Why is in OSPF LSA sequence nuber in range 0x80000001 to 0x7FFFFFFF

Question

Why is in OSPF LSA sequence number in range 0x80000001 to 0x7FFFFFFF. I suppose that it is for some historical reasons but cannot google it.

Answer 1

LSA sequence number is a signed 32-bit integer.

So 0x80000001 to 0x7fffffff basically covers the whole possible range except one number: 0x80000000, and this special number is reserved.

You can read this page of RFC 2328 for more details.

Answer 2

As a signed integer, the sequence number counts from -2 147 483 647 to 2 147 483 647.

If it would count from 0x00000001 to 0xffffffff, it would appear as a count from 1 to 2 147 483 647 and then from -2 147 483 648 to -1.