It's old and brittle. What other reason do you need? I'd personally replace it with PVC and additional sound insulation and never have to ever worry about it again.

I'd replace the damaged section and wait and see. Cast iron soil pipes are commonly said to have a 100+ year lifespan but that's a minimum and many are functioning well beyond that time period. If it cracks again, it'll be time eonough to change out the whole stack.

Depending on the size of the crack, I have seen rubber gaskets put on with clamps at top and bottom. Since the pipe only holds water for a brief time, you should have no problems after that. You might also try some epoxy filler--it worked nicely on a cracked copper water pipe for me.

In regards to the service life of Cast iron, there are some cast iron soil pipes in Europe that were installed by the Romans in the 3rd or 4th century and they are still working fine.

Before we begin the discussion of the why & how of your problem I want to caution you very carefully that repairing a cast iron vertical stack is not by any means a DIY project.

You stack is made with cast iron "hub & spigot" pipe, which means the pipe is simply cut off straigth across the pipe on the discharge end, leaving was is known in the plumbing trade as a "Spigot" end. On the upstream or input end it has a broadened cup shape,called a "Hub", that a spigot end is set into then the hub is packed with oakum, which appears like frayed hemp rope that is impregnated with an oil material. After the oakum is packed in they poured about a 1/2" to 3/4" thick layer of molten lead in the hub to complete the seal.

Hub & spigot CI pipe is extremely strong and generally it is capable of being self supporting when installed in a vertical stack however the codes do require additional supports at specified intervals as the stack rises. Generally as they attached horizontal branch lines to the stack they placed wooden blocking under the branch line close to the stack to provide the required support and in some cases they installed additional steel perf tape hangers. The problem here is that very often the steel perf tapes have long since rusted away and the stack is left supporting itself.

The next problem is that as the structure settles over the years it the wooden bracing often looses contact with the pipe, which again leaves the stack self supporting and often as the house settles a long branch line settles until it is actually causing a leverage force against the stack.

While CI is very strong it is also a brittle metal so as the forces of self supporting the weight plus any leverage forces are applied to the stack, the stack is rigid and in time the force on the vertical stack causes a section to crack vertically.

The proper course of action is to cut out the damaged section and replace it. The section can be replaced with no-hub cI or PVC but you must be aware that when you cut the pipe in all likelihood there is no support on that pipe above the cut and you run an extremely high risk of the entire remainder of the stack to come crashing down on you. Do not take this lightly as CI pipe weighs approximately 10 pounds per linear foot and a stack from the basement to the roof in a two story house can easily weigh in at 400 to 500lbs. Just imagine an V-8 engine block poised and ready to fall when you cut the pipe.

It really takes some serious measures to provide support to the stack before making the cut.

I once saw a guy cutting a short section out in a basement and the moment he completed the cut, the entire stack fell straight down in a heartbeat. Fast enough that it hit his foot before he had a chance to move and he had 4 toes instantly cut off, not to mention that it also ripped horizontal branch lines all the way up the stack so they had to open all the walls and replace the line all the way up to the roof...

This is one job that is best left to the pro's who have both the experience and the necessary equipment to support the stack while repairing it.

Modern cast iron waste pipes use the rubber gaskets you mentioned and the system is called "no-hub".

Here is a link that might be useful: no-hub images

On a walk through before purchasing our 1912 home, when the toilet upstairs was flushed, there was water running down the stack to the basement. They opened the wall to see how bad it was, and there were two or three big corroded holes (like 6"x12-18"), on the second floor section. We were told that it was probably due to the second floor bath not being used for years (like 10+ years). I didn't end up seeing any of the first floor section to know its condition. It ended up that the seller paid for replacement with pvc. It sounds like ours was quite a bit worse than yours though, it was definitely more than a crack!

"It ended up that the seller paid for replacement with pvc. "

Always go for the cheap repair.