How do prefix-lists work?

 

Prefix-lists are used to match on prefix and prefix-length pairs. Normal prefix-list syntax is as follows:

ip prefix-list LIST permit w.x.y.z/len 

Wherew.x.y.z is your exact prefix
And where len is your exact prefix-length

“ipprefix-list LIST permit 1.2.3.0/24″ would be an exact match for the prefix 1.2.3.0 with a subnet mask of 255.255.255.0. This does not match 1.2.0.0/24, nor does it match 1.2.3.4/32, nor anything in between.

Whenyou add the keywords “GE” and “LE” to the prefix-list, the “len” value changes its meaning. When using GE and LE, the len value specifies how many bits of the prefix you are checking, starting with the most significant bit.

ip prefix-list LIST permit 1.2.3.0/24 le 32 

Thismeans:
Check the first 24 bits of the prefix 1.2.3.0
Thesubnet mask must be less than or equal to 32

This equates to the access-list syntax:

access-list1 permit 1.2.3.0 0.0.0.255

ip prefix-list LIST permit 0.0.0.0/0 le 32

Thismeans:
Check the first 0 bits of the prefix 0.0.0.0
Thesubnet mask must be less than or equal to 32
This equates to anything

ipprefix-list LIST permit 0.0.0.0/0

This means:
Theexact prefix 0.0.0.0, with the exact prefix-length 0.
This is matching a default route.

ipprefix-list LIST permit 10.0.0.0/8 ge 21 le 29

This means:
Checkthe first 8 bits of the prefix 10.0.0.0
The subnet mask must be greater than or equal to 21, and less than or
equalto 29.

ip prefix-list CLASS_A permit 0.0.0.0/1 ge 8 le 8

Thismatches all class A addresses with classful masks. It means:
Check the first bit of the prefix, it must be a 0.
Thesubnet mask must be greater than or equal to 8, and less than or equal to 8. ( It is exactly 8 )

Whenusing the GE and LE values, you must satisfy the condition:

Len < GE <= LE

Therefore“ip prefix-list LIST permit 1.2.3.0/24 ge 8″ is not a valid list.

What you can not do with the prefix-list is match on arbitrary bits like you can in an access-list. Prefix-lists cannot be used to check if a number is even or odd, nor check if a number is divisible by 15, etc… Bit checking in a prefix-list is sequential, starting with the most significant (leftmost) bit.

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