Does a BGP prefix need to exactly match the record in RIPE/ARIN?

[Filip Knezevic]

Hello,

I have a BGP question.

One of our customers has a problem with latency to certain IP addresses, lets say, 11.11.11.11 and 11.11.12.131. Those IP addresses are all part of a /19 prefix record in ARIN, say 11.11.0.0/19. We have two upstream providers, and the neighbors are 1.1.1.1 and 2.2.2.2.

However when I do sh ip ro 11.11.11.11, it will say it's a part of 11.11.11.0/24 subnet. If you type in that address in ARIN, you will see it's announced as /19.

Now if you do sh ip bgp neigh 1.1.1.1 routes | i 11.11.0, it will print that it is only learning this prefix as 11.11.11.0/24, but not as 11.11.0.0./19.

I thought that maybe the prefix was advertised as more specific (g.e. le 19), but it should also be learned as /19?

Checking the other neighbor 2.2.2.2 will give the same results.

So I'm confused if this can create issues for the customer?

Should that prefix be learned as 19 (so it matches ARIN) and also as a more specific 24?

Or it can happen that ARIN operator from that company didn't change their records and advertised those subnets as /24s?

Thank you in advance.

If you guys want, I can post some sanatized command outputs. Thanks.

[Julio E. Moisa · Accepted solution]

Hi,

Please correct me If Im wrong, please verify the prefix into the BGP table: show ip bgp and verify how it is known. When you are advertising a prefix through BGP the subnet mask must match with the entry into the RIB.

[Filip Knezevic]

Hi Julio,

Thank you. You are right. Sh ip bgp x.x.x.x did the job. Now I know the advertising is not correct in my case and can start writing a letter to our Upstream. Thanks :)

[Julio E. Moisa]

Hi

You are welcome, have a great day!

:-)