this makes sense otherwise when a Magus walks into a foreign Aegis, then his Parma as a magical effect would also be resisted (as per wind of mundance silence), as many Tribunals are likey to be protected by an aegis, the younger magi, walk in, loose their parma and feel the full force of the gift. Accordingly Chaos ensues.
Again a Ward of Lvl 10 won't stop a lvl 15 beastie, even if it has a penetration of 100. As I see an Aegis as no different to a ward in this regard then An Aegis of level 25 won't stop a beastie of level 30 even if it has a high penetration. But I still see no reason which excludes the penetration.
Wizards communion becomes INCREDIBLY useful in this situation. Having 1 Maga per magnitude cast the spell means the level drops accordingly and the penetration skyrockets. If the main caster has a total + penetration of 30 all together then on wizards communion with 5 magi on a level 25 aegis means the spell goes form penetration 5 to penetration 25. There is flavour text about how major council meetings and feasts follow the casting. What could be more symbolic then the magi comming together the cast the spell that holds them all safe.
The only thing where I think it could be argued it DOESN'T need to penetrate (and I can be swayed either way here) is the casting total alteration. Is the spell suppressing the spell as cast, the ambient (fluid) vis or directly effecting the Magi. If it's the 1st 2 then no it won't need to penetrate, if its the 3rd then yes it would.
A
But what about the mastery "magic resistance" when it comes to aegis?
If the aegis has to penetrate, doesn't that mean that every battle magus will learn an aegis (level 20 or more), and spend 1 season to master it in that way, so that their effective MR is doubled against foreign aegis?
And, if that is the case, they can walk in enemy aegis during wizard's war, and vice versa. (According the fact that a parma of 4 (in vim), + a score of 5 mean 60 magic resistance, and is not hard to have... but having an aegis with 60 penetration is not that easy. (Let say a level aegis 40, that needs 8 magi in the communion and a initial casting total of 60+(40/8)= 65. Which need, let say 18 in vim, 18 in rego, 9 in stamina/artes/philo and a roll of 5, and penetration ability 5)
What effect does the Aegis have on the magus, that you'd want extra MR? They don't have MM and, except for the few personal spells they will be casting within the Aegis, there's nothing left to protect.
I don't want to live in a world were Aegis has to penetrate. 
Besides the fact that the idea of wards having to penetrate instead of the acting party having to beat the ward makes little to no sense to me outside of the game, the entire purpose of a Aegis is blown if you have to stack 60-100 levels of penetration on top of it.
Axiom is: personal spells are under the parma, since parma can't stop them to affect you.
Hypothesis: you are invisible in an enemy aegis, because the aegis has not penetrated you due to your aegis mastery magic resistance.
What happens when you go into magus A sanctum during the night, seize a dagger [which doesn't become invisible since it was not part of the initial spell, hence is not magically invisible and doesn't bounce off his parma] and kill him?
During a wizard's war, this is really bothering.
In my opinion, the aegis not penetrating prevented that trick: your magus would become visible and grogs - who are supposed to guard the sleeping magi - can see him.
The magus didn't use any spell inside covenant, but the fact that the aegis couldn't dispell his personal invisibility when he entered totally ruined its purpose.
Well, when those 'personal' spells start with a Ball of Abysmal Flames and follow up with a Clenching Grasp of the Crushed Heart, you really want an Aegis to do something. Because, you know, an Aegis is supposed to provide some defense during a Wizard War...
The point is those spells don't have MR so they would be effected by the aegis no matter what the penetration was.
In an aegis must penetrate, a hermetic breakthrough idea: give MR to spells.
Which give me good ideas of stories huhu.
Test: 4 magi use WC for an Aegis 40, rolling 62 as casting total. Penetration is 52. Attacker's Veil of Invisibility will fizzle if his casting total was less than (20 + 40/2 = 40), but only if his Vim MR is less than (52 / 2 = 26 with mastery).
