how can I filter all the routes from one AS?
I want to filter all routes from AS 6400 towards AS1000, how can I filter whole routes from one AS to another?
I do not want to be used as transit AS so I need to do that, I Googled and got some stuff about using prefix-list + route-map but the thing is AS6400 is an actual Internet active AS which hosts some 500000 plus routes, and it does not sound reasonable to write prefix list for that amount of route, so what should I do?
Best Answer
There are a couple ways to do this, without having to specify every single prefix that you're receiving from AS6400 in a prefix-list (I would personally advise against doing this because as you mentioned, the administrative overhead is high and the process will become exponentially more error-prone as the number of prefixes increases).
1) Tag routes you've received from AS6400 with the no export community. You would do this within a route-map:
This will tell R3 to not advertise the routes learned from AS6400 to AS1000 via the eBGP session to R1. This is your simplest option (note here that this will need to be an inbound filter applied on R4 on the eBGP session to R2/AS6400).
2) You could use an AS Path access list to determine which prefixes have an AS path that begins with 6400 and then you could use it on a BGP neighbor statement with a filter list, or you could use it in a route-map to deny advertising the prefixes on R3. This is less simple because it requires knowledge of regex (to be fair, the regex required here is somewhat simple) and it also depends on no one doing anything funny with their AS path, of which there's no real guarantee. Using a route-map, the configuration to implement would look something like this (assuming IOS):
Note that the above will need to be configured as an outbound filter on R3 for the eBGP session with AS1000.
Using the well-known 'no export' community is likely going to be your best bet, along with being very judicious with your outbound announcements to AS1000 and AS6400.