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ThinkMo EDU Share – network 61.What is the problem with the OSPF serial number?

蒂娜 1 Comment 01/06/2023

ThinkMo EDU Share – network 61.What is the problem with the OSPF serial number?

The start value of OSPF LSA Sequence Number is 0x80000001, and the end value is 0x7FFFFFFF. You may be confused, how can the start number be greater than the end number?

The upper limit of the number can be adjusted arbitrarily

In fact, this is because the initial 8 in the starting value represents a negative number, which means -7FFFFFFF, please refer to the following description to understand:

Playing games in the early days was a kind of self-cultivation and improvement of computer ability . With the help of these tools , experts in human manipulation can turn the dreams in the game into reality, as long as the upper limit of various possible numbers and the limit of the protagonist’s ability can be adjusted arbitrarily.

Byte range

Without further ado, let’s see what we’re going to talk about. A byte (byte) data can represent a range from 0 (0000 0000) to 255 (1111 1111), so how to represent negative numbers ?

There are only 0 and 1 in the computer system, and there are no additional symbols that can be used to represent the so-called positive/negative numbers, so a workaround can be used. We use the leftmost number in the byte group to represent positive or negative , 0 is positive and 1 is negative.

For example, if 0111 1111 is converted to decimal, it is 127, and if 1111 1111 is converted to decimal, it is -127, so we know that the representation range of a byte can range from -127 to 127 .

Reciprocal

What we talked about above is the so-called original code representation, but the data in the computer is stored in complement , only in this way can the burden on the CPU be reduced.

When it comes to complements, we have to mention the countdown . In the computer, the reciprocal is stipulated as follows: if it is a positive number, it will remain unchanged according to the original code notation , for example, 127 is still 0111 1111, and if it is a negative number, then the first number is 1, and the other numbers are Inversely (that is, 0 becomes 1, 1 becomes 0), for example, in binary representation -127 (1111 1111), the reciprocal representation is 1000 0000.

Complement

same is true for the complement number. If it is a positive number , it will remain unchanged according to the original code representation . For example, 127 is still 0111 1111 ; Such as -127, first take the reciprocal to be 1000 0000, and then add 1, it is 1000 0001.
But 1000 0000 is special , we can use it to represent -128, so the range that can be represented by complement number is -128 to 127 .

Serial number N

“The sequence number -N (0x80000000) is reserved and unused , and -N+1 (0x80000001) represents the smallest (that is, the oldest) sequence number, which is defined as the constant InitialSequenceNumber .

A router uses the InitialSequenceNumber when it first generates any LSA, and increments the LSA sequence number by one each time a router generates a new LSA instance .

When attempting to increase the maximum sequence number N-1 (0x7fffffff, also defined as MaxSequenceNumber), the current LSA must be revoked from the routing domain first .


This is accomplished by prematurely aging the LSA (see Section 14.1) and reflooding it. When acknowledgments are received from all adjacent neighbors, a new instance with the InitialSequenceNumber as the sequence number is created.

Unexpectedly received LSA

unexpected new instance of an LSA is received during the flooding process , a router may forcibly increase its LSA sequence number. This should rarely happen, it may be that old LSAs sent by the router before the last reboot still exist in the AS, see Section 13.4 for more information. “

If the serial number reaches 0X7FFFFFFF, the current router terminates the LSA, then advertises the LSA with an AGE of 60 minutes (aging in advance), and resets its serial number to 0X80000001.

Notice

(The inverse of a negative number is to invert every bit of its original code except the sign bit [-3] inverse=[10000011] inverse=11111100 The complement of a negative number is to invert every bit of its original code except the sign bit Add 1 to the end.

[-3] complement = [10000011] complement = 11111101 a number and its complement are reversible_).

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