Prefix list question

[George1988]

What does it mean by 

ip prefix-list 0.0.0.0/0 le 23 

this prefix list is applied inbound on our internet routers in BGP

I am not sure what's the use of this prefix list ?

[Martin L]

There are Lots of applications, methods, ways for prefix list to be used.  there are 2 parts or even 3 parts in case of route-maps: where and when/what.  the usage of any prefix list will depends on where or  how is applied;  you gave us 1 part but not the other one.  you must look Route-maps and in BGP/OSPF/EIGRP routing section,  what commands are there to point to prefix list ? do you have any route-maps ?

note: you missing prefix id or name; ip prefix-list Xname 0.0.0.0/0 le 23 , then BGP section would apply prefix Xname with neighbor x.x.x.x filter-list prefix-list Xname in

mostly prefix list is applied to a route-map, then route-map is applied to BGP or other protocols.

example:

ip prefix-list PL seq 5 permit 8.8.8.0/24
route-map INET permit 5
match ip address prefix-list PL
set metric 21

route-map INET permit 50

router bgp 12345
address-family ipv4
neighbor 1.2.3.4  route-map INET in

example 2

router bgp 100
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 update-source Loopback0
neighbor 2.2.2.2 next-hop-self
neighbor 2.2.2.2 prefix-list MARTIN in  (this one here will filter routes from 2.2.2.2)

next you would look around for prefix-list named MARTIN- usually at the end of sh run before line console 0. 

you can use permit or deny -just like ACL- but prefix list gives you more flexibility then ACL.

Regards, ML
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[George1988]

This is the prefix-list 

ip prefix-list XYZ seq 10 permit 0.0.0.0/0 le 23

and it is applied inbound for a BGP neighbor 

neighbor X.X.X.X prefix-list XYZ in

My question is what does it mean by 0.0.0.0/0 le 23 ?

the router will accept any routes with a sub net mask of less than or equal to 23 ?

[Martin L]

Prefix-list are tricky; You have to search and find good explanation that you can understand; Youtube should have several videos.  I learnt them by doing lab samples from old CCNP Route book; Mr. Odom's definition says "1st part specify network range; while 2nd part identifies prefix length , aka mask or range from/to prefix. 

10.0.0.0/8 ge 20 le 22  matches network range of class A, 10.x.x.x/8 (where 1st 8 bits must be 10); then prefix length (subnet mask) is between /20 and /22 including 20 and 22  

Two main ones are: 0.0.0.0/0 >>Match only the default route and

0.0.0.0/0 le 32 <> Match ALL routes (like any any)

Other resources:  INE Blog How do prefix-lists work?

 examples IP prefix list revelation!

 https://learningnetwork.cisco.com/s/global-search/EIGRP%20Prefix-list

Regards, ML
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[Richard Burts · Accepted solution]

The original post asked 2 questions: 

1) what does this prefix list mean

2) what does this prefix list do

We have had good responses about 1) and I would like to make a suggestion about 2). 

Most Internet providers will not accept network advertisement smaller than /24. So the Internet routing table  has lots of /24, /23, /22, etc and is very large. What this prefix list does is to accept most of the network prefixes advertised in the Internet, but it does not accept any advertisement of /24. The result is that they would have a fairly large routing table, but not nearly as large as it would be with the additional /24s. 

Much of the time when an enterprise wants to connect to the Internet and to exchange routes with the ISP, There will be 3 options: 1) default route only, 2) limited prefixes (prefixes from the ISP and from associated AS). 3) full Internet routing table. I see this prefix list as creating a fourth option that fits between limited prefixes and full routes.

[Richard Burts]

This is an unusual way to use a prefix list and so this has been an interesting discussion. I am glad that our responses have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[paul driver]

Hello
That prefix-list will allow any networks with a prefix length of /23 or less.