• Filter Long AS Paths
  • Purpose
• Configuration Examples
  • BIRD
  • Junos
  • IOS-XR
  • OpenBGPD

Filter Long AS Paths

Purpose

Some networks go completely overboard on the number of pre-pending AS numbers. And as it is a known attack from the past for AS paths longer than 256 AS numbers .. you might want to filter on max number of AS numbers.

There are at the moment of writing this page some prefixes with about 40 ASn’s in the AS_Path. So a filter of more than 50 should give no additional filtered prefixes.

A safe number on the filter would be on 100 AS’s in the AS_PATH.

Configuration Examples

BIRD

function reject_long_aspaths()
{
    if ( bgp_path.len > 100 ) then {
        print "Reject: Too long AS path: ", net, " ", bgp_path;
        reject;
    }
}

Junos

policy-options {
  policy-statement bgp-import-policy {
  term no-long-paths {
      from as-path too-many-hops;
      then reject;
     }
   }
 }

 as-path too-many-hops ".{100,}";

Info about the original bug report : Link to kb article

IOS-XR

route-policy BGP_FILTER_IN
  if as-path length ge 100 then
    drop
  endif
end-policy

OpenBGPD

deny from any max-as-len 100