Test: a magus cast Aegis 40, no penetration. Attacker's Veil of Invisibility will fizzle if his casting total was less than (20 + 40/2 = 40). {Much easier as you could roll 32 and still get it up, but I wanted the same numbers.}
So, your problem with penetration is that it is much easier to get MR 26 than to get 60+ in a ritual?
I don't understand at all what you wrote 
The aegis effect is:
1- prevent might creature to enter (as a ward would)
2- anyone inside which was not invited or haven't cast the aegis suffer level/2 malus in casting total.
3- block and dispell foreign spell even if it couldn't be blocked.
4- item must beat a MR of the aegis level
So with the debate: two possibilities.
- if no penetration needed:
1-> compare might to aegis. If might >, the creature enters without trouble
2-> no problem. You are invited or not, you have cast the aegis or not.
3->no problem. Any spell of an uninvited magus or not present at the time of casting is blocked
4- no problem. Is penetration of item enough or not to beat aegis level?
If penetration is needed:
1-> does the aegis penetrate if the might < level of aegis? No change for might> aegis.
2-> If you are not invited and weren't present at the time of casting, this effect only apply if: you are magic creature or magus casting a non personal spell. (since spelL/power has no MR)
3->if the aegis can't beat your MR, any personal spell on you is unaffected, even if you are not invited and weren't there at the time of thea egis casting. (here is my example lying)
4- no problem. Item has no MR so it's the same.
My problem lies in 3.
And yes, I think it's way easier to have a big specific MR against aegis than to have a big penetration against specific individuals.
So here is a question? Does Parma have to penetrate? It is a magical thing created and sustained by magic right. What ever sort of ability generates Parma, it still comes crashing down under a Wind of Mundane Silence. That sounds like sustained by magic to me, and I don't think there is any specific exception written for Parma not needing to penetrate.
So Loganus the combat twink buffs himself up with super sharp diamond claws coming out of his knuckles using a personal spell and stabs you in the heart. Congratulations he doesn't have to worry about penetrating your Parma, because all that magic resistance is magically created not natural. Personal spells protected by Parma, awesome.
Wait but your Parma is also magical so they cancel each other out right? Nope if we are going there Parma is in fact protected by itself (again description of Wind of mundane silence as the example) So the bigger Parma always wins. Parma specialist stealth assassin here I come.
So a Magus, all his personal spells, all his cloths and any items literally "very close to him" (so enchant the hell out of those things) are unaffected by a Parma Magica lower than his own, no penetration necessary. On the plus side form resistance is apparently natural so there is that.
Parma is not a supernatural ability but an arcane one.
And if your post is to say that the aegis having to penetrate is absurd like would be a parma needing to penetrate another parma, yeah, i agree.
in fact, your whole post sound like a joke so I will just ignore it...
What does that matter it's clearly a magically sustained effect that can be brought down by Wind of Mundane silence. Things created by constant magic can't effect something protected by MR period.
Pretty much. Just pointing out the train of thought that says aegis needs to penetrate could have further perhaps unwanted consequences.
At your own peril. 
I still don't get it.
Take the example of Bonigus of Tytalus.
He's sitting in his own sanctum, with an AC to his filius Claudius of Tytalus, who's a member of another covenant.
Bonigus decides to cast Summoning the Distant Images on Claudis. HE has an AC and is generally an older magus, so penetration is not really a problem.
It is (ofcourse, obviously) scrying but "I was worried about some rumors that he'd begun experimenting with demons! As his parens it is my sworn duty to stop hm from any such thing! Luckily there was no truth to the rumors..."
If the Aegis does not need to penetrate, Bonigus just needs a high enough casting total.
But what happens if the Aegis must penetrate?
Nothing, a spell has no MR.
Penetration is only needed to stop MM creatures and to fizzle personal spells on magi. The only good point about penetration is that it forces you to cast Aegis as a group instead of being the sole responsibility of a specialist.
But since those who travel around the boundary with the caster get a "free" token and can give token... why wouldn't it be a group activity already, without having to penetrate.
