Re : Prefix-list.

Unanswered Question
Jul 9th, 2009
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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.

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Giuseppe Larosa Thu, 07/09/2009 - 09:01
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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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