Re : Prefix-list.
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07-09-2009 04:46 AM - edited 03-04-2019 05:22 AM
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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07-09-2009 09:01 AM
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