The fact that the spell say "The entire covenant usually participates in the Aegis ritual (2), which often ends with the partcipants walking around the perimeter (...) Magi do not need to be able to cast the spell to participe in the ritual, and, indeed, non hermetic wizards can participate as well. Only one magus needs to actually cast the spell" give me some arguments to decide I don't see anymore any doubt in it: the aegis must not penetrate.
Reason 1: if the aegis would have to penetrate, it would need a wizard's communion (since only high specialised specialist focused in ward can cast normal wards with enough penetration, and aegis are outside normal wards focus and can have a higher level than most wards) and casting a communion while doing a ritual needs a communion ritual with year duration (since I do think that you will agree that a formulaic communion is just not enough to sustain a year spell, and anyway, because the muto magic needs the same duration as the spell enhanced [see errata about MuVi]). Doing 2 rituals at the same time would inflict by each magnitude of aegis spell, 150 dice of concentration, with at least 4 magnitude (=> at least 600 concentration rolls). Ease factor could be 6 with MuVi magic.
But a stressing roll. Meaning, all statistics kept, the spell fails everytime.
You will say: but the caster of the aegis has just to NOT cast the wizard's communion, and concentrate on the aegis... well, why not, but I think it's only possible if your covenant is made of multiple people willing to devote themselves to knowing a wizard's communion high enough... why not.
But no.
Because the Wizard's communion spell explicitly states that the participants are caster since they gain the same number of warping points than the real caster should the spell botche.
If reason 1 is not enough to convince you, reason 2. The spell include participation of people. We could argue this is "in the communion". yet, the spell speak about the ritual.
Reason 3 : okay so by ritual we read "communion". Yet, communion is about hermetic magic. Can a mundane participate in the casting of a spell? Would you allow it, reasonably? I wouldn't. Too easy to cast level 50 spell with your 8 grogs if you and your friend do know a MuVi 50 wizard's communion.
If a mundane cannot, why? Because the spell say so... yeah but really, why?
Because he has not the gift or because he doesn't have his arts opened?
- if it's the gift, that would allow non hermetic magi to participate in communion.
- if it's the hermetic arts, it would allow only hermetic magi to participate in the communion.
Since we speak about "magi", I suspect it's the hermetic arts. Otherwise, we would use "gifted".
But the spell speak about "the entire covenant". So that mean grogs also. Thus, anyway, it means that grogs would have to be there in the "ritual= communion". We could say at this point: but ritual is not communion.
If so, then we are back to base. the magus is alone. It's irrealistic to think that every magus in charge of aegis can do a ReVi 40 ritual spell. So aegis is a matter of specialist only, which negates the argument about "The only good point about penetration is that it forces you to cast Aegis as a group instead of being the sole responsibility of a specialist.".
Summarizing:
- penetration on aegis mean that aegis is level 40 maximum reallisticly speaking, and yet we speak about labtexts of level 50 and more, which are ancestral, thus used by generations of magi not especially the uberspecialist which developed it.
- Thus need wizard's communion, ritual spell.
- But every magus actually casting the communion is casting the communion AND the aegis. This needs at least 600 rolls stressful. Only one botch fail both spell. Meaning that there is no chance the spell will ever succeed.
You aren't making any sense. You should read Wizard's Communion again.
First, WC is a MuVi spell, I don't think grogs could cast that.
Second, the text of Aegis makes me believe only wizards may participate in the ritual and distribute tokens.
Third, refer to Fenicil's rituals (HoH:TL p 76), which is the original use of WC. I think you'd have a hard time arguing we'd need a Year duration, ritual version of WC when it works fine as is for Fenicil's rituals.
Fourth, compare to MuVi Name of Power (TMRE p 76) "When using Names of Power in Hermetic theurgy, a maga does not cast them as actual spells." and "The caster may apply the bonus to the Casting Score of any spell (Spontaneous, Formulaic, or Ritual) ..."
BTW: I don't care which is RAW, Aegis with Penetration is too complex for no return.