Why IKE Lifetime and IPSec Lifetime is used? Can't we just run the VPN without re-negotiating?
Why re-negotiation in VPN is happening? What is a necessity?
Every key is susceptible to Brute Force. This is unavoidable.
If all you ever use is one key, and an attacker manages to brute force your one key, then all your data that you have ever sent or received is compromised.
To limit the scope of potential compromise, IPsec performs "rekey" operations, so that if a brute force is successful, at best only 8 hours of your data is compromised.
Moreover, the keys used in each direction are different, so if a single key is compromised (which is not trivial), only 8 hours of one side of the conversation is compromised.
What will happen in re-negotiation? Will it initialize the entire main mode and quick mode messages?
When Phase 2 re-keys, Quick mode is performed again.
When Phase 1 re-keys, Main mode is performed again.
Some details of what goes on within Quick mode and Main mode are outlined here:
In IPsec VPN, how is the pre-shared key encrypted?
Should IKE Phase 1 timer should match or the lowest will be chosen?
Should IPSec Phase 2 timers should match or the lowest will be chosen?
Between two peers, they agree to use the lowest configured timer. If one peer prefers a 86400 timeout for IKE/Isakmp, and the other prefers 43200, then they both agree to use 43200.
Same for Phase 2.