It won't affect it much at all. The beer may be slightly darker, and your final gravity may be just barely higher than it would have been.
This is because boiling can cause darkening of the malt sugars via maillard reactions (some people call this caramelization, but it's not true caramelization) which leads to slightly less fermentable sugars in your wort.
Apart from a minor difference in color, I'd be surprised if you could tell at all.
Some people say hop utilization can be affected, but I think that's been fairly debunked. Especially with the low hopping rates of porters, your IBUs won't be affected at all.
Edit: just noticed that you're brewing a porter. I take all back- the beer is so dark and malt-forward to begin with, you won't be able to tell the difference.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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