BGP Prefix-list vs BGP table

Answered Question
Jul 6th, 2010

Hi All

One quick question.. Does the subnet mask defined in a prefix list be the same as the entries on a BGP table or routing table ?

for eg if i have 10 * /32 routes on a particular router (10 different loopbacks), can i have a single prefix list with the aggregate address defined ? or do i need to have 10 different rules on the prefix list to allow the /32's ?

note - im not summarizing anywhere to convert the /32's to /24's.. they are just plain /32's on BGP table..

Thanks

Correct Answer by Giuseppe Larosa about 6 years 7 months ago

Hello Sblavanya,

>> ip prefix-list loops seq 5 permit 10.10.10.0/24 ge 32

to allow all 10 routes to be sent to the neighbouring router ?

if they are near to each other (like 10.10.10.1, 10.10.10.2, and so on)

you can even use a more specific network like 10.10.10.0/28 as I did in my example

if they spread over the whole /24 you need to use the /24

note: without the ge 32 option the prefix list looks for the exact route 10.10.10.0/24

Hope to help

Giuseppe

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Chetan Kumar Ress Tue, 07/06/2010 - 12:13

Hi

------>One quick question.. Does the subnet mask defined in a prefix list be the same as the entries on a BGP table or routing table ?

No , It is not requried. The network in routing table & define prefix can be different.

----- >for eg if i have 10 * /32 routes on a particular router (10 different loopbacks), can i have a single prefix list with the aggregate address defined ? or do i need to have 10 different rules on the prefix list to allow the /32's ?

Yes you can have single prefix-list to define all 10 loopback address. But here you can't create single prefix with /32 for all of them. You have to create less than /32 . ( prefix-list xx seq 10 permit 10.x.x.x/24 ) or what ever you want.

In your senario if you wnat to filter teh router without aggregation the you should go with Distribution-list. Prefix list give you to play with prefix not with IP route.

Here you can get an good example.

prefix-list xx seq 10 permit 63.1.0.0/16 ge 18

The statement above allows any route announcement in the range of 63.1.0.0 - 63.1.255.255 but that announcement must have a length greater than 18 bits in the mask. This permits you to allow announcements in the range, but not an announcement equalling the entire range (/16), or even announcements of half the range (/17). Only announcments with a length "greater than or equal to" /18 will be permitted

Regards

Chetan Kumar

Giuseppe Larosa Tue, 07/06/2010 - 12:29

Hello Sblavanya,

another possible solution could be:

ip prefix-list loops seq 5 permit x.y.z.0/28 ge 32

Hope to help

Giuseppe

sblavanya Tue, 07/06/2010 - 15:11

Hi Chetan/Guiseppe

Thanks a ton for your reply.. For eg in my case, lets assume we have the following loopbacks...

10.10.10.1/32 , 10.10.10.2/32.... -> 10.10.10.10/32

Im able to make this work by the following prefix-list:

ip prefix-list test seq xxx permit 10.10.10.1/32

ip prefix-list test seq xxx permit 10.10.10.2/32  etc

Guiseppe - as per your post can i just use one prefix-list with the following command ?

ip prefix-list loops seq 5 permit 10.10.10.0/24 ge 32

to allow all 10 routes to be sent to the neighbouring router ?

we tried---> ip prefix-list loops seq 5 permit 10.10.10.0/24 without "ge 32" and it didnt work, and now i know why

Regards

Correct Answer
Giuseppe Larosa Wed, 07/07/2010 - 11:54

Hello Sblavanya,

>> ip prefix-list loops seq 5 permit 10.10.10.0/24 ge 32

to allow all 10 routes to be sent to the neighbouring router ?

if they are near to each other (like 10.10.10.1, 10.10.10.2, and so on)

you can even use a more specific network like 10.10.10.0/28 as I did in my example

if they spread over the whole /24 you need to use the /24

note: without the ge 32 option the prefix list looks for the exact route 10.10.10.0/24

Hope to help

Giuseppe

sblavanya Fri, 07/09/2010 - 12:02

Thanks Guiseppe..

Ill try this and let you know if it worked good..

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