Re : Prefix-list.

Unanswered Question
Jul 9th, 2009

Hi,

If I have a prefix-list say :

ip prefix-list MJ seq 5 permit 10.0.0.0/8

and advertise it to the coperate BGP table whereby I have say only two /16's in the RIB that falls under 10.0.0.0 range.

My question is what would the impact of a prefix-list when it does not cover all /16's in it. How would the router receiving this prefix-list see this ? It looks like it would only see the /8 and the 2 /16's would be known to someone that knows it. More of an aggregation.

True ?

Thank you,

InternetB.

I have this problem too.
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Giuseppe Larosa Thu, 07/09/2009 - 09:01

Hello InternetB,

the prefix-list syntax allows for the following:

written as

ip prefix-list MJ seq 5 permit 10.0.0.0/8

this line permits only prefix 10/8 no subnets are allowed

written as

ip prefix-list MJ seq 5 permit 10.0.0.0/8 le 16

allows all prefixes within 10/8 block whose prefix len is less or equal to 16

written as

ip prefix-list MJ seq 5 permit 10.0.0.0/8 ge 16

it allows all prefixes within 10/8 block whose prefix lenght is greater or equal to 16.

second version allows

10.1.0.0/16

10.8.0.0/15

third version allows

10.1.0.0/16

10.2.0.0/17

10.255.255.254/32

so a reasonable choice could be

ip prefix-list MJ seq 5 permit 10.0.0.0/8 ge 16 le 20

to be noted the prefix list allows existing routes it doesn't allow for example to create a 10/8 aggregate for this you need the aggregate address command

Hope to help

Giuseppe

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